Perhaps there is nothing so difficult for the imagination as to teach it to feel about space as modern science compels us to think.
We said that we know about what is happening in the brain exactly what naive realism thinks it knows about what is happening in the world. This remark may have seemed cryptic; it must now be expanded and expounded.
The gist of the matter is that percepts are in our heads; that percepts are what we can know with most certainty; and that percepts contain what naive realism thinks it knows about the world.
But when I say that my percepts are in my head, I am saying something which is ambiguous until the differen kinds of space have been explained, for the statement is only true in connection with physical space. There is also a space in our percepts, and of this space the statement would not be true. When I say that there is space in our percepts, I mean nothing at all difficult to understand. I mean - to take the sense of sight, which is the most important in this connection - that in what we see at one time there is up and down, right and left, inside and outside. If we see, say, a circle on a blackboard, all these relations exist within what we see. The circle has a top half and a bottom half, a right-hand half and a left-hand half, an inside and an outside. Those relations alone are enough to make up a space of sorts. But the space of every-day life is filled out with what we derive from touch and movement - how a thing feels when we touch it, and what movements are necessary in order to grasp it. Other elements also come into the genesis of the space in which everybody believes who has not been troubled by philosophy; but it is unnecessary for our purposes to go into this question any more deeply. The point that concerns us is that a man's percepts are private to himself: what I see, no one else sees; what I hear, no one else hears; what I touch, no one else touches; and so on. True, others hear and see something very like what I hear and see, if they are suitably placed; but there are always differences. Sounds are less loud at a distance; objects change their visual appearance according to the laws of perspective. Therefore it is impossible for two people at the same time to have exactly identical percepts. It follows that the space of percepts, like the percepts, must be private; there are as many perceptual spaces as there are percipients. My percept of a table is outside my percept of my head, in my perceptual space; but it does not follow that it is outside my head as a physical object in physical space. Physical space is neutral and public: in this space, all my percepts are in my head, even the most distant star as I see it. Physical and perceptual space have relations, but they are not identical, and failure to grasp the difference between them is a potent source of confusion.
To say that you see a star when you see the light that has come from it is no more correct than to say that you see New Zealand when you see a New Zealander in London. Your perception when (as we say) you see a star is causally connected, in the first instance, with what happens in the brain, the optic nerve, and the eye, then with a light-wave which, according to physics, can be traced back to the star as its source. Your sensations will be closely similar if the light comes from a lamp at the top of a mast. The physical space in which you believe the "real" star to be is an elaborate inference; what is given is the private space in which the speck of light you see is situated. It is still an open question whether the space of sight has depth, or is merely a surface, as Berkeley contended. This does not matter for our purposes. Even if we admit that sight alone shows a difference between an object a few inches from the eyes and an object several feet distant, yet you certainly cannot, by sight alone, see that a cloud is less distant than a fixed star, though you may infer that it is, because it can hide the star. The world of astronomy, from the point of view of sight, is a surface. If you were put in a dark room with little holes cut in the ceiling in the pattern of the stars letting light come through, there would be nothing in your immediate visual data to show that you were not "seeing the stars". This illustrates what I mean by saying that what you see is not "out there" in the sense of physics.
We learn in infancy that we can sometimes touch objects we see, and sometimes not. When we cannot touch them at once, we can sometimes do so by walking to them. That is to say, we learn to correlate sensations of sight with sensations of touch, and sometimes with sensations of movement followed by sensations of touch. In this way we locate our sensations in a three-dimensional world. Those which involve sight alone we think of as "external", but there is no justification for this view. What you see when you see a star is just as internal as what you feel when you feel a headache. That is to say, it is internal from the standpoint of physical space. It is distant in your private space, because it is not associated with sensations of touch, and cannot be associated with them by means of any journey you can perform.
Your own body, as known to you through direct experience, is quite different from your own body as considered in physics. You know more about your own body than about any other through direct experience, because your own body can give you a number of sensations that no other body can, for instance all kinds of bodily pains. But you still know it only through sensations; apart from inference, it is a bundle of sensations, and therefore quite different, prima facie, from what physics calls a body.
Most of the things you see are outside what you see when (as one says) you see your own body. That is to say: you see certain other patches of colour, differently situated in visual space, and say you are seeing things outside your body. But from the point of view of physics, all that you see must count as inside your body; what goes on elsewhere can only be inferred. Thus the whole space of your sensible world with all its percepts counts as one tiny region from the point of view of physics.
There is no direct spatial relation between what one person sees and what another sees, because no two ever see exactly the same object. Each person carries about a private space of his own, which can be located in physical space by indirect methods, but which contains no place in common with another person's private space. This shows how entirely physical space is a matter of inference and construction.
To make the matter definite, let us suppose that a physiologist is observing a living brain - no longer an impossible supposition, as it would have been formerly. It is natural to suppose that what the physiologist sees is in the brain he is observing. But if we are speaking of physical space, what the physiologist sees is in his own brain. It is in no sense in the brain that he is observing, though it is in the percept of that brain, which occupies a part of the physiologist's perceptual space. Causal continuity makes the matter perfectly evident: light waves travel from the brain that is being observed to the eye of the physiologist, at which they only arrive after an interval of time, which is finite though short. The physiologist sees what he is observing only after the light waves have reached his eye; therefore the event which constitutes his seeing comes at the end of a series of events which travel from the observed brain into the brain of the physiologist. We cannot, without a preposterous kind of discontinuity, suppose that the physiologist's percept, which comes at the end of this series, is anywhere else but in the physiologist's head.
This question is very important, and must be understood if metaphysics is ever to be got straight. The traditional dualism of mind and matter, which I regard as mistaken, is intimately connected with confusions on this point. So long as we adhere to the conventional notions of mind and matter, we are condemned to a view of perception which is miraculous. We suppose that a physical process starts from a visible object, travels to the eye, there changes into another physical process, causes yet another physical process in the optic nerve, finally produces some effect in the brain, simultaneously with which we see the object from which the process started, the seeing being something "mental", totally different in character from the physical processes which precede and accompany it. This view is so queer that metaphysicians have invented all sorts of theories designed to substitute something less incredible. But nobody notices an elementary confusion.
To return to the physiologist observing another man's brain: what the physiologist sees is by no means identical with what happens in the brain he is observing, but is a somewhat remote effect. From what he sees, therefore, he cannot judge whether what is happening in the brain he is observing is, or is not, the sort of event that he would call "mental". When he says that certain physical events in the brain are accompanied by mental events, he is thinking of physical events as if they were what he sees. He does not see a mental event in the brain he is observing, and therefore, supposes that there is in that brain a physical process that he can observe and a mental process which he cannot. This is a complete mistake. In the strict sense, he cannot observe anything in the other brain, but only the percepts which he himself has when he is suitably related to that brain (eye to microscope, etc.). We first identify physical processes with our percepts, and then, since our percepts are not other people's thoughts, we argue that the physical processes in their brains are something quite different from their thoughts. In fact, everything that we can directly observe of the physical world happens inside our heads, and consists of "mental" events in at least one sense of the word "mental". It also consists of events which form part of the physical world. The development of this point of view will lead us to the conclusion that the distinction between mind and matter is illusory. The stuff of the world may be called physical or mental or both or neither, as we please; in fact, the words serve no purpose. There is only one definition of the words that is unobjectionable: "physical" is what is dealt with by physics, and "mental" is what is dealt with by psychology. When, accordingly, I speak of "physical" space, I mean the space that occurs in physics.
It is extrordinarily difficult to divest ourselves of the belief that the physical world is the world we perceive by sight and touch; even if, in our philosophic moments, we are aware that this is an error, we nevertheless fall into it again as soon as we are off our guard. The notion that what we see is "out there" in physical space is one which cannot survive while we are grasping the difference between what physics supposes to be really happening, and what our senses show us as happening; but it is sure to return and plague us when we begin to forget the argument. Only long reflection can make a radically new point of view familiar and easy.
Our illustrations hitherto have been taken from the sense of sight; let us now take one from the sense of touch. Suppose that, with your eyes shut, you let your finger-tip press against a hard table. What is really happening? The physicist says that your finger-tip and the table consist, roughly speaking, of vast numbers of electrons and protons; more correctly, each electron and proton is to be thought of as a collection of processes of radiation, but we can ignore this for our present purposes. Although you think you are touching the table, no electron or proton in your finger every really touches an electron or proton in the table, because this would develop an infinite force. When you press, repulsions are set up between parts of your finger and parts of the table. If you try to press upon a liquid or a gas, there is room in it for the parts that are repelled to get away. But if you press a hard solid, the electrons and protons that try to get away, because electrical froces from your finger repel them, are unable to do so, because they are crowded close to others which elbow them back to more or less their original position, like people in a dense crowd. Therefore the more you press the more they repel your finger. The repulsion consists of electrical forces, which set up in the nerves a current whose nature is not very definitely known. This current runs into the brain, and there has effects which, so far as the physiologist is concerned, are almost wholly conjectural. But there is one effect which is not conjectural, and that is the sensation of touch. This effect, owing to physiological inference or perhaps to a reflex, is associated by us with the finger-tip. But the sensation is the same if, by artificial means, the parts of the nerve nearer the brain are suitably stimulated - e.g. if your hand has been amputated and the right nerves are skilfully manipulated. Thus our confidence that touch affords evidence of the existence of bodies at the place which we think is being touched is quite misplaced. As a rule we are right, but we can be wrong; there is nothing of the nature of an infallible revelation about the matter. And even in the most favorable case, the perception of touch is something very different from the mad dance of electrons and protons trying to jazz out of each other's way, which is what physics maintains is really taking place at your finger-tip. Or, at least, it seems very different. But as we shall see, the knowledge we derive from physics is so abstract that we are not warranted in saying that what goes on in the physical world is, or is not, intrinsically very different from the events that we know through our own experiences.
-Bertrand Russell (1927) Chapter XIII Physical and Perceptual Space
