It is not, then, surprising that the Academic and Peripatetic schools utterly failed to carry on the great movement inaugurated by their respective founders. The successors of Plato first lost themselves in a labyrinth of Pythagorean mysticism, and then sank into the position of mere moral instructors. The history of that remarkable revolution by which the Academy regained a foremost place in Greek thought, will form the subject of a future chapter: here we may anticipate so far as to observe that it was effected by taking up and presenting in its original purity a tradition of older date than Platonism, though presented under a new aspect and mixed with other elements by Plato. The heirs of Aristotle, after staggering on a few paces under the immense burden of his encyclopaedic bequest, came to a dead halt, and contented themselves with keeping the treasure safe until the time should arrive for its appropriation and reinvestment by a stronger speculative race. No sooner did the two imperial systems lose their ascendency than the germs which they had temporarily overshadowed sprang up into vigorous vitality, and for more than five centuries dominated the whole course not only of Greek but of European thought. Of these by far the most important was the naturalistic idea, the belief that physical science might be substituted for religious superstitions and local conventions as an impregnable basis of conduct. In a former chapter[1] we endeavoured to show that, while there are traces of this idea in the philosophy of Heracleitus, and while its roots stretch far back into the literature and popular faith of Greece, it was formulated for the first time by the two great Sophists, Prodicus and Hippias, who, in the momentous division between Nature and Law, placed themselves—Hippias more particularly—on the side of Nature. Two causes led to the temporary discredit of their teaching. One was the perversion by which natural right became the watchword of those who, like Plato’s Callicles, held that nothing should stand between the strong man and the gratification of his desire for pleasure or for power. The other was the keen criticism of the Humanists, the friends of social convention, who held with Protagoras that Nature was unknowable, or with Gorgias that she did not exist, or with Socrates that her laws were the secret of the gods. It was in particular the overwhelming personal influence of Socrates which triumphed. He drew away from the Sophists their strongest disciple, Antisthenes, and convinced him that philosophy was valuable only in so far as it became a life-renovating power, and that, viewed in this light, it had no relation to anything outside ourselves. But just as Socrates had discarded the physical speculations of former teachers, so also did Antisthenes discard the dialectic which Socrates had substituted for them, even to the extent of denying that definition was possible.[2] Yet he seems to have kept