Leucippus or Leukippos (bright horse ?) (5th century BC) was the originator of atomism, the philosophical belief that everything is composed entirely of various imperishable, indivisible elements called atoms.

There are no existing writings which we can attribute to Leucippus, since his writings seem to have been enfolded into the work of his famous student Democritus (q.v. for more on atomism). In fact, it is virtually impossible to identify any views about which Democritus and Leucippus disagreed.

Leucippus was born at Miletus (or some said Elea, for his philosophy is associated with the Eleatic philosophers), a contemporary of Zeno, Empedocles and Anaxagoras of the Ionian school of philosophy. His fame was so completely overshadowed by that of Democritus, who systematized his views on atoms, that Epicurus doubted his very existence, according to Diogenes Laertius x. 7. However Aristotle and Theophrastus explicitly credit Leucippus with the invention of Atomism.

G. S. Kirk, J. E. Raven, M. Schofield , The Presocratic Philosophers: A Critical History with a Selection of Texts , Cambridge University Press;