the fourth book of the Metaphysics, he has to deal with Protagorean scepticism, Aristotle uses, as one reductio ad absurdum, the consideration that, were this Protagorean doctrine true, “There would exist nothing in case beings with souls vanished from the world. For then,” he says, “sense-perceptions would cease.” “That,” he continues, “perceivable objects and sense-percepts would then vanish, is perhaps true, for all this latter existence (i.e. as we should say, the existence of color, odors, etc.) is a state of a sentient being; but that the substrata upon which sense is based, should not persist, even were there no sense-perception, is impossible. For sense-perception is not a perception of itself, but there is some other over and above perception; and this other must necessarily be prior to perception. For what moves, is prior in nature to what is moved. And if one says that these two principles (subject and object, moved and mover) are related to each other, the same result still holds true.” That in all this Aristotle admits interrelation, and recognizes no independence as absolute, is true, but here is one of the central difficulties of Aristotle’s system.
Later Realism only makes this sundering of knowledge and object more express, as scepticism has to be faced, and as the idea of the individual Self gets more sharply contrasted with all ideas of outer things. The Cartesian dualism of extended and of thinking substance derives its extreme character from considerations with which the problem of knowledge has not a little to do. Occasionalism is an instance of the translation of a logical independence of essence into the assertion of a real causal independence.
