You cannot step in the same river twice.
Heraclitus of Ephesus, also known as the Weeping Philosopher and Heraclitus the Obscure, has left us only a few philosophical sentences. Due to this lack of original writing, Heraclitus’ philosophy remains hard to characterize. His belief seems to have been that the universe is in a constant state of flux, as this famous quote indicates. By the time that you attempt to step into the river a second time, the waters of the river will have moved on and so, the river will not be the same one you stepped into the first time. The sentence also has a second meaning; you cannot step into the same river again because you are no longer the same as the person who took the first step. The question of how identity is preserved over time is one which still animates philosophers today.