?427–?347 bc, Greek philosopher: with his teacher Socrates and his pupil Aristotle, he is regarded as the initiator of western philosophy. His influential theory of ideas, which makes a distinction between objects of sense perception and the universal ideas or forms of which they are an expression, is formulated in such dialogues as Phaedo, Symposium, and The Republic. Other works include The Apology and Laws

a crater in the NW quadrant of the moon, about 100 km in diameter, that has a conspicuous dark floor