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.