Democritus was an influential pre-Socratic philosopher who formulated an atomic theory of the cosmos. He proposed that everything is composed of indivisible atoms moving through empty space, differing in shape and size. Democritus followed in the tradition of Leucippus, who is widely considered the first philosopher to develop the theory of atomism. Democritus believed that knowledge could be achieved through reasoning and inductive thinking to grasp truth beyond subjective sense perceptions.