1911 Encyclopædia Britannica/Lethe
LETHE (“Oblivion”), in Greek mythology, the daughter of Eris (Hesiod, Theog. 227) and the personification of forgetfulness. It is also the name of a river in the infernal regions. Those initiated in the mysteries were taught to distinguish two streams in the lower world, one of memory and one of oblivion. Directions for this purpose, written on a gold plate, have been found in a tomb at Petilia, and near Lebadeia, at the oracle of Trophonius, which was counted an entrance to the lower world, the two springs Mnemosyne and Lethe were shown (Pausanias ix. 39. 8). This thought begins to appear in literature in the end of the 5th century B.C., when Aristophanes (Frogs, 186) speaks of the plain of Lethe. Plato (Rep. x.) embodies the idea in one of his finest myths.