Consider the effects of this omission upon the theory of evolution. We are told that the world-process moves towards an end, and that this end is the self-realization of reason, and that it is proximately attained in man, because man is a reasoning being. So far this is quite intelligible. But this implies that each step in evolution is higher than the last because it approaches nearer to {337} the end of the world-process. And as that end is the realization of reason, this is equivalent to saying that each step is higher than the last because it is more rational. But how is sensation more rational than nutrition? Why should it not be the other way about? Nutrition passes through sensation into human reason. But why should not sensation pass through nutrition into human reason? Why should not the order be reversed? We cannot explain. And such an admission is absolutely fatal to any philosophy of evolution. The whole object of such a philosophy is to make it clear to us why the higher form is higher, and why the lower is lower: why, for example, nutrition must, as lower, come first, and sensation second, and not _vice versa_. If we can see no reason why the order should not be reversed, this simply means that our philosophy of evolution has failed in its main point. It means that we cannot see any real difference between lower and higher, and that therefore we have merely change without development, since it is indifferent whether A passes into B, or B into A. The only way in which Aristotle could have surmounted these difficulties would have been to prove that sensation is a development of reason which goes beyond nutrition. And he could only do this by showing that sensation logically arises out of nutrition. For a logical development is the same as a rational development. He ought to have logically deduced sensation from nutrition, and so with all the other forms. As it is, all that can be said is that Aristotle was the founder of a philosophy of evolution because he saw that evolution implies movement towards an end, and because he attempted to point out the different stages in the attainment of that end, {338} but that he failed rationally to develop the doctrine stage by stage. As neither the principle of form in general was shown to be necessary, nor were the particular forms deduced from each other, we have to conclude that Aristotle like Plato, _named_ a self-explanatory principle, reason or form, as ultimate principle of things, but failed to show in detail that it is self-explanatory. Yet, in spite these defects, the philosophy of Aristotle is one of the greatest philosophies that the world has ever seen, or is ever likely to see. If it does not solve all problems, it does render the world more intelligible to us than it was before. {339} CHAPTER XIV THE GENERAL CHARACTER OF POST-ARISTOTELIAN PHILOSOPHY