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 noticed 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 there is in that brain a physical process which 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.