(7) But by far the most important of all Aristotle's objections to the ideal theory, and that which, to all intents and purposes, sums up all the others, is that it assumes that Ideas are the essences of things, and yet places those essences outside the things themselves. The essence of a thing must be in it, and not outside it. But Plato separated Ideas from things, and placed the Ideas away somewhere in a mysterious world of their own. The Idea, as the universal, can only exist in the particular. Possibly the reality in all horses is the universal horse, but the universal horse is not something that exists by itself and independently of individual horses. Hence Plato was led into the absurdity of talking as if, besides the individual horses we know, there is somewhere another individual called the horse-in-general, or as if besides white objects there is a thing called {265} whiteness. And this is in fact the supreme self-contradiction of the theory of Ideas, that it begins by saying that the universal is real, and the particular unreal, but ends by degrading the universal again into a particular. This is the same thing as saying that Plato's mistake lay in first (rightly) seeing that existence is not reality, but then (wrongly) going on to imagine that the reality is an existence. Out of this last objection grows Aristotle's own philosophy, the fundamental principle of which is that the universal is indeed the absolute reality, but that it is a universal which exists only in the particular. What is reality? What is substance? This is the first question for the metaphysician. Now substance is what has an independent existence of its own; it is that whose being does not flow into it from any source outside itself. Consequently, substance is what is never a predicate; it is that to which all predicates are applied. Thus in the proposition, "Gold is heavy," gold is the subject, or substance, and "heavy" is its predicate. The heaviness is dependent for its existence on the gold, and it is therefore the latter, and not the former, that is the substance.