(Λεύκιππος), a Grecian philosopher, who is on all hands admitted to have been the founder of the atomic theory of the ancient philosophy. Where and when he was born we have no data for deciding. Miletus, Abdera, and Elis have been assigned as his birth-place; the first, apparently, for no other reason than that it was the birth-place of several natural philosophers; the second, because Democritus, who carried out his theory of atoms, came from that town; Elis, because he was looked upon as a disciple of the Eleatic school. The period when he lived is equally uncertain. He is called the teacher of Democritus (D. L. 9.34), the disciple of Parmenides (Simplic. Phys. fol. 7, a), or, according to other accounts, of Zeno, of Melissus, nay even of Pythagoras (Simplic. l. c ; D. L. 9.30; Tzetz. Chil. 2.930; Iamblich. Vit. Pyth. 104). From the circumstance that Parmenides and Anaxagoras had objected to some doctrines which we find connected with the atomic theory, and from the obscurity that hangs over the personal history and doctrines of Leucippus, Ritter (Geschichte d. Phil. vol. i. book 6.100.2) is inclined to believe that Leucippus lived at a time when intercourse between the learned of the different Grecian states was unfrequent. With regard to his philosophical system it is impossible to speak with precision or certainty, as Aristotle and the other writers who mention him, either speak of him in conjunction with Democritus, or attribute to him doctrines which are in like manner attributed to Democritus. Diogenes Laertius (9.30-33) attempts an exposition of some of his leading doctrines. Some notices will also be found in Aristotle (De Anima, 1.2), Plutarch (De Placitis Phil. 17, p. 883), and Cicero (de Nat. Deor. 1.24). For an account of the general features of the atomic theory, as developed by Democritus, the reader is referred to that article.