Every possible idea therefore may be said to be used existentially, for every possible idea qualifies and is true of a real world. And the number of real worlds, in a word, is indefinite. Every idea therefore in a sense is true, and is true of reality. The question with every idea is how far and in what sense is it true. The question is always whether, qualifying reality in one sense, the idea qualifies reality in another sense also. For, true in one world, an idea may be false in another world.