We have seen how the idea of Nature, first evolved by physical philosophy, was taken by some, at least, among the Sophists as a basis for their ethical teaching; then how an interpretation utterly opposed to theirs was put on it by practical men, and how this second interpretation was so generalised by the younger rhetoricians as to involve the denial of all morality whatever. Meanwhile, another equally important conception, destined to come into speedy and prolonged antagonism with the idea of Nature, and like it to exercise a powerful influence on ethical reflection, had almost contemporaneously been elaborated out of the materials which earlier speculation supplied. From Parmenides and Heracleitus down, every philosopher who had propounded a theory of the world, had also more or less peremptorily insisted on the fact that his theory differed widely from common belief. Those who held that change is impossible, and those who taught that everything is incessantly changing; those who asserted the indestructibility of matter, and those who denied its continuity; those who took away objective reality from every quality except extension and resistance, and those who affirmed that the smallest molecules partook more or less of every attribute that is revealed to sense—all these, however much they might disagree among themselves, agreed in declaring that the received opinions of mankind were an utter delusion. Thus, a sharp distinction came to be drawn between the misleading sense-impressions and the objective reality to which thought alone could penetrate. It was by combining these two elements, sensation and thought, that the idea of mind was originally constituted. And mind when so understood could not well be accounted for by any of the materialistic hypotheses at first proposed. The senses must differ profoundly from that of which they give such an unfaithful report; while reason, which Anaxagoras had so carefully differentiated from every other form of existence, carried back its distinction to the subjective sphere, and became clothed with a new spirituality when reintegrated in the consciousness of man. The first result of this separation between man and the world was a complete breach with the old physical philosophy, shown, on the one hand, by an abandonment of speculative studies, on the other, by a substitution of convention for Nature as the recognised standard of right. Both consequences were drawn by Protagoras, the most eminent of the Sophists. We have now to consider more particularly what was his part in the great drama of which we are attempting to give an intelligible account.