A river in Hades whose waters caused forgetfulness. It was on the banks of another Underworld river called the Styx that the shades, or ghostly remains, of the dead congregated to seek passage to the Afterlife.
Unless they bribed Charon to ferry them across the stream, they wandered aimlessly on the near bank forever. But those who made it across the Styx did not have much more to anticipate. Once they had drunk from the waters of Lethe, they were left with nothing to reminisce about for eternity. The Lethe had therefore been dubbed the stream of oblivion; the others were the Styx, Akheron, Pyriphlegethon and Kokytos.
‘He [Aithalides, son of Hermes, gifted with unfailing memory] has long since been lost in the inexorable waters of the Akheron, yet even so, Lethe (Forgetfulness) has not overwhelmed his soul [ie unlike the other dead he remembers his past lives and retains his memory in the underworld].’
– Apollonius Rhodius, Argonautica 1. 642 ff (trans. Rieu) (C3rd C.E.)
In modern philosophy, Martin Heidegger used the term lēthē to symbolize the “concealment of Being” or “forgetting of Being” that he saw as a major problem of modern philosophy. Examples are found in his books on Nietzsche and Parmenides.