What does it mean?
The water in a river is always moving, so it is never the same river – and you are never the same person either. Everything is always changing.
Who said it?
Heraclitus: Ancient Greek philosopher (c. 535–475 BC).
These exact words are widely credited to Heraclitus, but they don’t appear in their known work. They may be a later paraphrase or a misattribution.