Democritus (c.460-c.370 BCE) was an Ancient Greek philosopher, one of the most important of the pre-Socratics. His most important contribution to Western civilization was his atomic theory of the universe. For this he is also known as the Father of Modern Science.
Democritus was born in Thrace (modern-day Bulgaria) and was known to have traveled widely in his life, visiting Egypt and Babylon. He also traveled extensively throughout Greece, learning everything he could about natural philosophy. Unlike Plato and Aristotle, Democritus belonged to the atomist school, which attempted to explain the nature of the universe without resorting to a prime mover, or divine creator.
Democritus taught that everything is made of atoms, which held to be the smallest indivisible particle. Between the atoms is empty space. Atoms are indestructible and always in motion. Democritus also held that the shape of atoms determined the solidity of a material. Water atoms, for example, were smooth and slippery, while iron atoms were held together by hooks.
Democritus also taught that one could not know truth, since perception is subjective. There were two types of knowledge, genuine and secret. Secret knowledge comes through the senses and is therefore insufficient. Genuine knowledge comes through the intellect; it is the reasoned elaboration of secret knowledge gained though the senses.