  Individual substances, of course, are parts of, or rather, members of a world. In other words, a world is a set of individual substances; or, as Leibniz puts in the opening line of On the Ultimate Origination of Things, a world is a “collection of finite things.” (G VII 302/AG 149) More specifically, Leibniz tells Bourguet, “the universe is only a certain kind of collection of compossibles; and the actual universe is the collection of all possible existents, that is, of those things that form the richest composite.” (G III 573) In saying that a world is a set of compossible things, however, Leibniz is saying that a world is a kind of collection of things that God could bring into existence. For not even God can bring into existence a world in which there is some contradiction among its members or their properties. But this opens up the question: Just what is meant by a contradiction in the case of the members of a world?