Lethe (pronounced lee'thee) is the river of forgetfulness or oblivion. It is one of the rivers that circles Hades in Greek mythology. The other rivers are:

Spirits of the dead drank of its waters to forget the sorrows of former life before entering the Elysian Fields (Elysium). They again drank of the river Lethe before entering a new life.

They had much, of course, to say to each other. Anchises led Aeneas to Lethe, the river of forgetfulness, of which the souls on their way to live again in the world about must all drink. "A draught of long oblivion," Anchises said. And he showed his son those who were to be their descendants, his own and Aeneas' now waiting by the river for their time to drink and lose the memory of what in former lives they had done and suffered. A magnificent company they were--the future Romans, the masters of the world. One by one Anchises pointed them out, and told of the deeds they would do gave his son instructions how he would best establish his home in Italy and how he could avoid or endure all the hardships that lay before him.

From the Aeneid

Plato also mentions the River Lethe in his Republic. Following is a passage from the last part of The Republic: Book X. The bold added is mine to show refrences to Lethe.

All the souls had now chosen their lives, and they went in the order of their choice to Lachesis, who sent with them the genius whom they had severally chosen, to be the guardian of their lives and the fulfiller of the choice: this genius led the souls first to Clotho, and drew them within the revolution of the spindle impelled by her hand, thus ratifying the destiny of each; and then, when they were fastened to this, carried them to Atropos, who spun the threads and made them irreversible, whence without turning round they passed beneath the throne of Necessity; and when they had all passed, they marched on in a scorching heat to the plain of Forgetfulness, which was a barren waste destitute of trees and verdure; and then towards evening they encamped by the river of Unmindfulness, whose water no vessel can hold; of this they were all obliged to drink a certain quantity, and those who were not saved by wisdom drank more than was necessary; and each one as he drank forgot all things. Now after they had gone to rest, about the middle of the night there was a thunderstorm and earthquake, and then in an instant they were driven upwards in all manner of ways to their birth, like stars shooting. He himself was hindered from drinking the water. But in what manner or by what means he returned to the body he could not say; only, in the morning, awaking suddenly, he found himself lying on the pyre.

References in the Divine Comedy:

The English word 'lethargic' is derived from this river. In chemistry, pyrolythic has origins in lethe.

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