Thus far the material process of transmission. The psychical process has still to come. Psychology is a science, not less than physiology or physics; and psychology has much to say on the subject of perception. It is true that scientific explorers whose point of departure is introspective; who concern themselves primarily with ideas, conceptions, sensations, and so forth, rarely succeed in fitting their conclusions without a break to those of their colleagues who begin with the “external” causes of perception. The two tunnels, driven from opposite sides of the mountain, do not always meet under its crest. Still, we cannot on that account ignore the teaching of psychology on the genesis of perceptual experience regarded, not as the ground of knowledge, but as a natural product. I do not mean to attempt a summary of psychology from this point of view, any more than I have attempted a summary of physics or physiology. My argument is really independent, in this case as in the other, of particular systems. All I ask for is the admission that in perception there are conditions antecedently supplied by the perceiving consciousness which profoundly modify every perceptual experience—and that these conditions (unlike Kant’s forms) are natural growths, varying, like other natural growths, from individual to individual. This admission must, I think, be made by every empirical psychologist, to whatever school he happens to belong. If this statement seems obscure in its general and abstract form, consider a particular application of it. Let us assume, with many psychologists, that Will, in the form of selective attention, lies at the root of our perceptual activities; that we may therefore be said, in a sense, voluntarily to create the objects we perceive; that experience of the present is largely qualified by memories of the past, and that the perceptual mould into which our sensations are run is largely a social product—born of the intercourse between human beings, and, in its turn, rendering that intercourse possible. Is it not clear that, on assumptions like these, consciousness, so far from passively receiving the messages conveyed to it through physical and physiological channels, actively modifies their character? V But why, it may be asked, should these considerations involve any difficulty? And, if there be a difficulty, what is its exact character?