Democritus (460 - 370) BC (estimate)
Democritus was the Greek philosopher whose work anticipated modern atomic theory. Democritus believed that the universe was made up of atoms, and nothing else.
The Greek word atomon means "indivisible". Democritus taught that atoms were were absolutely small that they cannot broken down to smaller pieces, or compressed. However, Democritus also believed that different atoms also varied in size and shape. Thus, water was made of smooth, round atoms that did not stick to each other, and thus could flow smoothly. While iron was made of jagged atoms, which allows the atoms to latch on to each other, making iron more solid than water.
Sensation, he believed, were caused by the interaction of atoms. For example, the taste "sweet" was caused by large, smooth atoms, and the color "black" is caused by rough atoms.
Since Democritus believed that everything in the universe was composed of atoms moving through the void (kenon), Democritus' philosophy did not leave room for the supernatural. Belief in gods, Democritus said, was triggered by a natural tendency of people to attribute to supernatural causes things that they could not explain.
However, Democritus believed in a highest good, a state of "cheerfulness" which could be attained by living in moderation, free from fear and superstition. Democritus would later be called the "Laughing Philosopher", in contrast to Heraclitus, the Weeping Philosopher.
References:
Democritus, britannica.com
Democritus, encarta.com