This is where the word "lethargy" comes from. The souls of the dead, in preparation to return to life in a new body, must drink from the river Lethe. And so they are made to forget:
All they had learned, all of their passions, all they had worked and suffered to create. We drink the waters of oblivion, ambition and accomplshments are washed away, and so we forget who and what we truly are.
Virgil, Aeneid 6. 705 ff (trans. Day-Lewis) (Roman epic C1st B.C.) :
"Now did Aeneas descry deep in a valley retiring, a wood, a secluded copse whose branches soughed in the wind, and the Lethe River drifting past the tranquil places. Hereabouts were flitting a multitude [of phantoms] without number . . . Aeneas moved by the sudden sight, asked in his ignorance what it might mean, what was that river over there and all that crowd of people swarming along its banks. Then [the ghost of] his father, Ankhises said:--`They are the souls who are destined for Reincarnation; and now at Lethe's stream they are drinking the waters that quench man's troubles, the deep draught of oblivion . . . They come in crowds to the river Lethe, so that you see, with memory washed out they may revisit the earth above.'"