In Greek mythology, Lethe is one of the five rivers of Hades that flow through the Underworld. Enshrouded in perpetual twilight and mist, it winds slowly round the cave of Somnus, the god of sleep. Its name translates literally as oblivion and all who drink its waters — as all souls destined for reincarnation must — instantly forget their past lives. Those condemned to wander the earth as ghosts return to Lethe each day as dawn breaks.
Nepenthe is a drug described by the ancients as engendering forgetfulness of sorrow and pain. Some modern scholars have speculated that it may have been a form of opium.