Pluto, The Night Planet

Greek philosopher, probably born in Athens, Greece of an aristocratic family. Little is known of his early life, but he was a devoted disciple of Socrates. He travelled widely, then in about 367 BC founded his Academy at Athens, where Aristotle was his most famous pupil. He remained there for the rest of his life, apart from visits to Syracuse, where he was involved in political experiments. His 30 or more dialogues are conventionally divided into three periods.

The early dialogues have Socrates as the principal character engaged in ironic and inconclusive interrogations about the definition of different moral virtues (piety in the Euthyphro, courage in the Laches, and so on). In the middle, highly literary dialogues, such as the Symposium, Gorgias, Phaedo, and Republic, he increasingly develops his own positive doctrines, such as the theory of knowledge as recollection, the immortality of the soul, the tripartite division of the soul, and above all the theory of forms which contrasts the transient, material world of particulars (objects merely of perception, opinion, and belief) with the timeless, unchanging world of universals or forms (the true objects of knowledge). The Republic also describes Plato's celebrated political utopia, ruled by philosopher-kings who have mastered the discipline of dialectic. The third group of later dialogues (including the Parmenides, Theaetetus, and Sophist) represents a series of highly sophisticated criticisms of the metaphysical and logical assumptions of his middle period, and contain some of his most demanding and original work.