Leucippus
From Philosopedia
Leucippus (also Leukippos) (5th Century B.C.E.)
Epicurus declared that Leucippus (Leukippos) was an imaginary person, but others say he was one of Protagoras’s and also Democritus's teachers and was a contemporary of Anaxagoras, Empedocles, and Zeno.
According to Diogenes Laertius, he was a student of Parmenides's follower Zeno - Zeno is best known for paradoxes suggesting that motion is impossible because a magnitude can be divided into an infinite number of parts, each of which must be traversed; the fact that atomism is thought to have been formulated in response to these arguments may account for the story that Leucippus was a student of Zeno.
Aristotle believed it was Leucippus who originated the atomistic theory, although Democritus usually is credited.
At any rate, Leucippus of Miletus is said to have demonstrated that matter is composed of tiny, indivisible properties in constant motion, an idea later developed by Lucretius.
Two of his fragmentary works are believed by some to be The Great World-system and On Mind, although the latter may have been by Democritus.
