Leucippus c. 480 BC – c. 420 BC

Leucippus was an ancient Greek pre-Socratic philosopher and the founder, with his pupil Democritus, of the atomist tradition. Almost nothing survives of his biography or writings, and his very existence was doubted by Epicurus, but Aristotle and Theophrastus credit him with first formulating the view that the world consists of indivisible particles, atoms, moving in an empty void. His one preserved book title is The Great World System, and his single attested fragment articulates the principle that nothing happens at random but everything from reason and necessity. The fully developed atomism that descended from him through Democritus, Epicurus, and Lucretius shaped the early modern revival of corpuscular natural philosophy.

Leucippus is dated to the middle and later fifth century BC. Ancient sources connect him with Miletus and with Elea, and Aristotle and the doxographic tradition place him as the founder of atomism, with Democritus as his immediate pupil and successor. So little of his life and writing is secured that Epicurus could later deny that any philosopher named Leucippus had ever existed; modern scholarship treats the figure as historical but admits the near-impossibility of separating his contributions from those of Democritus.

Two works are attributed to him by ancient authorities, the Great World-System and a treatise On Mind, but only a single direct quotation survives — the principle that 'nothing happens in vain, but all from reason and by necessity'. Aristotle reports the basic atomist response to the Eleatic argument: there are infinitely many indivisible bodies of varying shape and size, and there is real void, in which they move and combine to form the phenomenal world.

Leucippus thus stands at the head of the atomist tradition that, through Democritus, Epicurus, Lucretius, and the early modern revival in Gassendi and Galileo, supplied European thought with one of its most durable models of matter. The date of his death is unknown; the tradition places him still active around 420 BC.