The pre-Socratic Leucippus taught or was an associate of Democritus. Democritus is thought to have also known the philosopher Anaxagoras.
Atomists believed that there were tiny indivisible particles that moved within the infinite void of space.
They were trying to explain how there could be change without accepting the difficult idea that something could come into existence out of nothing. The indivisible particles (atoms) posited by atomists could be reformed into anything.
Besides a theory of nature, Democritus was also responsible for an ethical theory, although it is difficult to tease out what he said from what others wrote, since none of his writing remains.
Source: Berryman, Sylvia, "Democritus", The Stanford Encyclopedia of Philosophy (Fall 2004 Edition), Edward N. Zalta (ed.)
