We find that the field of science has been surveyed from two different standpoints. Men of science have set out, on the one hand, to describe the world as it would be with man left out. The result is what we call physical science. The world of physics is colourless, toneless, neither cold nor warm; its spaces are always of the same extent, its times are always of the same duration, its mass is invariable; it would be just what it is now if mankind were swept from the face of the earth. For what is light in the text-books of physics?—a train of electromagnetic waves; and sound is a vibratory motion of air or water; and heat is a dance of molecules; and all these things are independent of man. But men of science have tried, on the other hand, to describe =the world as it is in man’s experience=, as it appears with man left in; and the result of this endeavour is psychology. The world of psychology contains looks and tones and feels; it is the world of dark and light, of noise and silence, of rough and smooth; its space is sometimes large and sometimes small, as everyone knows who in adult life has gone back to his childhood’s home; its time is sometimes short and sometimes long; it has no invariables. It contains also the thoughts, emotions, memories, imaginations, volitions that you naturally ascribe to mind; it contains, that is, so much of these things as belongs to the sphere of observable fact. It is obviously very different from the world of physics, though both worlds alike have been opened up to us by science, by the impersonal and disinterested search for facts. So we have a world of matter and a world of mind. The physicist, however, describes and measures the various phases of energy, without assuming any material substance in the background, any matter of which this energy is the manifestation. Matter, if the word is to be used at all, is simply the inclusive name for all the forms of energy. And the psychologist, in the same way, describes and measures—so far as he is able to measure—the phenomena of his world, without assuming any active or perduring mind in the background; for him, mind is simply the =inclusive name of all these phenomena=. That is the first rough answer to our question. Much more must be said, if the answer is to be precise; but even as it is we have travelled a long way from the little man living inside the head!