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 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.