In Greek mythology, Lethe (Greek: ?, L?th?; Ancient Greek: [l:t:], Modern Greek: ['li?i]) was one of the five rivers of the underworld of Hades. Also known as the Ameles potamos (river of unmindfulness), the Lethe flowed around the cave of Hypnos and through the Underworld, where all those who drank from it experienced complete forgetfulness. Lethe was also the name of the Greek spirit of forgetfulness and oblivion, with whom the river was often identified.
In Classical Greek, the word lethe (?) literally means "oblivion", "forgetfulness", or "concealment". It is related to the Greek word for "truth", aletheia (?), which through the privative alpha literally means "un-forgetfulness" or "un-concealment".
Lethe, the river of forgetfulness, is one of the five rivers of the Greek underworld; the other four are Acheron (the river of sorrow), Cocytus (the river of lamentation), Phlegethon (the river of fire) and Styx (the river that separates Earth and the Underworld). According to Statius, it bordered Elysium, the final resting place of the virtuous. Ovid wrote that the river flowed through the cave of Hypnos, god of sleep, where its murmuring would induce drowsiness.
The shades of the dead were required to drink the waters of the Lethe in order to forget their earthly life. In the Aeneid, Virgil (VI.703-751) writes that it is only when the dead have had their memories erased by the Lethe that they may be reincarnated.
Lethe was also the name of the personification of forgetfulness and oblivion, with whom the river was often associated. Hesiod's Theogony identifies her as the daughter of Eris ("strife"), and the sister of Ponos ("Hardship"), Limos ("Starvation"), Algea ("Pains"), Hysminai ("Battles"), Makhai ("Wars"), Phonoi ("Murders"), Androktasiai ("Manslaughters"), Neikea ("Quarrels"), Pseudea ("Lies"), Logoi ("Stories"), Amphillogiai ("Disputes"), Dysnomia ("Lawlessness"), Ate ("Ruin"), and Horkos ("Oath").
Some ancient Greeks believed that souls were made to drink from the river before being reincarnated, so they would not remember their past lives. The Myth of Er in Book X of Plato's Republic tells of the dead arriving at a barren waste called the "plain of Lethe", through which the river Ameles ("careless") runs. "Of this they were all obliged to drink a certain quantity," Plato wrote, "and those who were not saved by wisdom drank more than was necessary; and each one as he drank forgot all things." A few mystery religions 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.
These two rivers are attested in several verse inscriptions on gold plates dating to the 4th century BC and onward, found at Thurii in Southern Italy and elsewhere throughout the Greek world. There were rivers of Lethe and Mnemosyne at the oracular shrine of Trophonius in Boeotia, from which worshippers would drink before making oracular consultations with the god.
More recently, Martin Heidegger used "l?th?" 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 (Vol 1, p. 194) and on Parmenides.
Many ancient Greek poems mention or describe Lethe. The river is also referenced in more recent novels and poetry. Simonides of Ceos, an ancient Greek lyrical poet, references Lethe in the sixty-seventh fragment of one of his poems. Publius Ovidius Naso, known as Ovid, in his description of the Underworld in his Metamorphoses, includes a description of Lethe as a stream that puts people to sleep. Aeneas, the protagonist of Virgil's epic Latin poem, Aeneid, travels to Lethe to meet the ghost of his father in Book VI of the poem.
"The souls that throng the flood
Are those to whom, by fate, are other bodies ow'd:
In Lethe's lake they long oblivion taste,
Of future life secure, forgetful of the past."
Virgil also writes about Lethe in his didactic hexameter poem, the Georgics.
In the Purgatorio, the second cantica of Dante Alighieri's Divine Comedy, the Lethe is located in the Earthly Paradise atop the Mountain of Purgatory. Dante, held in the arms of Matilda, is immersed in the Lethe so that he may wipe out all memory of sin (Purg. XXXI). After being washed in the Lethe, penitents are washed in the Eunoe, a river of Dante's own invention. The Lethe is also mentioned in the Inferno, the first part of the Comedy, as flowing down to Hell from Purgatory to be frozen in the ice around Satan, "the last lost vestiges of the sins of the saved" (Inf. XXXIV.130).
The French poet Charles Baudelaire referred to the river in his poem "Spleen". The final line is "Où coule au lieu de sang l'eau verte du Léthé" which one translator renders as "...in whose veins flows the green water of Lethe..." (the reference offers a few more English translations).  The same poet also wrote a poem called "Lethe".
The English poet John Keats references the river in his famous poem "Ode to a Nightengale". The first four lines of the poem are:
My heart aches, and a drowsy numbness pains
My sense, as though of hemlock I had drunk,
Or emptied some dull opiate to the drains
One minute past, and Lethe-wards had sunk:
Amongst authors in Antiquity, the tiny Lima river between Norte Region, Portugal, and Galicia, Spain, was said to have the same properties of memory loss as the legendary Lethe River, being mistaken for it. In 138 BCE, the Roman general Decimus Junius Brutus Callaicus sought to dispose of the myth, as it impeded his military campaigns in the area. He was said to have crossed the Lima and then called his soldiers from the other side, one by one, by name. The soldiers, astonished that their general remembered their names, crossed the river as well without fear. This act proved that the Lima was not as dangerous as the local myths described.
In Cádiz, Spain, the river Guadalete was originally named "Lethe" by local Greek and Phoenician colonists who, about to go to war, solved instead their differences by diplomacy and named the river Lethe to forever forget their former differences. When the Arabs conquered the region much later, their name for the river became Guadalete ("River Lethe" in Arabic).