The Entities of Psychology: Stimulus, Response and Relation

Page-ID: 25. Version 1.0. Last updated: 22 Nov 2023

The fundamental entities of psychology
- The “entities of psychology” principle
Stimulus
Response
Relation
Context
The Reasoning and further details
Alternative terms for the concept of “stimulus”
Alternative terms for the concept of “response”
Alternative terms for the concept of “relation”
Page information
References

The fundamental entities of psychology

The first principles of psychology must both include the fundamental entities of psychology (what all psychological phenomena are made of) and the fundamental laws of psychology (that explain how psychological phenomena behave or change). In physics, the fundamental entities are particles or fields, and the fundamental laws include forces like gravity and electromagnetism, and other laws like those in quantum mechanics and special and general relativity. We need to know what the fundamental entities are in order to understand what the fundamental laws apply to. What are the fundamental entities of psychology? The answer is: stimulus, response and relation. What it means to say that these are the fundamental entities is that any psychological phenomenon is made up of these entities. They are the smallest building blocks of psychology, from which everything else can be built. We can express this in terms of the following first principle:

The “entities of psychology” principle

Any psychological phenomenon is either a stimulus, a response, a relation, or a combination of these. There is nothing else.

In short, a stimulus is anything that is experienced as a coherent phenomenon. A response is any psychological event that is caused by one or several stimuli. A relation is the connection between stimuli and responses that specify which responses will be triggered when one or several particular stimuli occur. The figure below illustrates how stimulus, response and relation are connected.

Figure 1 – Stimulus, relation, response

For example, the stimulus could be the smell of food. The response could be to salivate. The relation would in that case be the connection between the smell of food and the salivation. The stimulus (the smell of food) causes the response (to salivate) to occur because there is a connection in the mind between the two.

Let us now look more closely at these entities and the properties that they have.

Stimulus

A stimulus is anything that is experienced as a coherent phenomenon. We can describe this in more loose terms as anything that is experienced as a “thing” or “event” or some kind of “unit”. For example, we experience a tree as a “thing” or coherent phenomenon, but we do not experience the combination of a cat, the movie Titanic, the sound of a fox and the eiffel tower as a “thing” or coherent phenomenon, instead, these things are experienced as four separate stimuli.

Stimulus elements are the parts of a stimulus that the stimulus consists of. For example, when we see the face of a person as a single stimulus, then the eyes, ears, mouth, nose, hair, etc are the stimulus elements of that stimulus. If we focus our attention towards any of the stimulus elements they can become stimuli of their own, but in the very moment where the whole face is the stimulus, the stimulus elements are not technically stimuli on their own.

There are two broad types of stimuli:

An external stimulus is a stimulus that exists outside the mind, out there in the world. In other words, the stimulus is external to the mind. For example, a tree, a song, a cat.

An internal stimulus is a stimulus that exists only inside the mind. In other words, the stimulus is internal to the mind. For example, a memory, a dream, a feeling.

Types of causal power
External and internal stimuli have different forms of causal power. External stimuli can cause things to happen even when we have no experience of them, while internal stimuli can only cause things to happen while we experience them. For example, since an apple is an external stimulus, it can fall from a tree and land on the ground, even if no one is there to see it. And since the thought of a unicorn is an internal stimulus, it can cause a person to think of a rainbow, which is another internal stimulus, but it cannot cause a physical rainbow to appear. Thanks to this, external and internal stimuli can bring about different types of psychological responses, and differ in how they do so.

Types of psychological responses
External stimuli can only cause one type of psychological response called “sensation”, and it does so by stimulating the senses of an organism via transmission of physical energies. For example, when light enters our eyes, the light energy stimulates the millions of receptors at the back of the eye and brings about millions of consciously experienced tiny dots of different colors and intensities. When a sound wave enters our ears, the sound energy can stimulate thousands of receptors inside the ear, resulting in a conscious experience of thousands of individual frequencies. External stimuli thus only cause raw sensory data to occur. Each sensory data (for example, each dot or frequency) is an internal stimulus or stimulus element, which corresponds to a particular physical energy in a specific location. But on its own it usually doesn’t make much sense. Instead of seeing a million dots, we would prefer to see objects, landscapes, animals, movements. Instead of hearing a thousand frequencies we would prefer to hear voices, songs, sounds. This is what happens after the external stimuli have been sensed by the mind and thus the creation of internal stimuli.

Each internal stimulus can be viewed on two different levels:
The content level: What the conscious experience is about
The meta level: What the conscious experience is itself

For example, let’s take the experience of hearing a song. The content of this experience is the song itself. This is an external stimulus, something that exists outside the mind. But the experience of hearing a song itself is an internal stimulus, this is the meta level of the stimulus. Another example is the experience of thinking about a unicorn. The content of this experience is in this case an internal stimulus, since the unicorn only exists in our minds (if we are not talking about a drawing of a unicorn or something like this). The experience itself is also an internal stimulus, i.e. a thought. So in this case, both the content level and the meta level describe internal stimuli.

Direct experience: When we experience an external or internal stimulus directly, for example when we perceive an external stimulus (like a tree or song) that is present in that moment, or experience an internal stimulus (like a feeling or mental image) that is present in that moment.

Indirect experience: When we experience an external or internal stimulus indirectly, for example when we remember something that we experienced before or when we are told about something that someone else has experienced.

Response

A response is any psychological event that is caused by one or several stimuli. For example, when we feel the taste of something sweet we start to salivate, when our eyes feel dry we blink, when we hear a song that we like we feel happy. There are different types of responses. The response may be a behavior (e.g. blinking, grabbing something with the hand), a physiological response (e.g. release of hormones, increasing heartbeat), learning (i.e. forming new relations between stimuli and responses), an emotion (e.g. fear, happiness), a memory (e.g. remembering the smell of roses, a song we heard a few years ago), a prediction (e.g. expecting our the next step of the stairs to be as high as the previous step). The response can be to activate something (e.g. cause a behavior), or to inhibit something (e.g. stop a behavior).

A response must be observed in some way in order for us to know that it occurred. There are three ways we can observe a response:

Direct experience by an outside observer. For example, when we see an organism move, or when we observe physiological reactions of the organism like crying or increased heart rate.
Direct experience from the inside, by the organism itself. For example, when we observe feelings or thoughts that we have.
Indirect experience where the response is inferred from its observable consequences. For example, we infer that a memory was created because at a later point in time we remember something.

The response to a stimulus can be to produce another stimulus. For example, when we see a wolf we have the response of being scared. The feeling of being scared is also a stimulus. So when we say that a stimulus causes another stimulus to occur, then what is really going on is that the occurrence of the second stimulus is a response to the first.

Relation

A relation is the connection between stimuli and responses that specify which responses will be triggered when one or several particular stimuli occur. When a particular stimulus causes a particular response to occur, then we can only explain this by expressing that there is a relation between the two. When one stimulus causes three responses to occur, then the stimulus has three different relations, each connected to a different response.

Context

Another concept that is useful to define, even though it is not a fundamental entity, is “context”. When a specific stimulus is in focus in one moment, all the other stimuli that are present in that moment make up the context for that stimulus. To clarify which stimuli we are talking about in one situation, we can say “context stimuli” or “contextual stimuli” for the stimuli that make up the context, and “focal stimulus” for the stimulus in focus.

The Reasoning and further details

How should we define “stimulus”?

The question

How should we define “stimulus”?

Why is this question important?

The term “stimulus” is central to psychology, and in particular in the unified theory. If we don’t have a good understanding of what a stimulus is, then the laws that are applied to stimuli will yield the wrong results. Trying to define the term “stimulus” is also part of a broader ambition to define what the fundamental entities of psychology are. In physics, for example, the fundamental entities are particles (to put it in rough terms), and the laws of physics apply to these fundamental entities. If we don’t know what the fundamental entities are in a field, then we won’t know how the laws are applied with any good accuracy. So defining the term “stimulus” correctly is therefore crucial in order to set up the field of psychology from first principles.

Notes on strategy

Establishing the first principles of a field means finding all of the fundamental entities of the field. This means that all phenomena within the field must be explained in terms of the fundamental entities. If the terms “stimulus”, “response” and “relation” are to be established as the fundamental entities of psychology, then all psychological phenomena must be able to be categorized as one of these, or a combination of them. When defining the term “stimulus” we must therefore make sure that this definition allows us to establish the full set of fundamental entities.

Observations

Behaviors occur as a response to sensory phenomena (For example, visual, auditory, taste, smell)
Responses are triggered by physical energies (for example, sound waves activate our auditory receptors, heat energy activates our heat-sensing receptors)
Stimulating a nerve with electricity can cause muscles to twitch.
A certain phenomenon that has been shown to cause a response, might not succeed in doing so every single time it occurs, but merely most of the time.
Responses can occur when an event/phenomenon is predicted to occur but does not appear. In other words, it is triggered by the surprise of the absence of stimulation. ( the response does not occur because of the activation of receptors, but rather the opposite).
The comparison (i.e. relation) between two phenomena or events can cause a response (i.e. not the activation of receptors per se, but a higher level comparison becomes a stimulus)
A response to a pattern (for example, a certain arrangement of dots, or objects) can be different from the response to when the individual parts occur on their own. This is true both of spatial patterns and temporal patterns (i.e. sequences).
When the phenomenon triggering a response is unchanging for a certain amount of time, the response eventually stops.
The greater the magnitude of a phenomenon that causes a response, the greater the magnitude of that response
We have experiences of phenomena or events around us.
There are things in the world that we can not experience directly, but merely conclude to exist via indirect observations (for example, very low frequencies and very high frequencies of light and sound)
We have experiences of phenomena or events that are merely in our minds (e.g. thoughts and feeling)
When we have certain experiences, certain responses may be caused to occur.
Even things we imagine in our mind can trigger responses.
There are certain experiences that do not cause a response to occur.
It may be that events or phenomena that are too weak or brief to be consciously experienced can trigger a response to a weak degree.

Hypotheses (alternatives for definition of “stimulus”)

There are a number of aspects of the definition of “stimulus” that need to be specified, this includes what type of phenomenon that a stimulus is, its level of granularity, whether it is something potential or actual, its degree of effectiveness, and the viewpoint from which it ought to be defined. A complete definition of stimulus must take a stance on the available hypotheses for all of these aspects.

[Regarding a stimulus’s type of phenomenon]
A stimulus is the cause of a response.
A stimulus is the object that is the cause of the response
A stimulus is the physical stimulation of the receptors of the organism, described in terms of the physical energies involved.
A stimulus is the activity of the receptors of the organism (regardless of what physical energies were involved in order for that activity to appear).
A stimulus is something that is experienced by the organism
A stimulus is anything we experience as a coherent phenomenon
A stimulus is anything that is treated by the mind as a coherent phenomenon

[Regarding a stimulus’ granularity]
A stimulus is a single cell, or “pixel”
OR
A stimulus is a pattern of cells or “pixels”

[Regarding a stimulus’ potentiality]
A stimulus is something that can cause a response/be experienced/activate receptors, etc
OR
A stimulus is something that does cause a response/ being experienced/activating receptors, etc.

[Regarding a stimulus’ effectiveness]
A stimulus is something that triggers a response
OR
A stimulus is something that merely motivates the individual to perform responses, and so the response isn’t automatically triggered

[Regarding a stimulus’s viewpoint]
A stimulus is defined from the viewpoint of the organism performing the response
OR
A stimulus is defined from the viewpoint of the outside observer

A stimulus in a given situation is something that is experienced as a more or less coherent phenomenon, i.e. as a “thing” or “unit”.
Or, in the case of subliminal perception: something would be experienced as a “thing” if it had occurred for long enough to be consciously experienced.

Evaluation of hypotheses

“A stimulus is the cause of a response” – partly correct

There are many observations where psychological responses are caused by the occurrence of some phenomenon or event. Defining “stimulus” as those phenomena or events which cause a psychological response would therefore make a lot of sense. But there are also a set of problems with this definition.

First, whenever we talk about causality, the situation is always more complex than it may initially seem. For example, if a car crashes we may try to uncover what caused the car to crash. We may discover that one of the wheels came off the car before the crash, and so we might want to say that what caused the car to crash was that the wheel came off. But the reason that the wheel came off was because it hadn’t been securely tightened, so we could equally say that what caused the car to crash was that the wheel hadn’t been securely tightened. But the reason that the wheel wasn’t securely tightened was that the car mechanic had failed to notice that the wheel was loose. So we may say that the car crashed because the mechanic had failed to notice the loose wheel. But the reason that the mechanic failed to notice that the wheel was loose was that the owner of the company had distracted the mechanic. And so on. Where do we stop? What was the “real” cause of the car crash? We could continue to go back in time until we reach the Big Bang, the birth of the universe, but it does not seem reasonable to say that the car crash was caused by the Big Bang. Similarly, we could discover that the car was traveling on a bumpy road when the wheel came off, and if the driver had chosen to drive on another road, the wheel would not have come off. So we could equally say that the car crash was caused by the bumpy road, or if we wish, by the decision of the driver to drive on the bumpy road. And once again we could then go back in time step by step to find the ultimate cause behind the car crash. We could also point out things like if there hadn’t been a tree where the car was driving, the crash wouldn’t have occurred either, so we could argue that it was the tree that caused the crash to occur. We could go on forever.

So trying to define “stimulus” as a cause isn’t really enough to tell us what that “thing” was that caused the response to occur. When we blink, this response can be said to be caused by the air that caused the eye to become dry, or we could define the cause as the “dryness” of the eye, or we could define it as the stimulation of the receptors in the eye that sensed the dryness or the air, or we could define it as the neurons that were activated by the stimulation of the receptors. Which of these alternatives is the correct stimulus if we choose to define “stimulus” as the cause of the response? In similarity with the example of the car, we could expand the options forever, perhaps the stimulus is the person’s decision to stare at the scenery, which resulted in the eye becoming dry. Or it is the movements of the wind across the globe that are the stimulus, since this is what caused the air to hit the person’s eye. And so on.

Merely defining “stimulus” as that phenomenon that causes a psychological response isn’t good enough. The definition is too general for us to be able to tell which phenomenon is the stimulus in a specific situation. There are too many things that could be said to be the cause of a response. Our definition isn’t precise enough to tell us which is the correct choice. Being able to cause a response may still be a central feature of stimuli, but it isn’t sufficient as the definition of “stimulus”. Perhaps we could just add some more details to this definition to solve this problem? There are some suggestions along these lines that we can take a closer look at.

“A stimulus is the object that is the cause of the response” – incorrect

One common definition of “stimulus” is that the stimulus is the object in the world outside us, that causes the response to occur. To take a common example, if someone sees a tree, then this tree is a stimulus that causes the person to see the tree, as well as any other response that the person performs as a result of seeing that tree. But there are some problems with defining stimulus this way. First of all, how do we know that it is the tree that is the relevant object in that example? The tree reflects light from the sun. Isn’t it then the sun that is the object that caused the response? Or if we see a tree in the reflection of a mirror, isn’t it the mirror that is the object that caused the response?

When “stimulus” is defined as the object that causes a response, it is sometimes specified that it is not just any object, but the object that is the “ultimate cause” of the response. But we know from our previous discussion that really the only ultimate cause of everything in the universe is the Big Bang so this definition would not allow us to specify the tree in our example as being a stimulus. Sometimes, “stimulus” is defined more loosely as “the distal object” that causes the response. But then how do we know which object is the “distal” one?

Even if we were able to solve that question, there are many other types of phenomena that are not objects but still cause psychological responses to occur. For example, a response may be triggered by a sound, an electric current, by a thought, by a dream, by a feeling, by a sensation. A response may even be triggered by the absence of something which we expected to occur.

So “stimulus” cannot be defined as the object that causes the response. There are just too many stimuli that aren’t objects, and even when there are objects we don’t know which of these to choose as the stimulus. Let us therefore look at some other possibilities.

“A stimulus is the physical stimulation of receptors” – incorrect

Another common definition is that a stimulus is the physical stimulation of the receptors of the organism, described in terms of the physical energies involved. For example, if a researcher stimulates a receptor by an electric current and this causes the leg of the animal to twitch, then this electric current would be the stimulus in this example. These kinds of experiments were some of the earliest within the field of neurology, and it is where the term “stimulus” was first used in the study of animal behavior. It makes sense in such experiments to isolate what kind of physical stimulation that causes responses in an animal, and give these a name in the form of “stimulus”. There is a clear causal connection between the physical stimulation (the stimulus) and the subsequent movement of the animal’s limbs (the response). The application of the physical energy is under control by the experimenter, and so it is clear that as long as all other factors are kept the same, the only thing that determines whether the response occurs or not is the presence of that physical energy. Thus, defining stimulus as the physical stimulation of receptors allows us to use the term “stimulus” to investigate what kinds of responses that are caused by the various stimuli. In other words, such a definition allows us to formulate laws of how different stimuli and responses relate to each other.

If we use this definition then we could say that when we blink as a response to a puff of air in our eyes, then the stimulus in this case is the kinetic energy in the air that stimulates the sensory receptors in the eye. When we pull our hand away from a hot stove, then we can say that the stimulus in this case is the heat energy from the stove that stimulates our heat receptors in our hand. So in some examples this definition seems to make sense. But there are also some problems with the definition.

First, in our everyday lives, millions of receptors are stimulated at the same time. In the human eye alone, there are more than 100 million receptors. So, for example, when someone sees a dog there are millions of small receptors in that person’s eye that are stimulated by light energy. If the person responds to the dog with fear and running away, we will want to understand what stimulus caused this response. Should we then say that in that moment there are a million stimuli, that the person reacted to the millions of physical energies in their eyes? This kind of answer does not provide sufficient information. Before the dog appeared, there were also a million receptors being stimulated in the person’s eye, but the person did not become fearful and run away. So what is the difference between the million stimuli in one situation versus the other?

One way of trying to answer this question is to say that the difference is the pattern. In the situation with the dog, the stimulation of receptors has one pattern, and in the situation without the dog, the stimulation of receptors has another pattern. From this we could try to define “stimulus” as the pattern of physical stimulation of receptors. Each individual physical stimulation is just a “stimulus element” that can be combined with other “stimulus elements” in different ways to form different “stimuli”. But we would still have to be able to describe what that pattern is in each of the situations, otherwise we won’t be able to recognize when one stimulus is present as opposed to another stimulus. This is a lot harder to describe than one may think. For example, two dogs can look very different. They can have different shapes, colors, sizes. So the pattern when we see one dog can be very different from the pattern when we see another dog, yet the person still has the same response, i.e. gets scared and runs away. Also, if we look at the same dog from different perspectives, for example, from the side, from the front, from above, from below, the pattern is always different. What’s even worse is that if a cardboard box approaches the person and someone says “there’s a dog inside” then the person may become scared and run away, even though the visual pattern of receptors has nothing in common at all with those of a dog since a cardboard box looks nothing like a dog.

So how can we explain that the same response occurs, even though the pattern of physical stimulation is completely different between situations? Simply talking about physical stimulation does not capture the central properties of a stimulus. The same cardboard box can both cause a person to stay calm and cause a person to be scared and run away, depending on what the person believes is inside the cardboard box. Adding the thought of there being a dog causes fear, even when the physical stimulation of receptors is the same. In fact, the person could just as well lie in a dark room with no physical stimulation at all: if that person started to think about dogs, they could become scared.

What seems to be the common thread between all the different situations where the same response is observed is how the person experiences a situation. So let us therefore look more closely at defining “stimulus” in terms of experiences. Before we do so, it may be worth pointing out the difference between neurology and psychology. Even though the definition of stimulus as physical stimulation does not work for psychology, it works better when describing experiments in neurology. We must remember that neurology and psychology are two different fields. Psychology is the study of the mind, of animals that are alive, that experience the world. Applying an electric current to a dead frog and noticing that the leg twitches is therefore not an example of psychology. It is an example of physiology. Exactly how to think of the term “stimulus” in such cases will be clearer once we have defined its meaning in psychology. So let us therefore turn to how the term “stimulus” must be defined in psychology.

“A stimulus is something that is experienced” – correct but insufficient

Something that is undeniable for anyone who is trying to make sense of the world is that they have experiences. We experience the world around us, we experience our bodies, and we experience things in our mind. Our experience consists of a number of “parts”. For example, when we go for a walk we may experience a path in a field, individual trees, a sky, a sun, our body, other people around us, the sounds of birds, the sound of traffic, the sound of our footsteps, the smell of the flowers, the feeling of wind against our body, the feeling of our body moving, a feeling of heat, a sense of balance, the position of our body parts, a feeling of happiness, a thought about where to go next, a memory of when we walked this path yesterday, and so on.

As we live our lives we can discover that some of the things or events we experience cause some kind of reaction in us. For example, when we feel pain in our hand we immediately pull it away from whatever was causing the pain. Or when we feel that our eyes are dry, we automatically blink. Thus we conclude that there are events that seem to be triggered in us by some of the things we experience. It therefore seems to make sense to define “stimulus” as any “thing” or “event” that we experience. And we can then study in what ways different stimuli (i.e. different experienced “things” or “events”) cause us to respond or react in some way.

Going back to the example of a dog: If we are scared of dogs, then whenever we have the experience that a dog is near us, then our fear and desire to run away is triggered. The stimulus is the experience of there being a dog, the response is the fear and running away. Compare this to the attempt to define a stimulus as the physical object that caused the response to occur. When we defined stimulus this way, we could not explain why a person got scared when they mistakenly thought that there was a dog nearby, when there actually was no such dog. What causes a response to occur isn’t the object or event per se, but our experience (for example our belief) that there is such an object or event. Of course, most of the time our experience will reflect what is actually there. When there are dogs around we will often be able to perceive them by seeing, hearing or feeling them, and so the fear will appear because we become consciously aware of the dogs. But sometimes we will not perceive that there is a dog, even though there is one, and our fear may not be triggered despite the object being present. And sometimes we will think there is a dog, even though there isn’t one, and our fear will be triggered all the same.

Defining “stimulus” in terms of conscious experience also helps us solve the problem of how we can have the same response to a stimulus, even when the pattern of stimulation of the receptors are completely different in one situation compared to another. When seeing a small black dog on our right hand side, the activation of our receptors is completely different from when we see a big white dog on our left hand side. Yet, we perceive them both as the same kind of animal: a dog. The explanation for this is that how we experience and respond to things depends not only on how the receptors in our senses are activated at that moment, nor on what is physically present around us at that moment, but also by our previous experiences, our learning history, and how we have learnt more general or abstract ideas. Let’s look at how that works.

Our mind is constantly trying to predict what stimuli will appear in the next moment, and via this mechanism it is able to connect things we experience to form more advanced concepts or experiences. So when we see a dog from one perspective, and then move around and see it from a slightly different perspective, we learn that these experiences are just two different perspectives of the same object. Through such experiences our mind builds up a more general experience or “idea” of the dog such that when we view the dog from one perspective we don’t experience just that perspective, instead we experience it as an object that has several possible perspectives. We see the whole dog “Fido” or “Bella”, not some two-dimensional image or a collection of colors and lines. And when we see one dog behave in a certain way, and at a later time see another dog behave in similar ways, we learn that these two dogs are the same type of animal. So when we see a dog we don’t just have an experience of that specific dog, but also have an experience of it being a dog among many. In other words, we don’t just experience it as being “Fido” or “Bella”, but also as “a dog” more generally. So for example, if we have been bitten by one dog, then we expect other dogs to bite us too, because of us having learnt that different dogs often behave in similar ways. So our personal history of learning things about the world causes our experience in any given moment to consist of more than is physically present at that moment.

So defining “stimulus” as something we experience allows us to explain how and when stimuli cause responses to occur when other attempts to define the term fail. But simply saying that a stimulus is something that is experienced is not a sufficiently detailed definition. Earlier we described our experience as consisting of a number of “parts”, and said that a stimulus is such a “part” of our experience. But what makes something a “part”? If we have a person standing in front of us, is that person a stimulus, or is the person’s hat, t-shirt, shorts and shoes individual stimuli? When are these things “parts” of something else, and when are they “things in themselves”? Is the person’s eyes stimuli or is it the person’s whole face, or head that is a stimulus? Is there one stimulus in front of us, or 1000 stimuli? If we can’t answer such questions then our definition of “stimulus” is not precise enough. Let us therefore see how we can make it more precise.

“A stimulus is anything we experience as a coherent phenomenon” – correct

What makes something that we experience one stimulus as opposed to a collection of several stimuli? How do we know how to divide up the world we experience in its “correct parts”? The answer is that our experience tells us the answer. When we experience the world around us, some things are experienced as more or less one “thing” or event, being more or less separate from other “things” or events. So a more precise definition of stimulus is that it is anything that is experienced as a coherent phenomenon, i.e. that is experienced as “a thing in itself” and not merely as a part of something or as just some jumbled nonsense. But how we experience this will depend on the specific situation.

When we see a person, we could experience the whole person as a single stimulus. For example, if we see a person from far away walking alone in a big field, then in contrast to the surroundings the person stands out and is experienced as separate from the surroundings. Thus the whole person is experienced as a single stimulus. We cannot see enough details of the person for any of these details to stand out and be perceived as separate stimuli. If we see a person from far away surrounded by a thousand other people, then instead we may perceive the whole crowd as a single stimulus, where the individual people are experienced more vaguely or indirectly as parts of that stimulus. If we see a person very close up and where their head lines up with ours, then when looking at their face we may experience the face or the head as a single stimulus, and the rest of the body more vaguely as a somewhat separate stimulus. If we focus our attention on the eyes of the person, then the eyes can be experienced as separate stimuli, while the rest of the face is experienced as another stimulus. If we see two people coming towards us that we have met many times before and know that they are a couple who do everything together, then we may see these two people as a single stimulus. “Oh, look here come the Hendersons!”

What makes something a stimulus in a specific situation is thus determined by factors like how it compares or contrasts to the surrounding stimuli, how we focus our attention, and our previous experiences involving the stimuli. When we experience something as a stimulus, we may call the parts or details of that stimulus “stimulus elements”. For example, when we see the face of a person as a single stimulus, then the eyes, ears, mouth, nose, hair, etc are the stimulus elements of that stimulus. If we focus our attention towards any of the stimulus elements they can become stimuli of their own, but in the moment where the whole face is the stimulus, the stimulus elements are not technically stimuli on their own. The concept of “stimulus” is thus very dynamic, always depending on the specific situation under interest. There is no absolute or “always true” stimulus that can be established “in general”. What counts as one stimulus for one individual, in one context, at one point in time, does not have to count as one stimulus for another individual, or in another context, or at another point in time. This fact can be hard to accept since in science and philosophy we often want to establish absolute truths and general statements. This doesn’t cause any problems for science or philosophy though, since the definition of stimulus is still an absolute and general truth, and describes phenomena to which absolute and general laws can be applied. It is only that what counts as a stimulus in a specific situation always is relative, because our experience is always affected by contextual factors.

Another concern may be that the boundaries of a stimulus often are fuzzy. For something to be a stimulus it isn’t necessary that the phenomenon is 100% coherent and 100% separate from all else. Instead, something can be more or less of a stimulus, depending on the degree to which it is coherent and separate from other phenomena. For example, a mountain can be a stimulus even though it isn’t entirely clear exactly where the mountain begins and the ground ends. The closer we are to the mountain, the harder it will usually be to point to the exact spot where the ground stops and the mountain begins. But the further away we are, the more the mountain is experienced as separate from the ground, as the area of uncertainty is less prominent at such distances. We will rarely be in a situation where there is a 100% clear cut between the mountain and everything else. Still, we can experience the mountain as a “thing in itself”, i.e. as a stimulus.

Summing all this up, technically it might be more precise to say that: A stimulus is something that is experienced as a more or less coherent phenomenon in a specified situation. But the less technical definition is good enough, as long as we understand the nuances of what it means. This definition is true of all stimuli, but it may be useful to know more than this general definition in order to understand properly what a stimulus is. We also need an understanding of two different types of stimuli: external stimuli and internal stimuli. These have very different properties, they cause very different types of responses, and do so through very different mechanisms. Let us therefore take a closer look at them.

External and internal stimulus
There are two broad types of stimuli:

An external stimulus is a stimulus that exists outside the mind, in other words, out there in the world. In other words, the stimulus is external to the mind.

An internal stimulus is a stimulus that exists only inside the mind. In other words, the stimulus is internal to the mind.

In order to make sense of these different types we must first remember that we have defined “stimulus” in such a way that when we use this term we are not referring to the experience itself, but to the phenomenon (the “thing” or “event”) that we are having an experience of. The definition states “A stimulus in anything that is experienced as a coherent phenomenon”. And so the things we experience can exist in different forms and have different properties. We may experience things that exist in the world outside our mind, like trees, the sky, the sound of a bird, and we may experience things that exist only inside our mind, like a feeling, a memory, a hallucination.

When a stimulus exists outside the mind it has a physical existence. It is made up of physical matter or energies, and follows the laws of physics. When a stimulus exists inside the mind it has a mental existence. It is made up of conscious experience, and follows the laws of psychology. Stimuli that exist outside our mind (external stimuli) continue to exist even when we are no longer experiencing them, while stimuli that exist inside our mind (internal stimuli) cease to exist when we are no longer experiencing them. For example, when we fall asleep we stop experiencing the house around us, but the house continues to exist all the same. But when we experience a magical house in our dreams and then wake up, the imagined magical house ceases to exist as soon as we stop thinking about it. It existed only in our mind.

External and internal stimuli also have different forms of causal power. External stimuli can cause things to happen even when we have no experience of them, while internal stimuli can only cause things to happen while we experience them. For example, since an apple is an external stimulus, it can fall from a tree and land on the ground, even if no one is there to see it. And since the thought of a unicorn is an internal stimulus, it can cause a person to think of a rainbow, which is another internal stimulus, but it cannot cause a physical rainbow to appear. Thanks to this, external and internal stimuli can bring about different types of psychological responses, and differ in how they do so.

External stimuli can only cause one type of psychological response called “sensation”, and it does so by stimulating the senses of an organism via transmission of physical energies. For example, when light enters our eyes, the light energy stimulates the millions of receptors at the back of the eye and brings about millions of consciously experienced tiny dots of different colors and intensities. When a sound wave enters our ears, the sound energy can stimulate thousands of receptors inside the ear, resulting in a conscious experience of thousands of individual frequencies. External stimuli thus only cause raw sensory data to occur. Each sensory data (for example, each dot or frequency) is an internal stimulus or stimulus element, which corresponds to a particular physical energy in a specific location. But on its own it usually doesn’t make much sense. Instead of seeing a million dots, we would prefer to see objects, landscapes, animals, movements. Instead of hearing a thousand frequencies we would prefer to hear voices, songs, sounds. This is what happens after the external stimuli have been sensed by the mind and thus the creation of internal stimuli, which we will turn to next.

Internal stimuli cause all the other types of responses aside from that of sensation. This includes responses like perception, behavior and emotion. When internal stimuli or stimulus elements have been created by external stimulation, they first set off the response of perception. Perception is the process where the individual stimulus elements of sensation are put together to more meaningful wholes. For example, when a number of black dots are placed next to each other in a line, this creates the experience of a black line, instead of just a collection of black dots. When we in a certain moment hear a number of frequencies that have a relationship with each other in the form of a harmonic series, this creates the experience of a tone with a certain timbre, instead of just a collection of frequencies. The process of perception happens so quickly and automatically that we almost never realize that there is a very short stage of perception where we experience a vast number of raw sensory stimuli. Another reason we seldom experience a collection of raw sensory stimuli is that the mind has the capacity to quite accurately predict which stimuli will appear in the next moment. This has the effect that the more meaningful parts, the more coherent wholes, are experienced first, and only the sensory data that don’t match the prediction are experienced as raw sensory stimuli for a short while before being perceptually processed. We will look at this in more detail when we get to the prediction and learning principles of the mind.

The perceptual experience that occurs as a result of this two-step process is the conscious experience of the external stimuli. In other words, when something outside the mind, like a tree, stimulates our receptors, for example by reflecting light towards us that is absorbed by the receptors in the eyes, then this creates internal stimuli which set off a perceptual process which in turn results in an experience of seeing the tree.

Now, at this point, it is really easy to be confused. Is the experience of seeing the tree an external stimulus or an internal stimulus? The way the process is described makes it sound like the external stimulus (the tree) caused an internal stimulus (the experience of the tree), and that internal stimulus is the external stimulus? What? So let’s slow it down a bit and be really careful here about what is going on. We must remember the definition of “stimulus”: A stimulus is anything that is experienced as a coherent phenomenon. And what is experienced depends on the specific situation or context. Let’s apply this to the above description. When we talk about something outside the mind (like a tree), then what is in focus in our conscious experience is that something (e.g. tree) that we know exists outside our mind. Thus, in this specific situation we are talking about an external stimulus. When we just a few moments later talk about an experience of seeing the tree, then what is in focus in our conscious experience is the experience itself of seeing a tree. An experience exists inside our mind, and thus in this specific situation we are talking about an internal stimulus. We have made a subtle shift in situation that can be hard to notice. First we talk about the tree (external stimulus), and then we talk about the experience of seeing a tree (internal stimulus). These two stimuli are of course very intimately linked. The content of the internal stimulus of “seeing a tree”, i.e. what this stimulus is about, is the external stimulus “the tree”.

This leads us to another fundamental property of stimuli: Each internal stimulus can be viewed on two different levels:
The content level: What the conscious experience is about
The meta level: What the conscious experience is itself

We can formulate the property another way from the perspective of external stimuli. Each external stimulus exists outside the mind, but every time we consciously experience this stimulus, this experience itself is an internal stimulus. Or to say this another way: an external stimulus can only be experienced in terms of it being the content of an internal stimulus.

It is okay to be really confused about this for a while. The distinction is very subtle, and in fact has been missed by philosophers for thousands of years. We are so used to thinking about the content of our experience that we tend to forget that whatever we are having an experience about, it is also an experience in itself, and we can shift focus between the content level and meta level for different purposes. In fact, this shift is central for the therapy of metacognitive therapy. Realizing the difference between a catastrophic scenario we are thinking about, and the fact that we are having thoughts about a catastrophic scenario. Regardless of how distressing the catastrophic scenario is, we still have the power to choose how we deal with our thoughts of the catastrophic scenario. Even if we can’t control the scenario, we can control our thinking.

An example that is a little more tricky is when we think of a memory, for example we could think about the last time we brushed our teeth. The event really took place (unless you are a person who has never brushed your teeth, in which case you might want to see a dentist). So in this sense, the content is about an external stimulus, but the situation is different from when we are actually standing in the bathroom with the toothbrush in our hand, and perceive the event directly. Another way to contrast these types of experience is to compare the situation of looking at what is in front of us in one moment, then closing our eyes and focusing on our memory of what we just saw when we had our eyes open. Both times we are focusing on the same external stimulus, but in the first moment we are focusing on the direct experience of the external stimulus (our perception), while in the second moment we are focusing on the indirect experience of the external stimulus (the memory). The same kind of distinction can be made when the content is an internal stimulus. For example, we can experience a feeling of happiness directly, and we can think about the memory of that feeling, in other words, the feeling is experienced only indirectly. Thus, the content of an internal stimulus can have two different types of relationships to its content:

So when we talk about perception, we are talking about an external stimulus being experienced directly. We can explain how this occurs by describing the two-step process that is involved.

The first step: An external stimulus (like a tree) causes a large set of internal stimuli (a collection of tiny dots) which each is about a tiny part of the external stimulus. In other words, the content of each tiny internal stimulus is a tiny external stimulus in the form of a “dot” in the world outside the mind. The large set of internal stimuli is thus the response to the external stimulus. And this occurs through the physical mechanism outside our mind where a large number of light waves travel from the external stimulus and transfer their energies to the receptors in the eye.

The second step: The large set of internal stimuli in turn cause a coherent internal stimulus to occur that is about the external stimulus as a whole. The larger, more coherent internal stimulus is thus the response to the collection of smaller internal stimuli (which thereby become stimulus elements, i.e. parts, of the coherent internal stimulus). This occurs via psychological mechanisms, i.e. through the activation of a network of relations in our mind between stimuli and responses (which we will look at more closely later).

Even though this explanation is the technically correct one, we can often use a shorter version when talking about perception to get the main point across:

An external stimulus (like a tree) causes an internal stimulus to appear (the experience of a tree), where the external stimulus (the tree) is the content of that internal stimulus (the experience of the tree).

A mix of external and internal stimulus elements

When we have an experience, parts of that experience may be external stimulus elements and parts may be internal stimulus elements. For example, we might see a cat from a distance and think that the cat is wearing a hat, but as we get closer we see that it was a flower that the cat was hiding under. In such a case, the part of the cat that we experienced as a hat was a misperception, an imagination, merely something that existed in our mind. While the rest of the cat was in fact as we perceived it to be. So the cat itself was an external stimulus (or stimulus element), while the hat was an internal stimulus (or stimulus element). Or if we see something in our home in the dark we may think that we see an intruder, but in fact it was just a coat hanging on a coat hanger. The coat is thus an external stimulus element, but the head, arms and legs that we think that we see are internal stimulus elements, mere imagination.

Being mistaken about a stimulus

What the examples show is that it is not always clear whether the content of our experience is something outside our mind (an external stimulus) or merely something inside our mind (an internal stimulus). Sometimes we can be almost completely wrong about what we experience. Seeing a bush from afar, it can sometimes look like a cow (this has happened to me on several occasions). But as we get closer we discover that it is in fact a bush, and not a cow. We were incorrect in our previous judgement. First we thought there was an external stimulus in the form of a cow, and then we thought there was an external stimulus in the form of a bush. When we come to the realization that it is in fact a bush, we realize that our initial experience was not a correct perception of something outside our mind, but instead more of a hallucination or imagination. The only parts of our initial experience that was actually correct and corresponded to the actual external stimulus were the colors and shapes.

That fact that we can be mistaken about stimuli may seem to pose a threat to how we have defined “stimulus”. If we have defined “stimulus” as something that is experienced as a coherent phenomenon then shouldn’t a stimulus always be “correct” according to such a definition? If we experience something as being a cow, and a stimulus is that which is experienced, then the stimulus must be a cow, right? How could we ever claim that this stimulus is incorrect if the cow fits the definition of “stimulus”? To see the solution to this question we must remember that saying that something is a stimulus is not the same as saying that the stimulus exists outside our mind the way we experience it. There is nothing in the definition of “stimulus” that says that whatever we experience is an external stimulus. Determining whether a stimulus is external or internal can only be done by collecting more experiences of the stimulus. When we see a stimulus from a closer distance, or from another perspective, we are able to see more details. These details allow us to determine the properties of a stimulus more accurately. When we approached what we thought were a cow we could see that it consisted of branches and leaves and not flesh and bone. When we think back to the previous moment when we thought we saw a cow, we can now see that it was in fact this bush that we were perceiving in an incorrect manner. So retroactively, we can now conclude that the cow-stimulus was an internal stimulus (or at least a stimulus where most of the stimulus elements are internal), while the bush-stimulus was the external stimulus present in that situation. We change our opinion on what the external stimulus was.

Describing this in another way, we can never deny that we have such and such experience, for example we can never deny that we have the experience of there being a cow, but we can deny whether the content of that experience really has the properties that we perceive them to have. In other words, we can deny whether the experienced cow is actually something outside our minds or merely our imagination. The fact that we can be mistaken about what the external stimulus is in a certain situation is of course unfortunate, but it does not mean that there is any problem with how we have defined the term “stimulus”. Defining “stimulus” in any other way would not have prevented us from making the mistake anyway.

A stimulus is defined from the perspective of a specific organism

The distinction between external and internal stimulus, and between content level and meta level, allowed us to show that even though the definition of “stimulus” is “anything that is experienced as a coherent phenomenon”, a stimulus can still be something that exists outside the mind. The fact that “experience” is a fundamental part of the definition doesn’t force us to get locked inside our head. But still, the fact remains that something can only be classified as a stimulus as long as it is experienced directly or indirectly by someone. This means that, technically, a stimulus is always defined from the perspective of a specific organism that experiences the phenomenon. This is an unusual property in science and philosophy, where we are used to thinking that scientific and philosophical phenomena are independent of who is talking about them. This property therefore needs to be investigated closely in order to provide us with a solid understanding of the nature of stimuli.

Even if I talk about an external stimulus like a tree, this external stimulus is experienced in a specific way by me. I see it from a specific perspective, I am aware of certain features of the tree, I have specific memories related to the tree. If I have only seen the tree from one side, and my friend has only seen the tree from another side, if I am color blind, but my friend is not, if I have good eye-sight, but my friend has blurry vision, then the tree appears very differently to me and my friend. Even though the tree is the same physical object, the properties that the tree appears to have according to me are very different from how my friend sees it. We agree on some properties, for example the fact that it is made of wood, that it is placed in this spot, that it has branches, and so on. But not on others, for example that the tree is colorful, that has intricate patterns on its leaves, and that it has a certain shape. In this kind of situation it is not the case that I am correct and my friend is wrong, we both have incomplete knowledge of the tree and so the specific perspectives we have differ wildly. Let us now say that the side of the tree that I see has a haunting look, like something out of a horror movie, while the side of the tree that my friend sees is more pleasant. So when I see the tree I get scared and run away, but my friend sits down in the grass and admires the beautiful tree. It would seem like we have very different responses to the same stimulus, but in fact there are two different stimuli, with overlapping stimulus elements and properties. One of the stimuli is a haunting tree, and the other is a pleasant tree. If me and my friend had switched sides, it is quite likely that our responses would have switched as well. My friend would have been scared and run away, and I would have sat down in the grass to admire the tree. Thus, in order to understand and explain responses of an organism, we must also define the stimulus from the perspective of that specific organism. It would have been incorrect of my friend to say that I got scared of a pleasant-looking tree, since the stimulus was not a pleasant-looking tree in my case. A stimulus must be a stimulus from the perspective of the organism if we want to explain the psychology of that organism.

Similarly, there can be situations where I and someone else may disagree on whether there is any stimulus at all. For example, my friend may be deaf and so when I hear a bird singing, my friend does not. So the bird song is a stimulus from my perspective, but not from my friend’s. To take another example, a bird can sense the magnetic field of earth, while I cannot. So from the bird’s perspective the magnetic field is a stimulus, but not from mine. Of course, I could use various technical instruments and discover that there is a magnetic field. In this case, the magnetic field would become a stimulus from my perspective too, although only experienced indirectly. Similarly, I could sign to my friend that there is a bird singing, and so the bird song would then become a stimulus from my friend’s perspective too, although only via indirect experience.

Knowing if something is a stimulus for another organism

Defining “stimulus” as anything that is experienced as a coherent phenomenon does bring about a great difficulty: we only have direct access to our own experience. The term “stimulus” is supposed to allow us to explain the psychology of all organisms, which includes ourselves, other people, animals and other organisms. The trouble is that we don’t have any way of looking into the conscious experience of anyone else. How could we then determine whether something is a stimulus when talking about another organism?

The problem of knowing what goes on inside someone else’s mind is one of the reasons why the field of behaviorism was created at the beginning of the 1900’s with the goal of making psychology a discipline where the mind was irrelevant. The field focused only on what could be observed from the outside. Whether a stimulus was experienced or not didn’t matter. And the only responses that were of interest were the organism’s visible behavior, not the organism’s feelings or thoughts, etc. It would of course have been great if it was possible to create a complete psychology this way, but unfortunately it is not. There are a number of reasons for this.

Firstly, a stimulus can be wholly internal, and so the stimulus cannot be observed from outside. For example, a person may fantasize about a magical world for hours. The fact that this person forgets to eat dinner as a result of this cannot be explained as a reaction to any external stimulus present in that situation. A classical behaviorist may try to solve this problem by explaining that these internal stimuli appear as a result of external stimuli that were present at previous times in the person’s life, and try to trace the causal chain to those stimuli. Thus, they might say that the person didn’t eat their dinner tonight because during the person’s childhood there were several books, films and stories that that the person read, saw or heard. This kind of answer isn’t particularly satisfying. The explanation only makes sense if we add that these events in turn inspired the person to fantasize about magical worlds, which the behaviorist cannot say, since this is something that goes on in the mind. Another problem is how to explain why a person with aphantasia, i.e. the inability to visualize things in their mind, doesn’t end up in a similar situation as the person with the ability to visualize. Even if two people had been exposed to the same external stimuli, the person with the ability to visualize things in their mind would spend time doing so, while the person with aphantasia would not. A classical behaviorist could not explain this difference in behavior, since the only difference between the two people is their inner experience, and classical behaviorism cannot talk about inner experience.

Secondly, classical behaviorism can never explain what happens in any specific situation, only what happens on average over a number of similar situations. In experiments of classical behaviorism a rat may be placed in a box and be presented with a lever that the rat can press to get some food. On some occasions the rat will press the lever, on other occasions it will not. What can be observed is that on average, a rat presses the lever more often if it results in food, compared to if it results in an electric shock. When limited only to what is visible to the observer, there is no way to tell the difference between a moment where the rat presses the lever, and when it does not, since everything in the surrounding remains the same. There is no external stimulus that shifts between a moment where the rat decides to press the lever, and when it does not. The difference is internal. For example, the rat’s level of curiosity may shift from one moment to another, it may be focused on or distracted by something else in the box for a while, the rat’s hunger may shift. These kinds of internal shifts average out when a situation is repeated 100 times, and allows the researcher to say something like “When the rat is presented with a reward it presses the lever 45 times per minute on average, but when the rat is presented with a punishment it presses the lever only 5 times per minute on average. Thus the reward caused the rat to press more often.” But in our everyday lives, the exact same situation rarely appears again and again. Instead, life evolves, with new situations appearing all the time. We want to be able to explain why someone did something in a specific situation, not what it most likely would have done on average if it had been in the situation 100 times. We want psychology to be an exact science. We can compare this to a field like physics where it would have been completely unacceptable if we couldn’t explain why an apple fell from a tree in a specific situation. Imagine if the physicist’s explanation was that when apples are in a situation of hanging on a branch, they will on average fall down as a result of gravity sometime during that year. We would demand the more exact explanation that a specific apple falls from the tree at the specific moment when the force of gravity acting on the apple is stronger than the electric forces connecting the apple to the branch.

Thirdly, classical behaviorism is not able to define the term “stimulus” in a coherent manner. The most common attempt to define “stimulus” in classical behaviorism is as something that can cause a response. We have already looked at the problem of defining “stimulus” this way. There are an infinite number of things that could be said to be the cause of some event. The definition is too general for us to be able to specify a specific phenomenon.

Let us therefore look at how we may deal with the issue of not being able to look directly into someone else’s mind, while holding on to the definition of “stimulus” as anything that is experienced as a coherent phenomenon.

I am not the only conscious being

There are good reasons to believe that I am not the only organism in the world who has conscious experience. The fact that people before my time have come up with words like “consciousness” and “experience” suggests that they had conscious experience and wanted to have words that refer to this. The fact that people today use such words, that others understand what I mean when I talk about having experiences, the fact that even small children talk about things like “What if my color red looks like your color green?”, suggests that people in general have conscious experience. Thanks to our ability to communicate with language we can tell other people about our experience. We don’t need to look directly into another person’s mind in order to figure out if they are conscious, we can just ask them about it. In the future we may have figured out what mechanisms in the brain are responsible for conscious experience, and then we could indirectly see whether some organism is conscious by looking at their brain activity. But even when we lack this scientific knowledge and technology, we can be rather certain that at least other humans have conscious experience. Other organisms are more difficult to deal with, since their lack of language means we cannot ask them.

Many people believe that animals that are similar to us humans have conscious experience. Monkeys, gorillas, dogs, cats. When we interact with such animals, they behave much like humans would. They seem to understand us, to have a mind of their own, and are able to empathize with us. We also know that their brain is similar to ours, and we know that the brain is the seat of consciousness. But as we look at animals and other organisms that are less similar to us, we are more unsure. Is a worm conscious? Many people do not believe that worms have conscious experience. But worms do have a brain, though it consists of very few neurons, compared to that of a human brain. Since we haven’t yet discovered what brain mechanisms correspond to conscious experience, there is not yet any way to know whether an organism like a worm has conscious experiences or not. It is possible that only organisms with a certain level of complexity in their brain structure have conscious experience. Or it could be that all organisms with at least one neuron have conscious experience. How are we to deal with this uncertainty, and could it cause any problem for our definition of “stimulus”?

Stimulus from several perspectives

Research in psychology often involves presenting something to an organism and observing how it reacts. For example, a researcher may place some food in front of an organism and see how it reacts. In these experiments, the thing that is presented to the organism is described by the researcher as “the stimulus”, and the way that the organism reacts to this stimulus is described by the researcher as “the response”. If the organism is conscious then the food is a stimulus from the perspective of the organism, but if the organism isn’t conscious then what has been described as “the stimulus” isn’t actually a stimulus according to our definition. Here is how we can deal with it.

If the organism doesn’t have conscious experience, then we aren’t actually talking about psychology. The defining property of psychology is the presence of a mind, a conscious being, some being that acts, that feels, that wants, etc. If an organism lacks such properties, then it isn’t much more than a physical system with causes and effects, like a calculator. If we observe a physical system, for example how a light turns on when we press a certain button, then we can observe that there is a cause (the pressing of a button) and an effect (the light turning on). We can observe that there is a law that this particular effect always occurs when the particular cause occurs, and so we could explain and predict certain events that we observe regarding the physical system. So if an organism is of such a simple kind that it lacks conscious experience, then describing how it responds to certain events is like describing a mere physical system. So let’s say that worms lack conscious experience. If we poke a worm with a stick and observe that it recoils in response, then we can say that the poking with a stick causes a recoiling effect. There isn’t really any need to say that the poking with a stick is a stimulus. It adds nothing more than it would for us to have said that the pressing of a button is a stimulus. If we want we could instead use general scientific terms like “independent variable”, the variable that is under our control, and observe how this affects the “dependent variable”, i.e. the effect.

But if we do decide to use the term “stimulus” when talking about an organism like a worm, because we are uncertain of whether the organism is conscious or not, then there is a way to interpret the use of this term that avoids any problem.

Different definitions in different fields

The term “stimulus” can have different definitions depending on the area of use. It is common for words to have slightly different meanings in different contexts. For example, the word “date” can refer to a day in the calendar, the act of determining how old something is, the event of going out with someone for romantic purposes, the person we are going out with, and even a sweet-tasting fruit. In science, where it is important to be very specific in what is meant by a word, it is common for words to have slightly different definitions depending on the specific scientific field. For example, the word “sex” within genetics refers to the presence of X or Y chromosomes (XX is female, XY is male). Within anatomy, however, it refers to the genitalia of the person. In the context of reproduction, “sex” refers to the ability to produce eggs or sperm. Within sociology it refers to the social construction of categories which includes identity and social roles. Sometimes the different definitions align, and sometimes they don’t. For example, a person with female genes can have male genitalia. It is not the case that one of the definitions is “the correct one”, and all others incorrect. They are different definitions that serve different purposes in different fields.

Thus, the term “stimulus” can be defined one way when we talk about psychology, and another way when we use it in other contexts. For example, within the field of physiology, “stimulus” is usually defined along the lines of “a thing or event that evokes a specific functional reaction in an organ or tissue”. So, for example, when the skin gets sunburned, the sun is the stimulus, and the sunburn is the response. This happens even if the person isn’t consciously aware of the sun (maybe the person has fallen asleep in the sun), because the effect of being sunburnt isn’t a psychological response, it is a physiological response. So for these kinds of causal effects it is fine that “stimulus” is defined differently.

But here is a major caveat: changing the definition of “stimulus” also changes what laws apply to the stimulus. The causal mechanisms of physiology are not generally the same as the causal mechanisms of psychology. As we will look at later, in psychology the stimulus is connected to responses via psychological relations, and the way that these relations are activated and changed are determined by psychological laws. When we talk about a stimulus in the physiological sense instead, the laws that apply to such stimuli are different. They don’t follow the laws of psychology. When the sun causes the skin to get sunburnt this happens as a result of ultraviolet light waves damaging the cells. This kind of effect abides by the laws of physiology instead. We can compare this to a situation where the person is feeling happy because the sun is shining. In this case the sun must be a stimulus in the psychological sense for that person, i.e. something that is experienced as a coherent phenomenon, in order to explain the psychological response (feeling happy). And the connection between the stimulus and response in this case abides the laws of psychology.

The interesting thing here is that even if “stimulus” is used in a non-psychological context, it is still also a stimulus in the psychological sense from the perspective of the observer. The only way for someone to give a description of a physiological cause and effect is if this person in some way experiences the causes and effects involved. So when the sun is described as a (physiological) stimulus that causes the skin to be sunburnt, the sun is also experienced as a coherent phenomenon by this observer, and therefore is a psychological stimulus from the observer’s perspective. In this sense, a stimulus is always primarily a psychological stimulus, regardless of whether it is then used to discuss how it affects something in a non-psychological way. We should also take note that the effect that is observed by the observer is also a stimulus from their perspective. The only way for the observer to talk about the sun causing a sunburn is if the observer has some kind of experience of the sunburn too, i.e. that is experienced as a coherent phenomenon. So from this perspective, when we describe a physiological stimulus causing a physiological response, both the stimulus and response are psychological stimuli to us in the first place. This means that we need to be careful when claiming that something is a stimulus. From whose perspective are we saying that it is a stimulus? From our perspective, or from someone or something else?

Thanks to the fact that we can define “stimulus” differently depending on the scientific field we are dealing with, we can use the term “stimulus” to describe something that caused some kind of effect in an organism, even when we are uncertain of whether the organism is conscious or not. If the organism is conscious then the stimulus is a psychological stimulus from the perspective of the organism, and the effect this stimulus has is a psychological response. If the organism is not conscious, then the stimulus is instead just a more vague physiological stimulus which has non-psychological effects or responses. Our uncertainty regarding the organism means we won’t be sure which definition is the correct one, and thus we won’t be sure which laws apply to the organism (psychological or merely physiological?). But it works well enough. The line between psychology and physiology is fuzzy. Our brain is a physiological system. Psychology is really just a certain type of physiology. So when we use the term “stimulus” in a situation where we aren’t sure whether we are dealing with psychology or not, then it is possible that we are still able to say some things of value all the same. But ideally, we should try to learn as much as possible about the specific organism to determine which laws apply to this organism, what abilities it has, and what kinds of things it can respond to. Only then can we give exact descriptions of what it means to say that something is a stimulus from the perspective of that organism. Using the term “stimulus” more vaguely is just a temporary solution to get some more general point across before we have such knowledge.

Distinguishing between a psychological definition of “stimulus” and one or several non-psychological definitions of “stimulus”, also allows us to use the terms “stimulus” and “response” when talking about machines, which we generally don’t think of as conscious, but as technology gets more advanced we are getting less sure of. Thus, we could have a “mechanical” definition of “stimulus” in the form of “a thing or event that acts as input to the mechanical system and can bring about effects on the mechanical system and its outputs”. Then we could describe a certain event where a computer is presented with a picture of an apple as consisting of a stimulus (the picture of an apple) and a response (whatever the computer does in response to being presented with the picture). As with the case of physiology, in order for us to be able to give accurate descriptions and explanations for how the brain responds to certain stimuli, we would need to acquire more knowledge about how the machine works. What laws apply to the machine? But lacking such knowledge we can use the term “stimulus” more generally to communicate something more vague like something being an input to the machine.

What should be noted is that to the extent that it is possible, we should always try to communicate clearly which definition of “stimulus” we are using in any given situation. If we say that something is a stimulus for a computer, or that a stimulus affected someone’s skin, a person who believes that the word “stimulus” is used in the psychological sense may be misled into thinking that the stimulus was experienced by the computer or organism. We want to avoid such misunderstandings. We should also always try to be clear about whose perspective we are talking about. If we are talking about someone or something else, then there are at least two possible perspectives. When we use the term “stimulus”, we could be talking about something being a stimulus from our perspective, or from the perspective of the other organism or machine.

A conscious organism that fails to experience a specific stimulus

We have looked at how to deal with organisms and machines that either have conscious experience or not. But another difficulty is how to deal with organisms that generally have conscious experience, but where we are unsure of whether they experienced a specific phenomenon.

Let’s say that a researcher plays a sound for a cat and observers how the cat reacts. The researcher describes this as presenting a stimulus (the sound) to the cat, and wants to observe the cat’s response to this stimulus. Does the cat turn its head towards the sound? Does it jump in the air as a result of being startled by the sound? The researcher finds that the cat doesn’t respond at all. It is actually the case that the cat is deaf and so it doesn’t perceive the sound. In this case we might want to say that the cat doesn’t respond to the stimulus. But if we define “stimulus” as something that is experienced, then the sound isn’t technically a stimulus for the cat, since it doesn’t experience the sound to begin with. So it would not really be correct to say that the cat does not respond to the stimulus in this case. How could we solve this issue?

As mentioned before, when someone is an observer, there are at least two possible perspectives from which a stimulus may be defined. A sound may be a stimulus from the perspective of the researcher, even though it is not a stimulus from the perspective of the cat. So when the researcher says that a cat does not respond to the stimulus because it is deaf, then in that context the researcher is talking about the sound from their perspective – as something that in their experience appears to be there. The researcher had hoped that the sound would also be something that the cat perceived and responded to, but since the cat could not hear the sound, it failed to become a stimulus from the cat’s perspective. If we wanted to describe the situation more accurately we would have to specify whose perspective we are talking about at each moment the word “stimulus” is used. For example something along the lines of:

“The stimulus (as defined from my perspective as the researcher) was presented to the cat. The cat did not respond to the stimulus (as defined from my perspective). Further investigation showed that the cat was deaf and that the sound never became a stimulus from the perspective of the cat. The lack of response, therefore, was not the result of the cat not having any reaction to a stimulus (from its perspective), but the fact that the sound never was a stimulus to begin with (from the perspective of the cat).”

This way of writing would, however, be way too convoluted and take too much time to be of practical use. If we have made sure that the reader understands the deeper nature of “stimulus” they will understand what is meant without specifying the perspective each time the word “stimulus” is used. For example, we could write:

“The stimulus was presented to the cat. The cat did not respond to the stimulus. Further investigation showed that the cat was deaf.”

Expressing objective facts

The fact that a stimulus can only be accurately defined by specifying a certain individual’s perspective puts into question our ability to talk about objective facts using our current terminology. Since objective facts are of great importance in science and philosophy our current definition of “stimulus” could therefore be a potential problem. How are we to understand and deal with the question of objective facts regarding our definition of “stimulus”?

Let us look at the previous example of a researcher presenting a stimulus to a cat and observing how the cat responds. It seems wrong to think about the sound presented to the cat as merely being a stimulus from the perspective of the researcher. It makes the experiment seem more subjective, as if everything that occurred in the experiment was merely a subjective experience or opinion. We want experiments to be objective, to be independent of the specific person who performed the experiment. How do we get such objectivity despite using a definition of “stimulus” that is dependent on the experience of an individual?

We can imagine that the researcher invites some fellow researchers to perform the experiment together. All of them hear the sound that is presented to the cat. The sound is therefore a stimulus from the perspective of each and every researcher in the room. This is one of the core features of external stimuli; they exist outside the mind and can therefore be experienced by other people and not just ourselves. Each individual researcher may experience the sound slightly differently depending on where they are in the room and how good their hearing is. For one researcher the sound may appear louder than for another, or appear to come from a slightly different place. But they all agree that a sound is there and has roughly the same properties as the others perceive it to have. So when the original researcher writes about the experiment then the sound can be described as a stimulus in a more general sense. The sound can be seen as a stimulus from the perspective of everyone who observed the experiment.

We can further imagine that the researcher video records the experiments and shows it to a group of students. When watching the video, the students hear the sound too. Therefore, the sound is a stimulus from the perspective of each student as well. The researcher is of course not surprised by this. When they wrote about the sound being a stimulus, they believed that anyone who would have had the opportunity to observe the experiment would have experienced the sound too, unless they had some problem with their hearing. Anyone could check this by watching the video recording. The sound really was there, it was not just a private experience in the mind of one researcher. Another way that the researcher could have gone about this is to record the sound with a device that displays the sound in the form of an audio chart, where the intensity of each frequency is presented. A person who understands how these audio charts work could similarly verify that there must have been a sound present during the experiment in order for the chart to display what it displays. Thus, the sound becomes a stimulus from the perspective of this person too. So even if the researcher had been the only person present to observe the experiment as it occurred, there are a number of ways for other people to experience this sound afterwards. This is why a researcher may write about there being a stimulus in more general terms, and not specify that it was merely a stimulus from their own perspective. It is implicitly understood that when we talk about external stimuli, then the presence of such a stimulus could at least in theory be independently verified. Anyone with the appropriate senses or technology would experience the sound, and thus the sound would be a stimulus from the perspective of all of these individuals.

Something can thus still be objective, even if it must always be experienced by specific individuals in order for anyone to know that it is an objective fact. There is of course always the chance that we are mistaken about objective facts. Even if everyone on earth believed that something is an objective fact, it could later turn out that they were wrong. Thus, something isn’t objective merely because everyone agrees on something, i.e. share similar experiences. For something to be an objective fact it must also be the case that the phenomenon that is experienced is the result of the most detailed and accurate observations, and the most correct interpretation and logical conclusions.

Explaining causality

As stated already at the beginning of the task of trying to define the term “stimulus”, one important property of a stimulus is its ability to cause responses to occur. Yet as we have defined “stimulus” now, there is no mention of causality. This means that according to this definition, when we say that there is a stimulus in some situation, this doesn’t necessarily mean that this stimulus causes a response to occur. So how do we describe the causal connection between stimuli and responses when we have defined “stimulus” the way we have? We have already established that identifying a cause of something is difficult to do in general, because there is really an infinite number of causes for any single event. As an example let’s try to explain the cause of a response where we see a tree. Some examples of causes for this response are: the tree, the position of our eyes, the sun, the light waves, the fact that we are here right now and not in another city, another country or another planet, the fact that our eyes are working, the fact that we are not wearing any blindfold, the existence of earth, our decision not to close our eyes at this moment, the birth of our solar system, the fact that we were born, and so on. All these things needed to have happened for us to perceive the tree. So any attempt to explain the general and complete description of what caused our response to occur will fail. But given how we have defined “stimulus”, it turns out that we can avoid this problem. There are two facts that allow us to determine whether a stimulus is the cause of a response or not:

First, we have already established that there are two types of stimuli: external and internal stimuli. Internal stimuli only exist while they are experienced. This means that internal stimuli always have to be experienced the very moment they cause a response, since they cannot cause a response without existing. External stimuli on the other hand can cause things to occur even when they are not consciously experienced. They exist even when we aren’t consciously aware of them. But external stimuli can only produce one type of response: sensation (which leads to perception). This means that whenever they cause a response to occur, we will experience this external stimulus as a result. This means that any stimulus that causes a psychological response, whether external or internal, will always be present in our conscious experience, either at the exact moment they cause the response to occur, or with a short delay.

This greatly decreases the number of available options for singling out the cause. Taking some of the possible causes we listed above, it cannot, for example, be the case that the fact that we were born is the stimulus that causes us to see the tree. The fact that we were born is not some phenomenon that is present at that moment in our conscious experience. Nor can the sun be the stimulus that causes us to see the tree, since the sun is not experienced by us as a coherent phenomenon at that moment. The only possible candidates for being the stimulus that caused us to see a tree, are the phenomena present in our experience at that moment, and the tree is now one of few alternatives. Let us say that at that moment we also saw a road, a field, a mountain. We heard the wind, the song of a bird, and the sound of our footsteps. We felt the wind in our hair, the movements in our body as we walked. We now know that the cause of our response of seeing a tree must be one of these stimuli. We have therefore greatly limited the number of possible alternatives.

Second, whenever we have a psychological response, it is always the case that before the response occurs, and after the response has occurred, there is a time where the psychological response is not present. Given that the response is caused by a stimulus in a lawful manner, this means that there must be a change in which stimuli are present when we compare the moment where the response occurs and when it does not. With careful observation we can determine which stimulus or set of stimuli that must be present for the response to occur. In the case of us seeing a tree, we can figure out that this psychological response only occurs when the tree is present, and does not occur when it is not present. The other stimuli do not have this kind of relationship with the response. For example, the response occurs when the field is present, but also it fails to occur at other moments where the field is present. This means that the field cannot be the stimulus that caused the particular response we are investigating. The same goes for the mountain, the feeling of wind in our hair etc. The only stimulus that is always present when the response occurs, and is not present when the response does not occur, is in this case the tree.

We can use the same reasoning and apply it to an example from scientific experiments. A common experiment in psychology is to place a rat in a special type of box. The box is very simple, and always looks the same. The rat is allowed to explore the box over several occasions and get used to it. Then something is added to the box that wasn’t there before. For example, the floor may get an electric current so that the rat gets a mild electric shock. The researchers observe how the rat’s behavior changes. What they usually observe is that the rat jumps in the air when the electric shock occurs. How do we identify what stimulus caused the response to occur? First, we can establish that the stimulus in question must be present in the rat’s experience. Second, we must look at how these stimuli change over time, and identify which stimulus best explains the occurrence of the response. Since the rat did not jump when all the stimuli except the electric shock was present, these are not likely the cause of the jump. Instead, the electric shock must be the stimulus that caused the response.

The fact that a stimulus can cause a response is not part of our definition of “stimulus”, but this doesn’t stop us from analyzing if, when, and how a stimulus can cause a response to occur. The fact that stimuli can cause responses to occur is still a central property of stimuli, even though this fact isn’t stated already in its definition. Not including this property in the definition isn’t a disadvantage, in fact, it is an advantage. It allows us to identify stimuli without first analyzing its causal effects, and then look at its causal effect in the next step.

Another advantage of the definition is that a stimulus (like the tree we saw) can be a stimulus even at a moment when it is not actively causing a response. For example, when we go home and tell someone about the tree, this tree remains a stimulus. When using a definition where causality is part of the definition the tree stops being a stimulus as soon as it stops causing the response, since it is part of that definition that a stimulus causes a response.

There is some debate, however, on whether a stimulus causes a response (also called “triggers a response”) or whether a stimulus merely “elicits” a response, i.e. that the stimulus doesn’t necessarily cause the response to occur, but more sort of “suggests” to the organism to perform the response. The idea is that this later type of effect then allows the organism to choose whether to perform the response or not, while the first type is unavoidable. We will not go any deeper into this matter at this point, since it doesn’t affect how we define “stimulus”. But we will look at later why the correct way of describing it is in terms of “cause” or “trigger”.

Subliminal stimuli

We have looked at how the “experience”-definition of “stimulus” solves a number of different challenges. But there is yet another challenge that we must look at before we can conclude our analysis: how to deal with subconscious and unconscious processes. Although still hotly debated, there are some studies that seem to show that a stimulus that is presented for such a short time that it isn’t consciously experienced can still elicit a response. Such stimuli are called “subliminal stimuli” because they are below the threshold of conscious experience. If it is the case that subliminal stimuli can cause responses to occur then this could cause problems for our attempt to define “stimulus” in terms of something that is experienced. There are a few things that we therefore need to look closer at.

First of all, we know that it takes some milliseconds before a stimulus becomes consciously experienced, but it is not an either-or situation. It is not the case that either the stimulus is consciously experienced in its entirety, or it is not consciously experienced at all. Instead there is a continuum between the two end-points. The reason it takes time for our mind to consciously experience a stimulus is because it has to piece together all the individual stimulus elements that first appear as we sense the external stimulus. At the level of sensation, there is just the activation of a collection of discrete individual receptors, like dots or frequencies. For us to perceive something more meaningful the different stimulus elements must be connected. A collection of black dots must be united into a line. A collection of lines must be united to form the outline of an object. The outline and the colors inside the outline must be united in order to form an object. And so on. If a stimulus is presented for a longer time, the mind may get further along in this process to piece it all together. But if the stimulus isn’t presented for long enough for the whole process to be completed, we will experience something in between. For example, if we are presented with a red circle and a green triangle for a shorter time than is necessary to fully process them, then we may end up with the experience of a circle, a triangle, something red and something green. The different parts haven’t been fully united. We don’t know which color belongs to which shape. If we are forced to guess we may get it wrong.

So when we say that a stimulus is subliminal, and claim that it isn’t consciously experienced, this is quite misleading. There is no clear border between conscious and non-conscious. There is no single “threshold” that a stimulus either surpasses or doesn’t surpass. Instead it’s a matter of degree. Why then do many experiments seem to show that there is a single threshold, and that a stimulus either is consciously experienced fully, or it is not experienced at all? In order to answer this question we must understand what it is we do when we introspect. People who participate in experiments regarding subliminal stimuli are often asked whether they consciously experienced the stimulus or not. They are given a forced choice between yes and no and so they must determine whether their experience best fits the “yes”-category or the “no”-category. In order for participants to give the answer to such a question they must introspect: they must look into their conscious experience and analyze what they find. In order to introspect they must recall what they experienced a moment ago. In other words, they must try to bring back the experience that occurred in that short period of time that the stimulus was presented. But because of the psychological laws that determine our ability to recall past experiences, we run into problems when trying to do so. In the case where we were presented with a red circle and a green triangle for just enough time to experience a circle, a triangle, something red and something green, then these are just four stimuli to bring back to memory. We can report that we had such an experience, although it is a rather strange one, and may be frustrated by the fact that we don’t know whether it was the circle that was red and the triangle green, or vice versa. But if a stimulus is presented for an even shorter time, then what we had the chance to experience in that moment was a million unconnected dots. It is impossible for us to keep such a great number of stimuli in our working memory, and the very short time interval they were present for means that the learnt association to these stimuli is extremely small. This makes it near impossible for us to bring back such an experience when trying to remember afterwards.

To get an idea of exactly how difficult it is to be able to recall such an experience we can compare it to a different type of situation. Imagine you were standing on a hill where you had a view of a city, a forest or anything with a lot of tiny details. If you were given the chance to look at the scenery for a while, if you then closed your eyes and tried to remember what you had seen, there are very few details that you would be able to recall. You are more likely to remember the “gist”, the “general picture”, and then some of the details in that picture. But in the subliminal experiments your mind isn’t given enough time to form such a “gist” or “general picture”, all there would be is details. Even if you were able to remember quite a few details, you would have no idea how they related to each other, and thus understand the overall picture of what you are seeing. It is like being given 1000 puzzle pieces, and just by looking at the jumbled mess of pieces you were asked to say what the whole puzzle would look like when complete.

So when participants are asked whether they experienced a subliminal stimulus or not, they are more likely to say they didn’t experience it. But when later research has been more careful about how they ask participants about stimuli in these kinds of experiments, allowing participants to describe in more detail what their experience is like, then more nuanced answers are given. Most participants report that they do experience something when the stimulus is presented, but that it is hard to tell what exactly that was.

So a better way of understanding “subliminal stimuli” is as stimuli that aren’t presented for long enough to be fully processed, and the degree to which they are processed will affect to what extent an external stimulus will be experienced as one single coherent stimulus, or as several unconnected stimulus elements. If a subliminal stimulus is able to cause a response to occur then this is not an example of an unconscious stimulus causing a response, it is instead an example of simpler stimulus elements causing a response. (This also explains why the responses that sometimes have been shown to be caused by subliminal stimuli are very weak.) The stimulus is still experienced, even when it is subliminal, and so subliminal stimuli do not cause any problems for our definition of “stimulus” as something that is experienced as a coherent phenomenon.

“A stimulus is anything that is treated by the mind as a coherent phenomenon” – insufficient

It seems our definition of “stimulus” succeeds in solving all the challenges we have thrown at it. But say we are not completely satisfied all the same. Perhaps we worry that there could still be situations where a stimulus is not experienced at all but still manages to cause a response. Or perhaps we still feel uneasy about assuming that other organisms are conscious. Could there be any other way of solving all the challenges we have looked at while still avoiding to specify in the definition that a stimulus is consciously experienced? One way of trying to do so is to attempt to define “stimulus” as anything that is treated by the mind or brain as a coherent phenomenon.

At first glance, this definition is merely a more general definition, which still includes all the cases where a stimulus is consciously experienced, since if we experience something as a coherent phenomenon then this means that our mind must have treated whatever we experienced as a coherent phenomenon too. The only difference seems to be that the “treat”-definition also includes cases where an organism isn’t conscious (or where we merely aren’t sure). For example, a machine that is given a certain input may not consciously experience this input while still showing behavior that seems remarkably similar to living organisms. But with the “treat”-definition we can take a more technical look at what happens in the machine and conclude that the input was treated as a coherent phenomenon, and thus may be called a (psychological) “stimulus”, despite the machine’s lack of conscious experience. So it seems that the “treat”-definition should be able to solve the same challenges as the “experience”-definition, but with the advantage of not having to infer any conscious experience. But it isn’t quite that simple.

A major difference between the two definitions is in terms of how we know whether something is a coherent phenomenon or not. With the “experience”-definition this was simple: If something was experienced as a coherent phenomenon, then it was a coherent phenomenon. With the “treat”-definition this is much harder. First we must ask what we mean by “treat”? Is it in terms of what the brain activity looks like? Is it in terms of the effects that it has on the subsequent responses? Then we must ask what we mean by “anything” in this case. The definition says that a stimulus is anything that is treated as a coherent phenomenon, but what kinds of things is it that the mind or brain is supposed to treat? Is it any input to the mind or brain from the outside? That would exclude all internal stimuli that the “experience”-definition was able to capture. Is it then any activity in the mind or brain that “anything” refers to? In that case we instead lose our ability to talk about things outside the mind or brain, since the activity is wholly inside the organism. The “anything” must therefore include both the input and the internal activity of the organism’s mind or brain. The question therefore becomes: how can we tell whether the input or internal activity is treated as a coherent phenomenon? This is a very difficult question.

Since the mind and the brain are intimately connected, merely being two sides of the same coin, then whatever occurs in the mind must also be expressed in the brain. If we have an experience of something as a coherent phenomenon, the brain activity corresponding to this experience must have properties that correspond to this kind of coherence. Unfortunately, we don’t know enough about the brain to be able to determine what that brain activity is like. We have no brain scanning method where we are able to look into someone’s brain and see that one input or brain activity is a coherent phenomenon or not. So defining “stimulus” in terms of something that is treated by the mind or brain as a coherent phenomenon doesn’t really help us now if we are to rely on such brain scanning methods. However, in the future we probably will have the required tools and knowledge. So in theory, the “treat”-definition could work. But let us take a moment to think about what would be required for us to develop the required tools and knowledge. How will we figure out what brain activity corresponds to having an experience of something being a coherent phenomenon? There are three ways we may go about it:

-self-report,
-observing responses
-analyzing activity on its own

Using the method of self-report involves asking the person about their experience and looking at what the brain activity is like. The brain activity is then taken to correspond to the experience that the person described. Thus, if a person reports when something is experienced as a coherent phenomenon and when it is not, we could determine which brain activity corresponds to a phenomenon being experienced as a coherent unit. We could subsequently use this knowledge to determine what it means for a brain to treat something as a coherent phenomenon. But given that this knowledge was acquired via self-report we would only know what the brain activity looks like when conscious experience is involved. The point of defining “stimulus” as something that is treated and not experienced as a coherent phenomenon was so that we could talk about situations where conscious experience isn’t necessarily present, but where something is still “treated” as a coherent phenomenon on an unconscious level. But as soon as we observe activity that differs from the previously observed activity (i.e. when the organism had an experience of something as a coherent phenomenon) we no longer know whether something is still “treated” as a coherent phenomenon in an unconscious way, because we have no self-report to match that activity to. Thus, the purpose of defining “stimulus” in the “treated” way is lost when we use this method to try to uncover the corresponding brain activity this way. It doesn’t give us any advantage of saying that something is “treated” as a coherent phenomenon if we already know that when a brain shows signs of treating something that way, then it is also consciously experienced. Thus, we want to find a way of figuring out if something is treated as a coherent phenomenon without being dependent on being told by a person what their conscious experience is. We could try to do this by observing the responses of an organism instead.

If we present an organism with different stimuli (as defined from our perspective) we could observe how the organism responds as we change the types of stimuli, or the ways in which they are presented. If the organism responds the same way when the stimulus is (from our perspective) on its own, part of something “bigger”, presented from a different perspective or when only “a part” of the stimulus is presented, we could infer that all these variations must be treated as the same stimulus from the perspective of the organism. We acquire this knowledge without knowing whether the organism had a conscious experience of these stimuli. Thus, we avoid the problem that arose when only using self-report. But there are also some problems with using this method. Say we present a (video of a) wolf to a sheep, the sheep shows signs of being scared. Then we show only the head of a wolf, and again the sheep shows signs of being scared. Then we show the sheep a group of 10 wolves, and the sheep is scared once again. Even though the sheep shows the same response it is wrong to conclude that all these different stimuli (from our perspective) are treated as the same coherent phenomenon from the sheep’s perspective. It is more likely that the sheep’s experience is like ours: in the first case it experiences “a scary wolf”, in the second “a scary wolf head” (which is something different from a whole wolf), in the third case 10 separate “scary wolves” or “a group of scary wolves”. The fact that all these different stimuli have the same property “being scary” is not the same as being experienced as the same coherent phenomenon. The problem is that visible responses of an organism are usually very generic. The same response can occur to many different types of stimuli. We cannot reverse engineer what the specific stimulus was merely by observing the visible responses of an organism. Also, this method doesn’t allow us to learn anything about stimuli that are merely internal, for example whether things in a fantasy are experienced as coherent phenomena or not. Since we aren’t in control of presenting such stimuli we cannot observe how they match particular responses. So let us therefore see if our third method can help us.

If we had the tools to observe the brain activity of an organism in great detail, there could be things that we notice about this activity that makes some sense, even in the absence of self-report or visible responses. For example, we might discover that some neuronal activity is synchronized. We might then conclude that neuronal activity that is synchronized is “treated as a coherent phenomenon” by the brain. To make sure we are able to include external stimuli with this definition, we would also have to find a way to determine that some particular synchronized activity is about something in the outside world. In order for “synchronized activity” to be a useful way of defining “stimulus” then this kind of activity must also result in something that makes sense to call a “response”. If the synchronized activity does not clearly result in things like physical behavior, physiological reactions, or other responses, then “stimulus” in this case does not fulfill one of its major purposes, and hence isn’t a useful concept. The concept of “stimulus” must still be able to be involved in psychological laws of how stimuli can cause responses. We would also have to observe whether other synchronized activity causes other synchronized activity and in this way conclude internal stimuli and responses are explained by the definition. Step by step we could build up a coherent way of defining “external stimulus”, “internal stimulus” and “responses”, that all abide by the laws of psychology, but it would require technological advances and luck, in a sense, since we don’t know whether there are such clear markers of neuronal activity that seem reasonable to think of as “coherent”.

So in theory, it could be possible to find a way of observing whether an organism “treats” something as a coherent phenomenon without relying on self-report, but we don’t yet know if that is possible. At least it is no easy way out to switch to such a definition, and it is unclear whether the pros outweigh the cons. For organisms where we do know that they have conscious experience, like humans, it adds nothing to define stimulus as “anything that is treated as a coherent phenomenon”. Instead this would just take us one step away from what is both more intuitive and more accessible. Any advantage we could get from such a definition would only arise for organisms or machines where we don’t know whether they have conscious experience or not. For this reason we might as well think of the “experience”-definition as applying to the psychology of humans (and other organisms that we are sure are conscious), and use the “treat”-definition for organisms and machines where we are not so sure, just like we have previously suggested is an option.

It is also interesting to note that if we were able to discover some type of synchronized activity that could be used to study stimuli without relying on the “experience”-definition, then this activity would be experienced by us, the observers, as a coherent phenomenon. If we didn’t perceive the synchronized activity as a coherent phenomenon we wouldn’t be able to talk about it as a “stimulus”, and discuss its implications. So on the level of a metadiscussion about defining “stimulus” in some way or other, our experience of something as a coherent phenomenon is always primary.

How should we define “response”?

The question

How should we define “response”?

Why is this question important?

The term “response” is central to psychology, and in particular in the unified theory. If we don’t have a good understanding of what a response is, then the laws that are applied to responses will yield the wrong results. Trying to define the term “response” is also part of a broader ambition to define what the fundamental entities of psychology are. In physics, for example, the fundamental entities are particles (to put it in rough terms), and the laws of physics apply to these fundamental entities. If we don’t know what the fundamental entities are in a field, then we won’t know how the laws are applied with any good accuracy. So defining the term “response” correctly is therefore crucial in order to set up the field of psychology from first principles.

Notes on strategy

Establishing the first principles of a field means finding all of the fundamental entities of the field. This means that all phenomena within the field must be explained in terms of the fundamental entities. If the terms “stimulus”, “response” and “relation” are to be established as the fundamental entities of psychology, then all psychological phenomena must be able to be categorized as one of these, or a combination of them. When defining the term “response” we must therefore make sure that this definition allows us to establish the full set of fundamental entities.

Observations

–same observations as those concerning the definition of “stimulus”–

Hypotheses (alternatives for definition of “response”)

A response is anything that is caused by a stimulus
A response is any behavior of an organism
A response is any psychological event
A response is any psychological event that is caused by one or several stimuli

Evaluation of hypotheses

A response is anything that is caused by a stimulus – insufficient

When discussing the definition of “stimulus” we pointed out that one of the most common ways to try to define this term was “anything that causes a response”, or some variation of this. Similarly, the most common way of trying to define “response” is “anything that is caused by a stimulus”. We could see that there were a number of problems when trying to define “stimulus” in terms of its ability to cause a response. Do we then have the same problem if we try to define “response” in terms of it being caused by a stimulus? Well not exactly. If we have managed to identify a stimulus by using the more correct “experience”-definition, we can succeed in observing that “things” or “events” are caused by these stimuli, and we can categorize these as “responses”. We don’t get into the same problem of dealing with infinite causes since we have already solved that issue with how we chose to define “stimulus”. Still, there are some things with this definition of “response” that are problematic.

To begin with, when we say that a response is “anything that is caused by a stimulus” what do we mean by “anything”? An external stimulus exists outside the mind and can cause many different things to occur, not just psychological ones. A rolling ball can cause another ball to start rolling when they collide. According to our definition of “response”, the event where another ball starts rolling would also classify as a response, since this event is an example of “anything”. But what we are interested in is psychological responses. We want to know how stimuli affect an organism, not how it affects other objects. For this reason, the word “anything” is not precise enough. We need to specify that we are talking about psychological events.

Another point that is worth considering concerns the use of “cause” in the definition. Even though it doesn’t give result to any contradiction or philosophical impossibility to define “response” in terms of them being caused by a stimulus, there are reasons why it is not desirable to do so if possible to avoid. The reason for this is that if we define “response” this way, the only way we can identify a response is if we can observe that it is caused by a stimulus. The property of being caused by a stimulus becomes the central property of a response if we define it this way. But most of the time we are not aware of what caused a response to occur. What we are aware of instead is the fact that some psychological event occurred. This is the primary identifiable property. Even if we knew that all psychological events are caused by stimuli, and therefore would know that a specific response was caused by a stimulus even when we don’t know which stimulus that caused it, then it would still mean that we would have a definition that does not give us practical guidance on how to identify it. For this reason, the “cause”-definition of “response” is not optimal.

Also, if we define “response” as something that is caused by a stimulus, and claim that this category includes all psychological events, then we are already in this definition presupposing that all responses occur by being caused by stimuli. This is of course not wrong. The fundamental law of psychology in fact states that all responses are caused by stimuli. But we don’t want to make any suppositions about laws of psychology when trying to define the entities that the laws are supposed to apply to. It would be like defining electrons as particles that are attracted to protons. Sure, electrons are attracted to protons. Electrons have negative charge, and protons have positive charge, and positive and negative particles attract each other. But if we defined electrons this way we would be mixing a description of what electrons are in themselves, and the laws that apply to the electron. We must define entities separately from the laws that apply to them.

On a last note, a response may not always be caused by a single stimulus. Sometimes several stimuli need to occur at the same time in order for a response to occur. Therefore, it would be incorrect to say that a response is caused by “a stimulus”, when in fact it may be caused by several stimuli.

A response is any behavior of an organism – incorrect

In order to define “response” without making any supposition about how responses occur, or mixing definitions and laws of psychology, we would have to exclude any mention of causality in the definition. One popular way of doing so is by defining “response” as any behavior that an organism performs. This way of viewing “response” comes from behaviorism. This field of psychology originally only studied visible behavior, i.e. either when the organism moves its limbs, or when displaying a physiological reaction like salivating, crying, or increasing its heart rate. For classical behaviorists these were the only responses that mattered since they weren’t interested in anything going on inside the mind. The response had to be able to be observed from the outside, otherwise it was deemed unscientific by behaviorists. For this reason it seemed sufficient to define response this way.

The problem is that a lot of responses are not visible to outside observers. Thoughts, feelings, memories, predictions, decisions, etc are all inside the mind. For this reason, more modern behaviorism has expanded what is included under the term “behavior”. Things like feelings and thoughts are thus examples of behavior under the revised definition. The reasoning for why such things can be included under the term “behavior” and be considered to be in line with behaviorist philosophy, is that while not being observable by outside observers, they are observable by the person or organism themself. When we have thoughts or feelings, we can observe that this is so. Thus, “behavior” under such a definition is thought of as remaining an objective phenomenon, something that can be observed, and therefore requires no speculation. Behavior that can be observed from the outside is called “overt behavior”, and behavior that can only be observed from the inside is “covert behavior”.

Unfortunately, this doesn’t include all relevant psychological events that can be caused by stimuli. There are events that cannot be directly observed by either an outside observer or the organism itself. The most clear example of this is the formation of new connections (or relations) between stimuli and responses. The formation of these connections cannot be directly observed. We only discover that they have formed when we at a later time notice that a stimulus causes a response which it didn’t previously cause. For example, when we have an emotional experience in a place (such as being positively surprised, or being scared by something), and then return to this place at a later time, we discover that we feel that same emotion again. We thus conclude that some kind of association has been formed between the place and the emotion. Another example of a psychological event that we don’t observe directly is the prediction of what we are about to experience next. The mind does this in response to every stimulus we experience, constantly trying to predict the next set of stimuli, but we usually don’t notice it until our prediction fails, and feel surprised by what happened. Evidently we had predicted something else would happen!

These types of events are central to the functioning of our mind, and if we fail to include them among the list of responses, then we will not be able to create a complete psychological theory. One attempt to solve this problem is to expand the definition of “behavior” even further and create a third category of behavior which includes events that are only observed indirectly in the form of the consequences that these events have. But doing so would defeat the purpose of using the term “behavior” to begin with. Unconsciously forming a new connection between stimuli and responses is far from what most people think of when they hear the term “behavior”. It makes more sense to drop this term and instead use the term “psychological event”, which more accurately describes what we actually mean.

A response is any psychological event – insufficient

A psychological event is anything that occurs in, or is produced by, the mind. Whether it is an action we perform, a physiological reaction in our body, the sensing or perceiving of the outside world, a feeling, a memory, a decision, a thought, a dream, a prediction, the formation of new connections between stimuli and responses. By defining a response as any psychological event, there is nothing that is left out. The term “response” thus includes everything which we might be interested in understanding how it comes about, concerning the mind. And since we have made no mention of these psychological events being caused in any certain way we have avoided making any conflation between description of the entity itself and any laws that apply to it. So we should be fine then? Well, unfortunately no.

As we have previously established, the first principles of psychology are really pseudo-first principles. The reason for this is that the first principles of psychology involve only the mind perspective, and not the brain perspective, which instead is explained by the first principles of neuroscience. The first principles of psychology concern how the mind functions when the mind “works as it should”, when the things we experience and the things we do follow the internal logic and psychological laws of the mind. There are instances where this doesn’t occur, and instead can only be explained by neuroscience or neurobiology. For example, when we suffer brain damage, for example as a result of a stroke or an accident, this changes how the person functions. We cannot explain such a phenomenon in terms of a stimulus causing the person to form new connections between stimuli and responses. We can only explain it in terms of neurons being damaged, causing neural connections to break. And when we are exhausted after a stressful period and notice that we have trouble concentrating, feeling like our mind is in a bit of a fog, then we can’t explain this as the mind responding to a certain stimulus. We can only explain this in terms of neurons needing energy to function properly, and when the brain hasn’t received enough energy, the neurons have trouble working correctly until we have been able to relax and build up energy again. And if we take a psychoactive drug and experience strange, mind-bending things, then yet again we can’t explain this as a response to a stimulus. We can only explain this in terms of the drug crossing the blood-brain barrier, causing the synapses between neurons to behave differently. These explanations belong to neuroscience.

The first principles of psychology can only explain what happens when there is no external physical influence on the mind where the biological components are affected through non-psychological mechanisms. Most of the time there is no such external influence. The mind usually works as it should. And even when there are times where the brain is affected directly via non-psychological mechanisms like drugs, brain damage or brain stimulation, once we have established what stimuli or responses or relations were changed through this influence, we can then apply the laws of psychology to explain the further consequences of how the mind works. So it is okay that the first principles can’t explain 100% of all psychological events, but it means that we have to be more specific when defining “response” to make sure that we don’t include things that can only be explained by neuroscience. How could we do that?

A response is any psychological event that is caused by one or several stimuli – correct

There are several possible ways we may try to define “response” to show that it includes all psychological events to which the laws of psychology apply. One way is to try to say that a response is any “natural” psychological event, but a word like “natural” can have many different meanings. Another attempt could be that a response is any psychological event that occurs when the mind works as it should. But a phrase like “as it should” can also be interpreted in many different ways. It is not clear what is meant by this. It turns out that the best way of capturing that a psychological event can be explained by laws of psychology, and not just by laws of neuroscience, is in fact by specifying that the psychological event was caused by one or several (psychological) stimuli. When a response occurs this way, we know that it does so as a result of following the psychological laws that establish how stimuli and responses relate to each other. The first principles of psychology will apply to any psychological event if it is that type of event that occurs as a response to stimuli. So despite our willingness to avoid talk of causal mechanisms in the definition of “response”, we need to define it this way in order to be as accurate as possible.

So this leads us to defining “response” as “any psychological event that is caused by one or several stimuli”, where we have also specified, as mentioned before, that a response can be caused by either one or several stimuli.

When defining “stimulus” we talked about the causal effects of stimuli. In that discussion we briefly mentioned the question of whether a stimulus automatically causes (or triggers) a response, or merely elicits (“suggests”) a response that is under control of the organism to perform or not. We didn’t need to go into any details of that discussion when defining “stimulus”, since we defined that term without any mention of causality. But now that we are defining the term “response”, we do mention causality in our definition. Our ambition with our definition of “response” was, however, to try and avoid mixing in a description of psychological laws in our definition, and instead try to obtain a definition that is purely descriptive of the entity itself. Now that we had to mention “causality” in our definition all the same, we could purposely avoid any further specification of what we mean by “cause” in this definition. We could leave it open whether the causal effect is in the form of an automatic trigger, or whether it is more in the form of eliciting a response that doesn’t necessarily occur. It is only when we discuss the fundamental law of psychology that we have to make a final statement about this. We will be able to see that the correct way of viewing psychological causality is in terms of the automatic trigger, although the complexity of the interplay between several stimuli and responses does create situations where in practice we get an emergent effect where the organism can choose not to respond to a stimulus. More about that later.

What, then, makes an event “one response” as opposed to “several responses”? When we see a wolf, and as a response to this we feel scared, increase the activity of our sympathetic nervous system, and run away, are these things “one response”, or is it a set of “three responses”? When we talked about stimuli, we had a set of criteria that allowed us to determine whether we were dealing with one stimulus or several. When a phenomenon was experienced as coherent in itself, and as separate or distinct from other phenomena, then we could conclude that this phenomenon was a single stimulus. Are there any similar criteria for responses? In order to figure out the answer, we must begin by looking at how we know that a response occurs at all.

When an external object causes us to have a perception of that object, how do we know that the perceptual response occurred? When something causes us to have an emotional reaction, how do we know that the emotional response occurred? When the wind causes our eyes to blink, how do we know that the blink response occurred? When something causes us to remember something we did a few years ago, how do we know that the memory response occurred? The answer is: we experience the response. It is only when a response in some way or other enters our conscious experience that we can have an idea that the response occurred. Imagine moving your arm, but you cannot see it, cannot feel it, you cannot hear it, it doesn’t affect your balance, etc. In such a situation you wouldn’t know if you moved your arm or not. Another way of expressing this is that you can only become aware of a response through the stimuli that the response produces. When an external stimulus causes a perceptual response, we become aware of the fact that the stimulus caused this response by noticing this perceptual stimulus that was created. When something causes us to have an emotional reaction, we become aware of it by the feelings that were caused by the emotional response. When the wind causes us to blink, we only know about this because we can feel and see our eyes blinking. And so on.

Thus, the only way for us to determine the number of responses that occur, is to observe the number of stimuli that are produced by the response. Given what we know about stimuli, we can quickly see that there is no general, objective answer to this question. Instead, the answer will depend on the context. There can be situations where we experience the fear, the physiological activation, and our desire to run away as one coherent phenomenon, and thus as one stimulus, and thus as one response. In other situations we may experience these as separate, although related, responses.

Remember that stimuli can be experienced directly and indirectly. This means that whether we treat responses as one or several are not only determined by how they are experienced in the very moment when they occur, it can also be determined by how they are conceptualized through a more theoretical lens. For example, by studying the body and mind more carefully we can learn that an emotional response both involves a physiological reaction, a sense of positivity or negativity, a sense of having our attention pulled towards the stimulus that causes the emotional response, and a desire to move in some way or other. This kind of knowledge influences how we experience responses. Although we just feel one blurry sense of fear in a situation, we can a few minutes later reflect back on the situation and identify that there were several simultaneous responses: a feeling of negativity, the activation of the body, an urgency to run away. In this situation, then, we have an experience of there being three different responses to the same stimulus.

Thanks to our ability to observe phenomena indirectly, responses that are never experienced while they occur can still be known. The formation of memories is the paradigm example of this type of response. When learning new connections we never experience this directly. We may experience the feeling of surprise, as we come across something new, or the feeling of interest as we are told something we find interesting. But the actual connection that is formed is not itself experienced at that moment. Instead we become aware of this indirectly. For example, when we discover that we are able to do something we couldn’t before, or when we are able to bring something to memory. These are the moments when we realize that something occurred in the previous situation: a memory was formed, a connection was made. Because of our ability to observe consequences of hidden responses, we can experience these responses indirectly.

Contrasting this with the “behavior”-definition, a “response” is under our new definition not something that needs to be directly observable. It is okay for a response to be inferred from observable consequences. There is no reason to think of these types of stimuli to be considered less scientific just because we cannot see them with “our own eyes”. Very few things within the field of physics would be considered scientific if we used such a narrow view of what it takes for something to be scientific. Most phenomena in physics cannot be directly observed, but rather have been inferred from its observable consequences. No one has seen an atom, an electron or a proton. If we excluded such concepts on the basis that they are not directly observable, the whole field of physics would collapse. Likewise, there is no reason to think that the science of psychology would be less scientific just because we include responses that aren’t necessarily directly observable.

It is important to keep in mind that we use the word “event” in our definition. Why not use the word “phenomenon” instead? Or say “event OR thing”? When we specify that a response is a psychological event we focus on the fact that the response is what occurs just in that moment when the stimulus causes it to occur. We can compare this to an example in physics to illustrate what exactly is meant by this. Let’s take the example of a ball rolling into another ball and causing this ball to start rolling. Technically what happens in such a situation is not that the first ball causes the second ball to have a certain speed for a while. What happens is that the first ball causes the second ball to accelerate. The force that the first ball exerts on the second is equal to the mass times the acceleration of that second ball. When the first ball stops exerting any force on the second ball, the second ball stops accelerating which in turn makes the ball have a certain speed. As this ball continues to roll with this speed, the first ball is no longer exerting any force. The second ball’s continued rolling at a certain speed is therefore not technically the causal effect of the first ball. This is an effect of the law of inertia that states that an object will continue to move at the same speed (velocity) unless acted upon by force. Thus, the first ball is only involved in the event where the second ball accelerates. The acceleration of the second ball is the effect or “response”.

Similarly, when a stimulus causes an emotional reaction, for example, then the response is technically just the activation of the emotion. If the emotion continues to be present for a while then this is not caused by the event where the initial stimulus appeared. The continued presence of the emotion is either the effect of the original stimulus continuing to be present, and therefore continuing to cause an emotional reaction, moment after moment. Or it is the effect of other mechanisms, like the physiological system taking its time to restabilize, and as it does so the sensations in the body cause an emotional reaction to continue to occur, until the physiological system is back to normal (to homeostasis).

There is an important point related to this, which concerns what is technically going on when a response involves the appearance of a stimulus. For example, we talked about an emotional response. An emotion is an internal stimulus. It is a phenomenon that we can experience. So when a stimulus causes an emotion to occur, we might be prone to saying that the stimulus caused an emotional stimulus to occur. But the fundamental law of psychology states that a stimulus causes a response. It does not state that a stimulus causes a stimulus. So technically when an emotional stimulus occurs, what happens is that a stimulus causes an emotional response: the event of activation of an emotion. And this activation involves the production of emotional stimuli. Thus, the stimulus that appears is a response to another stimulus. So when we say that a stimulus causes another stimulus to occur, then what is really going on is that the occurrence of the second stimulus is a response to the first.

The fact that we are defining “response” in terms of a type of event is also important for correctly understanding what is meant when we say that a memory or relation occurs as a response to a stimulus. Since a response is a psychological event, then what is meant by this is only that moment where the memory (i.e. relation) is formed. It is the creation or formation of a relation that is the response. Once it has been formed, it is no longer a response, even if that relation continues to exist in our mind and allows stimuli to cause responses to occur that they previously didn’t.

How should we define “relation”?

The question

How should we define “relation”?

Why is this question important?

In our attempt to find the fundamental entities of psychology, to which all psychological laws apply, the concepts “stimulus” and “response” have been at the forefront. These are the concepts that also have been discussed the most throughout history, and are the hardest to pin down. But these entities are not sufficient. We must also specify the connection between stimuli and responses to understand the causal relationship between them. This is where the concept of “relation” comes in. In order for us to set up the field of psychology from first principles, we must therefore also define what this concept is.

Observations

A response can sometimes be triggered by several different stimuli, present on their own
A response can sometimes only occur if several stimuli are present at the same time
One stimulus can sometimes trigger several responses at the same time
A stimulus can fail to trigger any response at all
What responses are triggered by a stimulus can change throughout the organism’s lifetime

Hypotheses (alternatives for definition of “relation”)

A relation is the connection between stimuli and responses that specify which responses will be triggered when one or several particular stimuli occur

Evaluation of hypotheses

A relation is the connection between stimuli and responses that specify which responses will be triggered when one or several particular stimuli occur – correct

The same response can sometimes be triggered by several different stimuli, when present on their own. For example, the response of fear can be triggered by a wolf, a wolf’s head, a cliff, an object coming quickly towards us, and so on. So when we have identified a response there can be several different stimuli that may have triggered this response. There is nothing inherent in the response that specifies which stimulus triggers it. Therefore we need some way to specify this. The same goes for stimuli. A stimulus can trigger one response when it is present on its own, while triggering another response when present together with other stimuli. For example, when we see a little ladybug walking along on the ground we may respond by thinking that it is cute. But when we find a ladybug in our food, we may be slightly disgusted in response. It is also the case that a stimulus can trigger one response at one point in time, while triggering another response at a later point in time as a result of learning or biological maturation. For example, stimuli that evoke sexual arousal do not do so before we have reached puberty. For these reasons there is nothing inherent in stimuli themselves that tells us which responses they will trigger.

In order for us to specify which responses will be triggered by which stimuli we therefore need something more than the concepts of “stimulus” and “response”. This is what the concept of “relation” does. A relation is that entity which connects stimuli with responses. When a particular stimulus causes a particular response to occur, then we can only explain this by expressing that there is a relation between the two. The stimulus causes the response to occur because it has a relation to that response. When one stimulus causes three responses to occur, then the stimulus has three different relations, each connected to a different response.

What makes something “one” relation as opposed to “several” relations? We have looked at the same question concerning stimuli and responses, and how it in these cases depends on the specific situation whether we establish the presence of one or more stimuli, one or more responses. Since a relation is the connection between stimuli and responses, whether we say that there is one relation or many depends on the number of stimuli and responses we have established as being present. Each stimulus must have a relation with every response that it triggers. And each response must have a relation with every stimulus that needs to be present for it to be triggered.

Alternative terms for the concept of “stimulus”

We have settled on a definition of the concept we call “stimulus”, but we may still ask whether “stimulus” is the best word to describe this concept. The word originally means “something that stimulates”. But now that we have defined “stimulus” as anything that is experienced as a coherent phenomenon, this concept or entity is no longer tied to the property of stimulating. When it comes to external stimuli, this way of viewing “stimulus” is not too far off, since external stimuli only can be experienced if they stimulate our mind in some way. But internal stimuli have no such property. Should we then use some other word to denote that concept?

One alternative is to simply call it a “phenomenon”. This term is closely related to the idea of being experienced. The whole field of phenomenology is about studying “phenomena” in the sense of “things as they appear in our conscious experience”. But the word can also be used in ways that is more vague and abstract compared to the word “stimulus” which brings to mind more concrete things like objects. Still, associating “stimulus” with more concrete phenomena like objects could be a disadvantage, since we also include very abstract phenomena within the concept of “stimulus”, like “something failing to appear despite our expectation that it would appear”. So it wouldn’t necessarily be a bad thing to use the word “phenomenon” in the regard. Another aspect is that we are specifically interested in phenomena that are experienced as coherent wholes. This distinction could be lost if we merely used the term “phenomenon”. On the other hand, if we say that there is one phenomenon, the only way for us to do so is to experience some phenomenon as more or less coherent, otherwise we wouldn’t choose to call something “a” or “one” phenomenon. So in practice it would probably work out the same.

So the main reason for using “stimulus” instead of “phenomenon” is perhaps how “stimulus” is more associated with “responses”. People who are familiar with the term “stimulus” will immediately expect a stimulus to have the ability to cause a response. Within the science of psychology, this is one of the most important properties of stimuli.

Alternative terms for the concept of “response”

When looking at how to best define the concept of “response” we came to the conclusion that a response refers not to any and all psychological events, but only those which are caused by a stimulus. For this reason it would not be a good idea to replace the word “response” with “psychological event”. Calling it “psychological event” instead would give the impression that we include all psychological events.

A different alternative to consider is the word “reaction”, but this word gives the sense that we are talking about an emotional reaction, or at least something that is consciously experienced.

The word “effect” is another candidate, but this is a general term that is used in contexts not only psychological. It would be hard to differentiate between a psychological effect and a non-psychological effect if we used such a word. Using the word “response” is more unique and is more easily associated with something psychological.

Alternative terms for the concept of “relation”

We used the word “connection” when describing what the concept of “relation” is. Could it be motivated to use the word “connection” as the term denoting the concept instead? One advantage of using the word “connection” is that it is more familiar and colloquial. People in general understand what is meant by the word. But a disadvantage is that it is quite a general term that is used in many types of contexts. If we want to make sure that someone understands that we are talking about a psychological connection, a connection between stimuli and responses, then using a more specific word like “relation” could be helpful. It could make it clearer that we are not talking about a neurological connection between two neurons (i.e. a synapse).

On the other hand, the word “relation” may be more closely linked with the word “relationship”. People in general may mistake the word “relation” to concern the relation between two people. It may also be understood as a quite general connection. When we say that something relates to something else in some way, or that two things are related, it is not always clear that we are talking about a causal direction. But an important property of relations is that they always specify a clear causal direction from stimulus to response. In this regard, however, an alternative like “connection” would not do any better. Connections are usually thought of as going in both directions. If A has a connection to B, then B has a connection to A. The fact that the word “relation” is less common can therefore make it easier to establish as a term that only denotes a causal direction in one way, and not the other.

A third alternative is the word “association”. This word is very similar to that of “connection”. The word “association”, however, is much more common in the cognitive branch of psychology. In this branch, the word is not used in the specific sense of a unidirectional connection, but rather in a bidirectional sense. For this reason, it would not be a good idea to use this word instead of “relation”.

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References

[The list of references will be expanded upon. If you want to help with this please contact A Wiser World.]

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