Ancient Greek Philosophy.

Empedocles believed that all matter was composed of various combinations and arrangements of four elements -- fire, air, water, and earth -- and that the forces of Love and Strife acted on those elements and caused them to mingle, separate, and recombine.

Thales of Miletus believed that all substances come from water and will eventually revert to water. He also made important advances in the field of mathematics and was a careful observer of the heavens.

Democritus suggested that all substance in the universe was made up of particles so minute that nothing smaller was possible. He regarded those particles (atoms) as unchangeable and indestructible, and as the only content of the universe besides the very space in which they existed.

Aristotle founded the Athenian school called the Lyceum. He was the first philosopher to analyze the process whereby certain propositions can be logically inferred to be true from the fact that certain other propositions are true -- all men are mortal; Socrates is a man; therefore, Socrates is mortal.