Secondly, his teleology turns out in the end to be only a new theory of mechanism. The only reason which induces him to introduce the Nous into the world, is because he cannot otherwise explain the origin of movement. It is only the first movement of things, the formation of the vortex, which he explains by mind. All subsequent process is explained by the action of the vortex itself, which draws the surrounding matter into itself. The Nous is thus nothing but another piece of mechanism to account for the first impulse to motion. He regards the Nous simply as a first cause, and thus the characteristic of all mechanism, to look back to first causes, to the beginning, rather than to the end of things for their explanation, appears here. Aristotle, as usual, puts the matter in a nutshell. "Anaxagoras," he says, "uses mind as a _deus ex machina_ to account for the formation of the world, and whenever he is at a loss to explain why anything necessarily is, he drags it in by force. But in other cases he assigns as a cause for things anything else in preference to mind." [Footnote 10] [Footnote 10; Aristotle, _Metaphysics_, book i, chap. iv.] {106} CHAPTER IX THE SOPHISTS The first period of Greek philosophy closes with Anaxagoras. His doctrine of the world-forming intelligence introduced a new principle into philosophy, the principle of the antithesis between corporeal matter and incorporeal mind, and therefore, by implication, the antithesis between nature and man. And if the first period of philosophy has for its problem the origin of the world, and the explanation of the being and becoming of nature, the second period of philosophy opens, in the Sophists, with the problem of the position of man in the universe. The teaching of the earlier philosophers was exclusively cosmological, that of the Sophists exclusively humanistic. Later in this second period, these two modes of thought come together and fructify one another. The problem of the mind and the problem of nature are subordinated as factors of the great, universal, all-embracing, world-systems of Plato and Aristotle.