Heraclitus c. 535 BC – c. 475 BC

Heraclitus of Ephesus was a Pre-Socratic Greek philosopher of the late sixth and early fifth centuries BC, known in antiquity as the Obscure for the difficulty of his sayings. His one book, conventionally titled On Nature, survives only in roughly 130 fragments quoted by later authors. He is best known for the doctrine that everything flows, that strife and the unity of opposites underlie reality, and that an underlying logos governs the cosmos. His thought decisively influenced Plato, the Stoics, and modern philosophers including Hegel, Nietzsche, and Heidegger. The most famous saying attributed to him, that one cannot step into the same river twice, captures his central preoccupation with change.