(Greek) Ληθη, "forgetfulness, oblivion."
1. A symbolic "river" in the underworld, which the dead drink of in order to forget their past.
2. The spirit (daimona) of forgetfulness and oblivion, also associated with the underworld river Lethe.
"After having incinerated the seeds of the “Ego” through the purification of our corruption in Purgatory, the Initiate then bathes in the rivers Lethe [forgetfulness] and Eunoe. [...] The dissolution of the ego is necessary because the ego is nothing but an addition of tenebrous entities. We have arrived at the conclusion that every human being must dissolve the ego, burn the seeds of the ego, and be bathed in the waters of Lethe in order to be finished with the memories of the past. Then after the Confirmation in the Light, one is welcomed into the White Brotherhood; there one signs papers and is warned that one must be careful." - Samael Aun Weor, Tarot and Kabbalah
"It is written that the Son of Man has to descend into the atomic infernos of Nature. It is written that the Son of Man has to ascend to heaven, after passing through the Purgatorial region. The Son of Man has to carefully submerge himself within the waters of Lethe in order to reconquer innocence. With great urgency, we need to forget the sinful and absurd past that is the origin of much bitterness. The Lethe and Eunoe are certainly, and without the least bit of doubt, a single river of clear and profound waters. On one side, its waters delectably descend, singing upon its rocky bed, carrying the marvelous virtue that erases the memory of sin, the remembrances of “myself.” Its name is Lethe. The other very holy and sublime shore has the delectable enchantment of fortifying virtues, and its name is Eunoe. It is obvious that the tenebrous remembrances of too many yesterdays must be erased, because to our own disgrace, they have the tendency of actualizing, projecting themselves into the future through the alley of the present. In the name of truth, I have to say that the profound work within the waters of Lethe is frightfully difficult and more bitter than bile. The matter of passing beyond the body, affections, and the mind is not easy. Too many beloved shadows live within time... The memories of desire persist; they refuse to die; they do not want to disappear." - Samael Aun Weor, Magic Runes