Democritus, presocratic Greek philosopher (born at Abdera in Thrace around 460 B.C.; lived to be very old, but died at an unknown date). Democritus was a student of Leucippus, and co-originator of the belief that everything is made up of various imperishable indivisible elements which he called atoms.
Aristotle tells us that this theory, commonly called atomism, was a reaction to Parmenides, who denied the existence of movement, change, or the void. Parmenides argued that the existence of a thing implied that it could not have "come into being", because "nothing comes from nothing". Moreover, he argued, movement was impossible, because one must move into "the void" and (as he identified "the void" with "nothing") the void does not exist and cannot be "moved into".
Democritus agreed that everything which is must be eternal, but denied that "the void" can be equated with nothing. This makes him the first thinker on record to argue for the existence of an entirely empty "void". In order to explain the change around us from basic, unchangeable substance he argued that there are various basic substances which always existed but can be rearranged into many different forms.
It is impossible to tell which of these ideas where unique to Democritus, and which are attributable to his teacher Leucippus.