In Plato’s Myth of Er, the river that causes souls to forget their previous lives before reincarnation is the River Lethe.
The Myth of Er appears at the end of Plato’s Republic. Er, a warrior, is said to have died in battle and returned to life after witnessing the judgment of souls. The story describes souls receiving rewards or punishments, choosing new lives, and then drinking from the River Lethe before being born again.
Lethe means forgetfulness in Greek. The river’s waters erase the souls’ memories of their earlier existence, although the myth makes an exception for Er, who is told to remember what he saw and report it to the living. Plato uses the story to explore justice, moral choice, and responsibility.
The River Lethe should not be confused with the River Styx, which is a different river in Greek mythology and is associated with oaths and the underworld. The Myth of Er is a philosophical narrative, not a uniform description shared by every ancient Greek tradition.