The advent of man with his faculty of Discourse may be regarded as marking another distinct stage in the evolutionary movement--a stage, moreover, the operations of which throw light upon the whole nature of cerebral representations. The faculty of rational Discourse, as Max Müller pointed out, is denominated in Greek by the word λόγος, applicable at once to the mental activity and to its appropriate expression in speech. Discourse is an instrument by means of which man has been enabled to construct his whole system of representations of the world in which he lives, the system of what is commonly called his Knowledge. Human Knowledge just is the body of man's representations of his Experience in the world of which he forms a part. It is not necessary to insist here on the gradual but remarkable growth and extension which Human Knowledge has undergone during the last two thousand years. Concurrently with its extension man's ability to control the forces of Nature has been enlarged and increased. At the same time, however, that extension has rendered possible false developments and aberrations to which the more limited representations of the brute are less liable. With the faculty of rational Discourse constantly striving to extend the bounds of Knowledge, man came in time to attempt to give an account not only of the immediate objects which surround him, but of the whole choir of Heaven and furniture of Earth. In this advance the Greeks took a leading part. When we first make acquaintance through historical records with the intellectual activity of the Greek mind, we find it engaged in the construction of various such schemes for an explanation of the world--usually called cosmogonies. It was at this stage of intellectual progress that what we might call an interruption occurred in the normal process of evolution. Great intellectual activity had for some time prevailed in the Greek communities; several men of conspicuous genius--notably Heracleitus and Parmenides--had carried speculation as to the origin and nature of the world to a height hitherto undreamt of. These achievements and the consciousness of continual progress had engendered in Athens particularly what might be called an epidemic of intellectual pride.