The materialist theory of knowledge amounts, then, to this statement, that the human organ of cognition radiates no metaphysical light, but is a piece of Nature which pictures other pieces of Nature whose essence is explained when we describe it and bring it in connection with the whole Universe as the one Reality and the real Unity. Such a description demands from the epistemologist or philosopher that he should treat his subject in the same precise way as the animal world is treated by the zoologist. Should I be reproached with not following this precept immediately, I would point to Rome which, too, was not built in one day.