You can never step into the same river twice; for new waters are always flowing on to you.

This saying is attributed to Heraclitus of Ephesus (fl. c. 500 BCE), a pre-Socratic Greek philosopher famous for emphasizing flux and the dynamic character of reality. Heraclitus’ work survives only in fragments quoted by later authors; the “river” image is known chiefly through later testimonia rather than a complete original text. In antiquity it became a shorthand for his doctrine that the world is in continual change and that stability is, at best, provisional. The formulation most familiar in English reflects later paraphrase and translation of Greek reports about Heraclitus’ thought, especially those preserved by writers such as Plato and, more explicitly, Simplicius.

The river metaphor expresses the idea that change is not an occasional disturbance of an otherwise stable world but its basic condition. Even if a river keeps its name and appears “the same,” its identity is constituted by ongoing movement: the water is different from moment to moment, and the person stepping in is also altered by time and experience. The quote thus challenges common assumptions about fixed essences and invites attention to processes rather than static things. It also implies a practical lesson: attempts to return to an earlier state—of a relationship, a society, or oneself—misunderstand time’s irreversibility and the continuous becoming that shapes all experience.

“You cannot step twice into the same river.”
“No man ever steps in the same river twice, for it’s not the same river and he’s not the same man.”
“Upon those who step into the same rivers, different and different waters flow.”