SOURCE: “Fifth-Century Atomism: Leucippus and Democritus,” in Philosophy before Socrates: An Introduction with Texts and Commentary, Hackett Publishing Company, Inc., 1994, pp. 303-43.

In the following excerpt, McKirahan explicates passages concerning atomic theory by Democritus and ancient Greek commentators.

The third and most ambitious response to the Eleatic challenge was the atomic theory, invented by Leucippus and developed by Democritus. Leucippus is a shadowy character¹ who we are told was from (a) Miletus, (b) Elea, and (c) Abdera², though these claims could simply reflect the facts that (a) his philosophy was strongly of the Ionian type, (b) he was keenly aware of the Eleatic challenge, and (c) his pupil Democritus was from Abdera. Of his dates we are equally in the dark. Democritus, born c. 460, was his student. It is likely that Leucippus proposed the atomic theory in the decade 440-430. He wrote works called The Great World System...