When the light from a fixed star reaches me, I see the star if it is night and I am looking in the right direction. The light started years ago, probably many years ago, but my reaction is primarily to something that is happening now. When my eyes are open, I see the star; when they are shut, I do not. Children discover at a fairly early age that they see nothing when their eyes are shut. They are aware of the difference between seeing and not seeing, and also of the difference between eyes open and eyes shut; gradually they discover that these two differences are correlated—I mean that they have expectations of which this is the intellectualist transcription. Again, children learn to name the colours, and to state correctly whether a thing is blue or red or yellow or what-not. They ought not to be sure that light of the appropriate wave-length started from the object. The sun looks red in a London fog, grass looks blue through blue spectacles, everything looks yellow to a person suffering from jaundice. But suppose you ask: What colour are you seeing? The person who answers, in these cases, red for the sun, blue for the grass, and yellow for the sick-room of the jaundiced patient, is answering quite truly. And in each of these cases he is stating something that he knows. What he knows in such cases is what I call a “percept”. I shall contend later that, from the standpoint of physics, a percept is in the brain; for the present, I am only concerned to say that a percept is what is most indubitable in our knowledge of the world.
To behaviourism as a metaphysic one may put the following dilemma. Either physics is valid in its main lines, or it is not. If it is not, we know nothing about the movements of matter; for physics is the result of the most serious and careful study of which the human intelligence has hitherto been capable. If, on the other hand, physics is valid in its main lines, any physical process starting either inside or outside the body will, if it reaches the brain, be different if the intervening medium is different; moreover two persons, initially very different, may become indistinguishable as they spread and grow fainter. On both grounds, what happens in the brain is not connected quite accurately with what happens elsewhere, and our perceptions are therefore infected with subjectivity on purely physical grounds. Even, therefore, when we assume the truth of physics, what we know most indubitably through perception is not the movements of matter, but certain events in ourselves which are connected, in a manner not quite invariable, with the movements of matter. To be specific, when Dr. Watson watches rats in mazes, what he knows, apart from difficult inferences, are certain events in himself. The behaviour of the rats can only be inferred by the help of physics, and is by no means to be accepted as something accurately knowable by direct observation.
I do not in fact entertain any doubts that physics is true in its main lines. The interpretation of physical formulæ is a matter as to which a considerable degree of uncertainty is possible; but we cannot well doubt that there is an interpretation which is true roughly and in the main. I shall come to the question of interpretation later; for the present, I shall assume that we may accept physics in its broad outlines, without troubling to consider how it is to be interpreted. On this basis, the above remarks on perception seem undeniable. We are often misled as to what is happening, either by peculiarities of the medium between the object and our bodies, or by unusual states of our bodies, or by a temporary or permanent abnormality in the brain. But in all these cases something is really happening, as to which, if we turn our attention to it, we can obtain knowledge that is not misleading. At one time when, owing to illness, I had been taking a great deal of quinine, I became hypersensitive to noise, so that when the nurse rustled the newspaper I thought she was spilling a scuttle of coals on the floor. The interpretation was mistaken, but it was quite true that I heard a loud noise. It is a commonplace that a man whose leg has been amputated can still feel pains in it; here again, he does really feel the pains, and is only mistaken in his belief that they come from his leg. A percept is an observable event, but its interpretation as knowledge of this or that event in the physical world is liable to be mistaken, for reasons which physics and physiology can make fairly clear.
The subjectivity of percepts is a matter of degree. They are more subjective when people are drunk or asleep than when they are sober and awake. They are more subjective in regard to distant objects than in regard to such as are near. They may acquire various peculiar kinds of subjectivity through injuries to the brain or to the nerves. When I speak of a percept as “subjective” I mean that the physiological inferences to which it gives rise are mistaken or vague. This is always the case to some extent, but much more so in some circumstances than in others. And the sort of defect that leads to mistakes must be distinguished from the sort that leads to vagueness. If you see a man a quarter of a mile away, you can see that it is a man if you have normal eyesight, but you probably cannot tell who it is, even if in fact it is some one you know well. This is vagueness in the percept: the inferences you draw are correct so far as they go, but they do not go very far. On the other hand, if you are seeing double and think there are two men, you have a case of mistake. Vagueness, to a greater or less extent, is universal and inevitable; mistakes, on the other hand, can usually be avoided by taking trouble and by not always trusting to physiological inference. Anybody can see double on purpose, by focussing on a distant object and noticing a near one; but this will not cause mistakes, since the man is aware of the subjective element in his double vision. Similarly we are not deceived by after-images, and only dogs are deceived by gramophones.
From what has been said in this chapter, it is clear that our knowledge of the physical world, if it is to be made as reliable as possible, must start from percepts, and must scrutinize the physiological inferences by which percepts are accompanied. Physiological inference is inference in the sense that it sometimes leads to error and physics gives reason to expect that percepts will, in certain circumstances, be more or less deceptive if taken as signs of something outside the brain. It is these facts that give a subjective cast to the philosophy of physics, at any rate in its beginnings. We cannot start cheerfully with a world of matter in motion, as to which any two sane and sober observers must agree. To some extent, each man dreams his own dream, and the disentangling of the dream element in our percepts is no easy matter. This is, indeed, the work that scientific physics undertakes to do.
[CHAPTER XIII]
PHYSICAL AND PERCEPTUAL SPACE
Perhaps there is nothing so difficult for the imagination as to teach it to feel about space as modern science compels us to think. This is the task which must be attempted in the present chapter.
We said in [Chapter XII] 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, which we spoke about at the end of last chapter, 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 different 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 everyday 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 persons 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.