But we must go somewhat more into details. We must see how far he applied this principle successfully to the unravelment of the great problems of philosophy. In lecturing upon the Eleatics, I said that any successful philosophy must satisfy at least two conditions. It must give such an account of the Absolute, that the Absolute is shown as capable of explaining the world. It must be possible to deduce the actual world of facts from the first principle. Secondly, not only must this first principle explain the world; it must also explain itself. It must be really ultimate, that is, we must not, in order to understand it, have to refer to anything beyond and outside it. If we have to do so then our ultimate is not an ultimate at all; our first principle {236} is not first. That thing by means of which we explain it must itself be the ultimate reality. And besides being ultimate, our principle must be wholly intelligible. It must not be a mere ultimate mystery; for to reduce the whole world to an ultimate mystery is clearly not to explain it. Our first principle must, in a word, be self-explanatory. Let us apply this two-fold test to Plato's system. Let us see, firstly, whether the principle of Ideas explains the world, and secondly, whether it explains itself. Does it explain the world? Is the actual existence of things, horses, trees, stars, men, explained by it? What, in the first place, is the relation between things and the Ideas? Things, says Plato, are "copies," or "imitations" of the Ideas. They "participate" in the Ideas. The Ideas are "archetypal" of things. Now all these phrases are mere poetic metaphors. They do not really tell us how things are related to Ideas. But suppose we ignore this, and assume, for the sake of argument, that we understand what is meant by "participation" and that things are, in the literal sense, "copies" of Ideas. The question still remains, why do such copies exist, how do they arise? Now, if this problem is to be solved, it is not enough to show, merely as a fact, that, by some mysterious act, copies of Ideas come into existence. There must be a reason for it, and this reason it is the business of philosophy to explain. This reason, too, must exist in the nature of the Ideas themselves, and not outside them. There must be, in the very nature of the Ideas, some inner necessity which forces them to reproduce themselves in things. This is what we {237} mean by saying that the Ideas are a sufficient explanation of the existence of things. But there is in Plato's Ideas no such necessity. The Ideas are defined as being the sole reality. They have already all reality in themselves. They are self-sufficient. They lack nothing. It is not necessary for them further to realize their being in the concrete manifestation of things, because they, as wholly real, need no realization. Why, then, should they not remain for ever simply as they are? Why should they go out of themselves into things? Why should they