Nothing happens at random; everything happens out of reason and by necessity.
This single line, quoted by Aetius from a book called On Mind, is the only text preserved as Leucippus' own. Everything else comes secondhand.
Theophrastus credited him with The Great World-System, though later writers assigned the same book to Democritus, and Epicurus is reported to have denied that any Leucippus ever existed. Aristotle had no such doubt: he names Leucippus first whenever he traces the atomist account of nature, and treats the move as a deliberate answer to Parmenides.
What the testimony agrees on is the shape of the idea. The full and the empty both exist. What is full is divided into shapes too small to see, unlimited in number, carried through the void, colliding and interlocking. Worlds form where enough of them gather, and dissolve again. No purpose directs any of it, and nothing in it is chance.
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