Plato (429-348 B.C.) was the only--one of the disciples of Socrates who represented the whole doctrines of his teacher. We owe to him that the ideas which Socrates awakened have been made the germ of one of the grandest systems of speculation that the world has ever seen, and that it has been conveyed to us in literary compositions which are unequaled in refinement of conception, or in vigor and gracefulness of style. At the age of nineteen he became one of the pupils and associates of Socrates, and did not leave him until that martyr of intellectual freedom drank the fatal cup of hemlock. He afterwards traveled in Asia Minor, in Egypt, in Italy, and Sicily, and made himself acquainted with all contemporary philosophy. During the latter part of his life he was engaged as a public lecturer on philosophy. His lectures were delivered in the gardens of the Academia, and they have left proof of their celebrity in the structure of language, which has derived from them a term now common to all places of instruction. Of the importance of the Socratic and Pythagorean elements in Plato's philosophy there can be no doubt; but he transmuted all he touched into his own forms of thought and language, and there was no branch of speculative literature which he had not mastered. By adopting the form of dialogue, in which all his extant works have come down to us, he was enabled to criticise the various systems of philosophy then current in Greece, and also to gratify his own dramatic genius, and his almost unrivaled power of keeping up an assumed character. The works of Plato have been divided into three classes: first, the elementary dialogues, or those which contain the germs of all that follows, of logic as the instrument of philosophy, and of ideas as its proper object; second, progressive dialogues, which treat of the distinction between philosophical and common knowledge, in their united application to the proposed and real sciences, ethics, and physics; third, the constructive dialogues, in which the practical is completely united with the speculative, with an appendix containing laws, epistles, etc.