gravitation. In that way Kepler's laws were explained, and not only Kepler's laws, but many other astronomical laws and facts. Thus the explanation of the many isolated facts consists in their reduction to the one law, and the explanation of the many laws consists in their reduction to the one more general law. As knowledge advances, the phenomena of the universe come to be explained by fewer and fewer, and wider and wider, general principles. Obviously the ultimate goal would be the explanation of all things by one principle. I do not mean to say that scientific men have this end consciously in view. But the point is that the monistic tendency is there. What is meant by the explanation is the reduction of all things to one principle. {66} In philosophy, in religion, and in science, then, we find this monistic tendency of thought. But it might be asked how we know that this universal tendency is right? How do we know that it is not merely a universal error? Is there no logical or philosophical basis for the belief that the ultimate explanation of things must be one? Now this is a subject which takes us far afield from Greek philosophy. The philosophical basis of monism was never thought out till the time of Spinoza. So we cannot go into it at length here. But, quite shortly, the question is--Is there any reason for believing that the ultimate explanation of things must be one? Now if we are to explain the universe, two conditions must be fulfilled. In the first place, the ultimate reality by which we attempt to explain everything must explain all the other things in the world. It must be possible to deduce the whole world from it. Secondly, the first principle must explain itself. It cannot be a principle which itself still requires explanation by something else. If it is itself not self-explanatory, but is an ultimate mystery, then even if we succeed in deducing the universe from it, nothing is thereby explained. This, for example, is precisely the defect of materialism. Even if we suppose it proved that all things, including mind, arise from matter, yet the objection remains that this explains nothing at all, for matter is not a self-explanatory existence. It is an unintelligible mystery. And to reduce the universe to an ultimate mystery is not to explain it. Again; some people think that the world is to be explained by what they call a "first cause." But why should any cause be the first? Why should we stop anywhere in the chain of causes? Every cause is {67} necessarily the effect of a prior cause. The child, who is told that God made the world, and who inquires who, in that case, made God, is asking a highly sensible question. Or suppose, in tracing back the chain of causes, we come upon one which we have reason to say is really the first, is anything explained thereby? Still we are left with an ultimate mystery. Whatever the principle of explanation is, it cannot be a principle of this kind. It must be a principle which