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.