Leucippus (fl. c.440 B.C.), Greek pre-Socratic philosopher credited with founding atomism, expounded in a work titled The Great World-system. Positing the existence of atoms and the void, he answered Eleatic arguments against change by allowing change of place. The arrangements and rearrangements of groups of atoms could account for macroscopic changes in the world, and indeed for the world itself. Little else is known of Leucippus. It is difficult to distinguish his contributions from those of his prolific follower Democritus. See also ANCIENT ATOMISM. D.W.G.