“Modern physics and physiology throw a new light upon the ancient problem of perception. If there is to be anything that can be called “perception,” it must be in some degree an effect of the object perceived, and it must more or less resemble the object if it is to be a source of knowledge of the object. . . . Light-waves travel from the sun to the earth, and in doing so obey their own laws. . . . When they reach our atmosphere, they suffer refraction, and some are more scattered than others. When they reach a human eye, all sorts of things happen which would not happen elsewhere, ending up with what we call “seeing the sun.” But although the sun of our visual experience is very different from the sun of the astronomer, it is still a source of knowledge as to the latter, because “seeing the sun” differs from “seeing the moon” in ways that are causally connected with the difference between the astronomer’s sun and the astronomer’s moon.”

—Bertrand Russell, A History of Western Philosophy, 1945. From the chapter entitled The Philosophy of Logical Analysis.