“In Greek mythology, Lethe (Greek: Λήθη, Lḗthē; Classical Greek [lɛː́tʰɛː]) was one of the five rivers of Hades. Also known as the Ameles potamos (river of un-mindfulness), the Lethe flowed through the Underworld, where all those who drank from it experienced complete forgetfulness. In Classical Greek, the word Lethe literally means “oblivion”, “forgetfulness”, or “concealment”.
It is related to the Greek word for “truth”, aletheia (ἀλήθεια), which through the privative alpha literally means “un-forgetfulness” or “un-concealment”.
Some ancient Greeks believed that souls were made to drink from the river before being reincarnated, so they would not remember their past lives. A few taught the existence of another river, the Mnemosyne; those who drank from the Mnemosyne would remember everything and attain omniscience.”