The fifth century before Christ was, in Greece, a time of intense intellectual ferment. One is reminded, in reading of it, of the splendid years of the Renaissance in Italy, of the awakening of the human mind to a vigorous life which cast off the bonds of tradition and insisted upon the right of free and unfettered development. Athens was the center of this intellectual activity. In this century arose the Sophists, public teachers who busied themselves with all departments of human knowledge, but seemed to lay no little emphasis upon certain questions that touched very nearly the life of man. Can man attain to truth at all--to a truth that is more than a mere truth to him, a seeming truth? Whence do the laws derive their authority? Is there such a thing as justice, as right? It was with such questions as these that the Sophists occupied themselves, and such questions as these have held the attention of mankind ever since. When they make their appearance in the life of a people or of an individual man, it means that there has been a rebirth, a birth into the life of reflection. When Socrates, that greatest of teachers, felt called upon to refute the arguments of these men, he met them, so to speak, on their own ground, recognizing that the subjects of which they discoursed were, indeed, matter for scientific investigation. His attitude seemed to many conservative persons in his day a dangerous one; he was regarded as an innovator; he taught men to think and to raise questions where, before, the traditions of the fathers had seemed a sufficient guide to men's actions. And, indeed, he could not do otherwise. Men had learned to reflect, and there had come into existence at least the beginnings of what we now sometimes rather loosely call the mental and moral sciences. In the works of Socrates' disciple Plato (428-347 B.C.) and in those of Plato's disciple Aristotle (384-322 B.C.), abundant justice is done to these fields of human activity. These two, the greatest among the Greek philosophers, differ from each other in many things, but it is worthy of remark that they both seem to regard the whole sphere of human knowledge as their province. Plato is much more interested in the moral sciences than in the physical, but he, nevertheless, feels called upon to give an account of how the world was made and out of what sort of elements. He evidently does not take his own account very seriously, and recognizes that he is on uncertain ground. But he does not consider the matter beyond his jurisdiction. As for Aristotle, that wonderful man seems to have found it possible to represent worthily every science known to his time, and to have marked out several new fields for his successors to cultivate. His philosophy covers physics, cosmology, zooelogy, logic, metaphysics, ethics, psychology, politics and economics, rhetoric and poetics.