Anaxagoras of Clazomenæ, following another path, pronounced that the really efficient agency in the universe is Mind. For this utterance he has been justly eulogized by the metaphysicians of all succeeding centuries. It was, in fact, the starting-point of what in German phraseology is called _Begriffsphilosophie_. Anaxagoras insisted on a point which had been neglected by his contemporaries--the form-giving activity of mind, as known to us immediately in the human reason--and asserted the impossibility of leaving this out of the account of the universe. But, as Socrates complained, he stopped here, and diverged into material explanations, talking about attraction and repulsion and homogeneous particles, without attempting to connect them with the action of his #Nous#. Democritus of Abdera, a little later in time than the thinkers who have hitherto been mentioned, was so attracted by the indefinite divisibility of matter that he explained the universe by the theory of a void in which an infinity of atoms moved and met in varied combination. It is well known that this hypothesis, the parent of the Epicurean and the Lucretian systems, has been the main-stay of materialism in all ages, and that it has lately been received into favor by some of the most advanced physicists. Yet it must not be imagined that the Atomism of Democritus was in any true sense scientific according to our acceptation of the term. Like the Infinite of Anaximander, the Mind of Anaxagoras, the Numbers of Pythagoras, the Fire of Heraclitus, his Plenum and Vacuum was a conjectural hypothesis founded upon no experiment or observation properly so called. All these early systems were freaks of fancy, shrewd guesses, poetic thoughts, in which abstractions from language, elementary refinements upon mythology, together with crude speculations about natural objects, were made the groundwork of dogmatism. At the same time thought at this period was both active and creative; nearly all the permanent problems which occur to human ignorance--the antitheses of a beginning and an ending, of being and not being, of rest and motion, of the continuous and the discrete, of the one and the many--the criterion of knowledge and opinion, the antagonism of the senses and the reason, the relation of the vital principle to inanimate existence--were posed in the course of animated controversy. Logic had not been formulated as a method. Philosophical terminology had not as yet been settled. But the logical faculty was working in full vigor, and language was being made to yield abstractions hitherto unapprehended.