We may examine what goes on between the perceiving person and the thing he perceives from either end; but it is by no means a matter of indifference with which end we begin. If we examine the relation of the perceiver to the perceived it does not seem convenient or accurate to describe that relation as a process. It is an experience, immediate and intuitive; not indeed infallible, but direct and self-sufficient. If I look at the sun, it is the sun I see, and not an image of the sun, nor a sensation which suggests the sun, or symbolises the sun. Still less do I see ethereal vibrations, or a retinal image, or a nervous reaction, or a cerebral disturbance. For, in the act of perceiving, no intermediate entities are themselves perceived. But now if we, as it were, turn round, and, beginning at the other end, consider the relation of the perceived to the perceiver, no similar statements can be made. We find ourselves concerned, not with an act of intuition, but with a physical process, which is complicated, which occupies time, which involves many stages. We have left behind cognition; we are plunged in causation. Experience is no longer the immediate apprehension of fact; it is the transmission of a message conveyed from the object to the percipient by relays of material messengers. As to how the transmission is effected explanations vary with the growth of science. They have been entirely altered more than once since the modern era began, and with each alteration they become more complicated. They depend, not on one branch of science only, but on many. Newtonian astronomy, solar physics, the theory of radiation, the optical properties of the atmosphere, the physiology of vision, the psychology of perception, and I daresay many other branches of research, have to be drawn upon: and all this to tell us what it is we see, and how it is we come to see it. III