The theory that the universe consists of tiny, invisible particles that cannot be destroyed. In Classical Athens (461-429 B.C.E.), the idea of atomism was part of the great philosophical debate over the nature of the matter that makes up the universe. The followers of Pythagoras, a philosopher of the previous century, declared that the essence of matter lies in the abstract concept of numbers, not in its substance. The Pythagoreans believed that, by understanding numbers, they could understand the nature of the universe. Opposed to the Pythagoreans were the materialists, who believed that the universe was essentially solid and material. Among the most radical materialists were the atomists.
The two principal exponents of atomism were Leucippus and Democritus. Both of them believed that the universe consisted of tiny particles that could not be seen or felt but whose existence could be deduced through the exercise of reason. The atomists declared that these tiny atomic particles are the ultimate basis of matter. There are an infinite number of these atoms, according to the atomists, and they are identical, indestructible, and indivisible. The only difference between a man and a rock in the atomist viewpoint is in the number and arrangement of their atoms.