This page describes the mind-body problem and its solution within the context of transcendental idealism. The solution to the problem was first published in the Critique of Pure Reason by Immanuel Kant in 1781.

Introduction to the Problem

The question which is to be asked is: What is the relationship between the brain and the mind?

The brain is a physical object, while the mind is not. The mind is related to experience, a mental phenomenon. The question, then, is more generally about the relation between the physical and the mental.

We suppose that the events and processes in the physical world are responsible for causing experience. When my hand is burnt (a physical event), I feel pain (a mental phenomenon), and it appears natural to regard the former as the cause of the latter.

Also, our minds apparently have some control over the course of physical events, since we can use volition to move our arms and legs around. However, the motion of our physical bodies is subject to the laws of physics, and so it would seem that the mechanistic progress of nature in accordance with physical laws must ultimately be able to account for our behaviour. Indeed, when we look for the physical cause of deliberate behaviour, we find one, in the form of electrochemical activity in the nervous system, which proceeds in accordance with the usual laws of physics. It is, then, not necessary to appeal to any mental phenomena to account for the behaviour of human bodies.


We can, then, as a hypothetical exercise, imagine a human body which behaves in accordance with the laws of physics, but which has no mental correspondent. Such a person would behave like any normal person, but would have no experience. Their body and its behaviour would be indistinguishable from the body and behaviour of a person who genuinely has experience. In fact, the person would claim that they really do have experience, and there would be no way for us to check their claim, since the only thing which we would be able to examine would be their body.

Such a hypothetical person is called a ‘zombie’. I think that it may have been the modern philosopher David Chalmers who first used the term in this way.

Three Questions

One question, which is perhaps a simple expression of the mind-body problem, is why we aren’t all zombies. That is, since our bodies would behave in exactly the same way as they currently do, even if we had no experience at all, because they move in accordance with the laws of physics, then why do we have experience? The physical world could continue to progress exactly as it does without any accompanying experience.

However, given the observation that we do have experience, another question arises, which is: How do the physical processes in the brain give rise to experience? If we accept it as an empirical result that our brains do cause experience, then what is the nature of this causation, and what other kinds of physical processes have the ability to cause experience, like our brains have?

We can invent thought-experiments for ourselves, such as simulating a person’s brain in a computer. Would the simulated brain have experience, or does it need to be a fleshy brain? If it needs to be a fleshy brain, we could imagine replacing the brain cells, one by one, with simulated versions of themselves, so that an approximately continuous transition is possible from a fully fleshy brain to a fully simulated brain, progressing through intermediate stages in which part of a brain is connected to a computer which simulates the missing fleshy part and provides the remaining fleshy part with exactly those signals which it would receive if the missing fleshy part were there. How much of the biological brain would need to be replaced before experience disappeared?

A third question is why do people claim to have experiences? One might suggest that they are very well aware of their own experience, and that they can simply report that fact through their behaviour and speech. However, a zombie would say the same thing, because his behaviour would be exactly the same as that of a real, experiencing person. If a zombie would claim to have experience, even when he doesn’t, then evidently he does not observe that he has experience (because he does not, in fact, have experience) and report this fact.

In fact, the physical processes which unfold in the zombie’s body, when he claims to have experience, are exactly the same as the physical processes in an experiencing person’s body when he makes a similar claim. Consequently, a zombie would claim to have experience for the same physical reason as a person who genuinely does have experience.

Panpsychism, Computationalism

From the observation that some physical processes can apparently cause experience, namely those processes in the brain, we can reflect on the following. All physical processes are pretty much alike - they involve the movement of atoms, molecules, electrons and so on. The particles move in various patterns through space as time passes.

If the motion of some of these particles in some patterns causes experience (a non-physical phenomenon), and motion in different patterns causes different experiences, then surely it cannot be only very specific patterns of particle motion which are associated with mental phenomena. A more natural situation would be if all particle motion was associated with mental phenomena, with complicated motions (such as the processes in our brains) being associated with complex experiences (such as ours). Simpler motions, such as the motion of matter in the brains of lower animals, would be associated with simpler experiences. Following this train of thought, we are led to the conclusion that even bricks and pens have experiences, although not very interesting ones.

This idea is called “panpsychism”.

A different approach is based on the idea that a simulated person would behave in all respects like a genuine experiencing person. If we suppose them to have experiences, then it would appear that computations alone are enough to produce phenomenal experience. Based on this view, a zombie is impossible, since the experience will be instantiated purely as a consequence of the information processing which his brain executes.

Taking this hypothesis, we can again invoke thought-experiments to see how credible the idea is. If it is merely computations which are the cause of experience, then can I cause an entity to have experiences simply by doing calculations on a blackboard with chalk? What if I make a mistake? What if the symbols describing the computation are, by some strange chance, etched into the side of a cliff by erosion? Would that cause experience?

Furthermore, all computations are in fact a series of table lookups. For example, for all logical operations (AND, OR, XOR, NOT etc.), the computation is always done by looking up the answer in a truth table. Any more complicated computation can always be performed with a lookup table as well. For example, an algorithm which factorizes numbers can be implemented with a lookup table which has the number to be factorized in one column and the factors in another column.

What if the computations which are used to output the behaviour of the simulated person are simple table lookups of this sort? When you put a question to such a simulated person (e.g. Do you have experience?), the answer (Yes), will be found by looking it up in a table. Does such a simulated person have experience? If not, how does replacing one table lookup with several consecutive ones make the difference between no-experience and experience?

Some Discussion

The mind-body problem is one of the central problems of philosophy. How does the brain cause experience? Chalmers has called this the `hard problem’, and it is to be distinguished from the easy problem, which is: How does the brain process information and give rise to complex behaviour? Although the second problem is not actually easy, it can at least be investigated scientifically, and we more or less know the answer already - nerve cells signal to one another, this processes information, and the output of the processing is sent to the muscles. The rest are mere details.

With the `hard problem’, though, we have no idea of how it could possibly be that a physical process, such as the functioning of the brain, could give rise to a phenomenon of an entirely different sort - experience. The gap between these two seems to be so vast that it is difficult, if not impossible, to see how there could be some kind of mechanism which brings the consequences of physical events out of the physical world into the mental world.

Even if we consider what kind of answer we would be satisfied with, we cannot imagine that any answer at all would be satisfactory. An answer would have to be of the form: When the physical event happens, such and such a mechanism is activated, and that causes experience. But that will not be satisfactory, because the mechanism will be either a part of the physical world, or it will have some other kind of existence, like the type of existence that experience has. In neither case would such a mechanism be acceptable as a bridge between one type of existence and the other.

Computationalism and panpsychism are merely conjectures about the circumstances in which experience arises, not about why or how it arises as a consequence of the physical situation involved. They therefore do not even attempt to answer the question at hand.

This problem, then, is a problem of such difficulty that it ranks alongside the deepest problems of philosophy, such as: Why is there anything at all, rather than nothing? How could the universe come from nothing? How did the physical universe come into being? Does time have a beginning or does it not?

Many of these questions also have this characteristic - that we cannot even conceive of an answer. For any answer must have the form: Such and such a thing happened, and then the universe began. An answer like that is obviously inadequate.

Transcendental Idealism

Before we can address these questions, we must first understand something about space and time. In all of these enquiries we must remember that we are not concerned with conjectures. A person who devotes his attention to conjectures will not be in a good situation to determine the truth.


We can look around and wave our hands around in space. The objects that we encounter are in space, and it is because of this that they can have the properties of extension and location. If anybody thinks it is appropriate to draw a distinction between the space which we encounter in experience and another space which is conjectured to exist independently of experience, then it is the former that we are concerned with here. We are not concerned with building conjectures on top of conjectures, because in that way we can never achieve any knowledge.

Consider, then, a thing which is extended and which is encountered in experience. As an example, consider an image. We are considering here that which is presented to the mind, namely the image, which can be thought of as identical to the experience of seeing the image. We are not considering something which is conjectured to exist independently of the mind, but are considering that which exists only as an attribute of the perceiving mind, namely the experience of seeing the image.

When the mind perceives the image, it is presented with information, because it is this image, and not a different one, which is presented. The information is presented in a certain format, namely video format, by which I mean that the experience is a visual one. Consequently, the image is a data structure which is presented to the mind, and it has both information (which image it is) and a format (because it is an image and not, for example, a sound). Whenever any experience is presented to the mind, whether it be a sight, a sound, pain, a concept, a judgment or any other experience, information is presented to the mind, and this must always have a format, because pure information is not a possible perception, but information must always be presented to the mind in a certain form.

Anything which is presented to the mind in this way provides data in a specific form, and is called (by Kant) a representation.

Now we must pay attention to the format of the image. The image has parts, which stand together in relations of relative position. The image is a combination of its parts. Considered as a representation, it has the following property: That the representation of the whole is a combination of the representations of the parts. For the whole image is not presented to the mind unless all of its parts are.

Furthermore, the representations of the parts must be combined together in a specific way to form the representation of the whole image. Specifically, in the representation of the whole image, the parts are represented as standing in relations of relative position. The parts are not, for example, merely presented to the mind simultaneously, without it being determined what relations the parts have to each other. Consequently, in order for the mind to perceive an image, the representations of the parts of the image must be combined together in a specific way, so that in the representation of the image, the parts are represented as standing in relations of position, in space.

From this it follows that relations of position are not discovered empirically, through observation of the way that things are. For the mind must already be able to combine the representations of the parts into a representation of the whole before anything extended or located can be perceived. Extension and location are therefore not discovered through experiment, but the mind must have the ability to represent things as extended and located before experiments can themselves be represented.

We call the world around us, which we encounter in experience, external, not because it is external to the mind, or exists independently of it, but because its parts are in space and stand in relations of position to one another.

It is also the case that with this metaphysical understanding of the properties of space we can say what extension is more clearly and certainly than can be achieved with mathematics. In mathematics, a segment of a line has few properties that the set of numbers between 0 and 1 lacks. For each pair of points in the line, there can be found a point in between, and the same is true of the numbers, and so on.

However, we know that a line segment is extended, while the numbers are not. From the point of view of mathematics, this property - extension, cannot be expressed, for the numbers have all the properties that the line segment has and more. But in metaphysics we can say what the line segment has which the numbers lack. The representation of the whole line segment is a combination of the representations of the parts of the line segment. For the representation of half of the line segment must be combined with the representation of the other half in order for the whole line segment to be represented, and if the representation of either half is missing, then the whole line segment is not there.

With the numbers, however, it is not the same, for the representation of the numbers between zero and one (in thought) is not a combination of the representations of other collections of numbers. And although the mathematicians speak of a distance between two numbers, this is nothing more than a poetical metaphor, for the numbers do not stand in relations of position, in space, but are represented only in thought.

The reason that mathematics cannot describe extension is precisely this: because mathematics does not consider the way in which the objects with which it deals are presented to the perceiving mind, while the question of whether an object is extended concerns precisely the way in which it is represented in the mind. It must be the case that the representation of the whole is a combination of the representations of the parts, if an object is to be represented as extended.

From this we see what extension and location are, and how they are related to the representations of the mind.

It is concluded from these considerations that if space is understood to be that in virtue of which it is possible for things to be extended and located, then space pertains only to the manner in which things are represented in the mind. Considered in isolation from experience, space is nothing at all, for it is only representations which can be spatial, and representations can be apprehended only in experience.

Here I must deal with objections which have been raised against this doctrine.

First, there is the objection of Bertrand Russell, who says, in his “History of Western Philosophy” that he does not understand why, if Kant’s view of space is correct, the locations of objects do not change when he looks away and looks back. He finds that people’s mouths are below their noses, and remain so throughout time, and considers this to be inexplicable if Kant’s understanding of space is taken to be true.

However he managed to reach this conclusion, Russell is confused. The question of why people’s mouths stay below their noses would still arise regardless of how we view space, either as a property of the things themselves or as a property of the manner in which we represent them. People’s mouths were below their noses yesterday and the day before, and we expect them to be so today because of that. The reason why we expect the mouths to be below the noses today is because of induction - the principle that leads us to expect that things will continue to be as they were in the past.

Russell’s objection then, amounts to this: That Kant has not explained why inductive reasoning works. But even if we suppose that space has nothing to do with the representations of the mind, the question of why inductive reasoning works remains. Furthermore, Kant does, in fact, have an explanation of why inductive reasoning works, and Russell has not himself even attempted an explanation, or addressed Kant’s. Kant’s explanation of induction, however, would have to be expounded at another time, because here we are dealing with space.

So much, then, for Russell’s objection.

Many people have objected that this view of space is simply solipsism, and is thus the same as the assertion that nothing exists. It is possible that this was in Russell’s mind also when he made his objection, since if he supposed Kant to be saying that nothing existed, this could be why he felt that pointing to the regularity of the world was a refutation.

There are two ways in which the conclusion of solipsism can be reached. The first is the dogmatic way, which asserts that one must always begin with the conjecture that the world (i.e. space) exists independently of the mind, and that any approach which does not do this is solipsistic.

This is the same as when a religious man declares mathematics to be an athiestical discipline, since it does not take as an axiom the existence of God. Many people condemn as unacceptable any doctrine which does not begin with conjectures coinciding with their opinions. Such a reaction happens when a person confuses a refusal to adopt a conjecture with the adoption of the opposite conjecture. As it was said earlier, we are not concerned with conjectures here.

Behind this objection and Russell’s objection (if it was based on the supposition that Kant’s view is solipsistic) is the idea that whatever is represented in the mind is arbitrary, and that one can just as easily represent mouths to be above as below noses. This idea is incorrect. If it were true, people would conjure up fantasies that they find pleasing and believe them to be true.

If truth is the agreement of a mental representation with the object represented, then the object must be regarded as that which is opposed to the representations of the mind being arbitrary. If a representation is arbitrary, then, we can not regard it as in any way constrained by the truth. Consequently, we cannot simply perceive what we wish, but must regard those representations of ours which are arbitrary as not representing any object distinct from themselves.

Other objections have centred on the role of geometry, but since no argument involving geometry has been used here, those objections can be ignored.


The characteristics of things represented in space are extension and location, but those of time are succession and duration.

A succession of perceptions is not a perception of succession. Consequently, we must regard the succession which we perceive in experience as one which is encoded within the representations of the mind, rather than regarding the succession of our experiences as a consequence of a succession of (e.g. physical) states of affairs.

As in the case of space, it can be seen that succession is not discovered empirically, through examination of the way that things are. In order for the mind to be able to represent an event as succeeding another one, it must already be able to combine the representations of the two events into a single representation in which the later event succeeds the earlier one. This manner of combining the representations cannot be learnt from any observed succession, because it is only by combining them in this way that anything can be regarded as successive.

Consequently, the time of which we are aware is not something which would remain in the absence of a perceiving mind. It is encoded within the representations which the mind apprehends. Also, the combination of representations of events in such a way that in the combined representation of the two events, one follows the other, can occur only in experience. Conversely, when one event is understood to succeed another, what is present in the mind is precisely that representation which combines the representations of the two events in the way just mentioned - so that one is earlier and the other is later.

From the doctrines of space and time, we conclude that all of the objects which are to be found in space and time are representations of the mind. Kant refers to them as appearances. As noted above, this does not mean that they are arbitrary, and nor does it mean that the objects in space and time aren’t real. Just as real pain is different from imagined pain, so a real object in space and time is different from an imagined one.

This also does not mean that the world doesn’t exist, and it doesn’t mean that the people that you meet don’t have experiences. It means that all that the mind deals with are its own representations, which, after all, is obvious, since how could a mind apprehend anything except for a representation which is presented to it?

The Mind-Body Problem

Here we come to the solution to the problem. It is seen that the question: How does the brain cause experience? is based on a mistaken assumption.

Since both space and time, along with all relations of location and succession, pertain only to representations which can be encountered in experience, the brain, as a spatiotemporal object, cannot be regarded as the cause of experience.

The mistaken assumption is that we can represent to ourselves things as they would exist in the absence of experience. Even by representing objects as existing in space and time, we are restricting the applicability of those representations to circumstances which arise in experience. Consequently, if we consider the brain as an object with the characteristics of extension and duration, then such a thing, since it is only an appearance in space and time, can have no existence outside of the mind, any more than a headache or an emotion can.

If we wish to understand how it can be that the physical world and mental phenomena mutually interact, as the case would appear to be when injury is regarded as the cause of pain, or when volition is regarded as the cause of action, then that can be understood in the following way. Since physical processes are appearances in space and time and are hence representations of the mind, it can be understood without difficulty that they can interact with pain or volition, which are likewise mental representations. That injury causes pain is the way that we regard the situation when, through experience, we have learnt that one invariably accompanies the other. It is in just the same way that all empirical physical laws are discovered and represented.

On the other hand, if we suppose that space and time, and the objects and processes therein, have an existence which is independent of experience, then it cannot be understood how the physical and mental world could be in any interaction. Nor could it be understood how we come to have knowledge of time and space, if they existed independently of experience, because the only things that we can have knowledge of are the representations of the mind.

Similarly, the question of why we are not all zombies is based on the same mistaken assumption - that we can represent to ourselves how things might exist independently of experience and consider what the consequences would be. Hence we imagine that physical bodies could exist in space and time, without there being any experience. However, when imagining such objects to exist in space and time, all that we can represent to ourselves is the manner in which they would be represented in a mind which perceived them. This is unavoidable, because it is only in experience that space and time can be encountered.

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