"No man ever steps in the same river twice, for it is not the same river and he is not the same man."
Info & reflection: The metaphor rests on two movements at once, not one: the river changes (the object), but whoever steps in is already someone else too (the subject). It isn't just "everything flows": the observer is not a fixed point from which to measure change. Every encounter, every rereading, every return to a thought is already a meeting between two things that are different from before.
Attribution: the idea is Heraclitus's (fragments B12 and B91), but "and he is not the same man" is a later paraphrase, not his words.