I mentioned incidentally in the first section that it is conceivable that failure to give adequate consideration to practical judgments may have a compromising effect upon the consideration of other types. I now intend to develop this remark with regard to sense perception as a form of knowledge. The topic is so bound up with a multitude of perplexing psychological and epistemological traditions that I have first to make it reasonably clear what it is and what it is not which I propose to discuss. I endeavored in an earlier series of papers[90] to point out that the question of the _material_ of sense perception is not, as such, a problem of the theory of knowledge at all, but simply a problem of the occurrence of a certain material--a problem of causal conditions and consequences. That is to say, the problem presented by an image[91] of a bent stick, or by a dream, or by "secondary" sensory qualities is properly a problem of physics--of conditions of occurrence, and not of logic, of truth or falsity, fact or fiction. That the existence of a red _quale_ is dependent upon disturbances of a certain velocity of a medium in connection with certain changes of the organism is not to be confused with the notion that red is a way of knowing, in some more or less adequate fashion, some more "real" object or else of knowing itself. The fact of causation--or functional dependence--no more makes the _quale_ an "appearance" to the mind of something more real than itself or of itself than it makes bubbles on the water a real fish transferred by some cognitive distortion into a region of appearance. With a little stretching we may use the term appearance in either case, but the term only means that the red _quale_ or the water-bubble is an _obvious_ or conspicuous thing from which we infer something else not so obvious.