LetheIn ancient Greek mythology, one of the several rivers of Hades. Drinking from the river Lethe ("(river of) forgetfulness" or "oblivion") caused complete forgetfulness. Some ancient Greeks believed that souls were made to drink from the river before being reincarnated, so they would not remember their past lives.
Some gnostic groups taught the existence of another river, the Mnemosyne; those who drank from the Mnemosyne would remember everything and attain omniscience. Initiates were taught that they would receive a choice of rivers to drink from after death, and to drink from Mnemosyne instead of Lethe.