Heraclitus

Heraclitus of Ephesus was one of the founders of dialectics. He was nicknamed 'the Obscure' for the complexity and aphoristic nature of his sayings.

Heraclitus' main idea is that everything flows, everything changes (panta rhei). You cannot step into the same river twice, for the waters are always different. He considered fire to be the primordial element — a symbol of eternal motion.

He also developed the doctrine of Logos — the universal law governing all that exists. The unity of opposites is the key to understanding the world.