Hippolytus of Rome, The Refutation of All Heresies I.11: Leucippus; His Atomic Theory

But Leucippus, an associate of Zeno, did not maintain the same opinion, but affirms things to be infinite, and always in motion, and that generation and change exist continuously. And he affirms plenitude and vacuum to be elements. And he asserts that worlds are produced when many bodies are congregated and flow together from the surrounding space to a common point, so that by mutual contact they made substances of the same figure and similar in form come into connection; and when thus intertwined, there are transmutations into other bodies, and that created things wax and wane through necessity. But what the nature of necessity is, (Parmenides) did not define.