Refining and elaborating Leucippus' ideas Democritos advanced the atomic theory, which asserted that the universe is composed of two indivisible elements, called the atom and the void. An infinite number of eternal and uncaused atoms differing from each other only in shape, arrangement, and magnitude, move through infintite space.

In theology Democritos attributed popular belief in the gods to a desire to explain extraordinary things (thunder or earthquakes) to superhuman entities.