Using scientific reasoning and observation, the Pre-Socratic philosopher Leucippus developed the theory of atomism, which posited that the whole universe is made up of solid atoms constantly moving through void, or empty space. Leucippus' theory of atomism was developed even further by his pupil, Democritus.

What did Leucippus and Democritus discover?

In the 5th century BCE, Leucippus and his pupil Democritus proposed that all matter was composed of small indivisible particles called atoms. Nothing whatsoever is known about Leucippus except that he was the teacher of Democritus. ... These are the main source of information on his teachings about atoms.

When did Leucippus discover the atomic theory?

The first proponents of an atomic theory were the Greek philosophers Leucippus and Democritus who proposed the following model in the fifth century B.C. 1. Matter is composed of atoms separated by empty space through which the atoms move.