Anaxagoras owes his importance in the history of philosophy to the theory of the Nous. This was the first time that a definite distinction had been made between the corporeal and incorporeal. Anaxagoras is the last philosopher of the first period of Greek philosophy. In the second chapter, [Footnote 9] I observed that this first period is characterized by the fact that in it the Greek mind looks only outward upon the external world. It attempts to explain the operations of nature. It had not yet learned to look inward upon itself. But the transition to the introspective study of mind is found in the Nous of {104} Anaxagoras. Mind is now brought to the fore as a problem for philosophy. To find reason, intelligence, mind, in all things, in the State, in the individual, in external nature, this is the characteristic of the second period of Greek philosophy. To have formulated the antithesis between mind and matter is the most important work of Anaxagoras. [Footnote 9: Pages 23-4.] Secondly, it is to the credit of Anaxagoras that he was the first to introduce the idea of teleology into philosophy. The system of the Atomists formed the logical completion of the mechanical theory of the world. The theory of mechanism seeks to explain all things by causes. But, as we saw, causation can explain nothing. The mechanism of the world shows us by what means events are brought about, but it does not explain why they are brought about at all. That can only be explained by showing the reason for things, by exhibiting all process as a means towards rational ends. To look to the beginning (cause) of things for their explanation is the theory of mechanism. To look to their ends for explanation of them is teleology. Anaxagoras was the first to have dimly seen this. And for this reason Aristotle praises him, and, contrasting him with the mechanists, Leucippus and Democritus, says that he appears like "a sober man among vain babblers." The new principle which he thus introduced into philosophy was developed, and formed the central idea of Plato and Aristotle. To have realized the twin antitheses of matter and mind, of mechanism and teleology, is the glory of Anaxagoras. But it is just here, in the development of these two ideas, that the defects of his system make their appearance. Firstly, he so separated matter and mind that {105} his philosophy ends in sheer dualism. He assumes the Nous and matter as existing from the beginning, side by side, as equally ultimate and underived principles. A monistic materialism would have derived the Nous from matter, and a monistic idealism would have derived matter from the Nous. But Anaxagoras does neither. Each is left, in his theory, an inexplicable ultimate mystery. His philosophy is, therefore, an irreconcilable dualism.