=The Divisions of Plato’s Philosophy.= Plato himself had no clear conception of an exact division of science, and did not confine himself in a single dialogue to a single science. Aristotle, however, distinguished in the philosophy of his master dialectic, ethics, and physics, and these divisions of Plato’s teaching have been traditionally adopted. The dialectic, as commonly used in his time, meant “the dialogue or conversation employed as a means of scientific investigation.” It was transformed by Plato to mean not logical but metaphysical discussion. Plato was concerned with the laws of Being rather than the laws of logic, and, as Being to him consisted of Ideas, his dialectic interest was to reduce experience by division and induction to some unity. Plato’s dialectic was not logical but methodological,――logical operations taken as a whole,――by means of which the Ideas and their relations to one another were to be found. The physics of Plato is of little value. It was an afterthought to satisfy the demands of his school. The world of nature phenomena could never be for Plato the object of true knowledge. Unfortunately, the teleological physics of Plato was regarded by the Hellenistic time and the Middle Ages as Plato’s most important achievement. Plato wrote entirely in the spirit of the Enlightenment, and his works show a great interest in man as a moral being, but little interest in physical nature. =Summary of Plato’s Doctrine.= The interpretation of Plato as set forth in what follows may be thus summarized: Plato began with the conceptual form of idealism, suggested by the logical method of Socrates, with the purpose of solving logical and ethical problems. He advanced to a teleological idealism, conditioned by the doctrines of Anaxagoras and the Pythagoreans, with the purpose of applying his doctrine to physical problems. =The Formation of Plato’s Metaphysics.= In his earliest period Plato made these very clear statements: (1) virtue is knowledge; (2) by knowledge is not meant sense-perceptions. In his final statement of his philosophy, as he bequeathed it to posterity, he only gave a new evaluation of these two early principles, although he expressed them in a highly complex form. “Virtue is knowledge” is the basis of agreement between Socrates and the Sophists; and “by knowledge is not meant sense-perceptions” is the basis of their opposition. During Plato’s early period he was acting as a faithful transcriber of Socrates in the presentation of this first principle: virtue is knowledge, is teachable, is one. During Plato’s second period he was called on to defend the second statement against the Sophists. Plato’s formation of his own theory begins at this point,――at the point where his defense of his master was keenest. From this time, for a full half-century, Plato developed the Socratic principles in a theory that went far beyond Socrates, but that was never untrue to him.