Heraclitus, (active around 500 BC)

Heraclitus of Ephesus (c. 535 – c. 475 BC) was a pre-Socratic Greek philosopher. He was a native of the city of Ephesus, then part of the Persian Empire. He was of distinguished parentage. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom.

Heraclitus is best known for his doctrines that things are constantly changing (universal flux), that opposites coincide (unity of opposites), and that fire is the basic material of the world. The exact interpretation of these doctrines is controversial, as is the inference often drawn from this theory that in the world as Heraclitus conceives it contradictory propositions must be true.

Heraclitus famous saying, "No man ever steps in the same river twice."  I used to illustrate his notion of flux (because the river is flowing a man never steps in the same water.)  He believed in the unity of opposites, stating that "the path up and down are one and the same", all existing entities being characterized by pairs of contrary properties. His cryptic utterance that "all entities come to be in accordance with this Logos" (literally, "word", "reason", or "account") has been the subject of numerous interpretations.

http://plato.stanford.edu/entries/heraclitus/ 3/4/12

http://en.wikipedia.org/wiki/Heraclitus 3/4/12