u/zensama

In Ancient Greek mythology, the Lethe is the river of forgetfulness and one of the five rivers of the Underworld ruled by Hades.

Its waters cause those who drink from it to experience complete forgetfulness of their earthly lives, a necessary step for reincarnation or entry into Elysium.

The river is also the name of the goddess personifying oblivion, who's primary function is erasure of memory.

In the Divine Comedy, Dante drinks from the Lethe in Purgatory to forget his sins before entering Heaven.

All this to say they knew at least two key components of PPT, reincarnation and memory wipe.