Heraclitus

Heraclitus wrote in deliberately obscure aphorisms — so much so that the ancients nicknamed him "the Riddler" and "the Weeping Philosopher." Behind the riddles is one of the most influential ideas in philosophy: reality is not stuff, it is process. Everything flows. Stability is the temporary shape that change happens to wear.

The river fragment is the most famous. The river is the same river only because its waters are different. Identity is dynamic. Opposites — hot and cold, day and night, war and peace — are not enemies but the necessary tension that holds the world together. He calls this principle the logos: the rational pattern by which the world hangs together.

Plato will spend his life refusing Heraclitus. Hegel will spend his life agreeing with him. Twentieth-century process philosophers still cite him.