Which river in the underworld causes forgetfulness?

The river in the underworld that causes forgetfulness is indeed Lethe. In Greek mythology, Lethe is one of the five rivers of the underworld, and its waters are said to induce forgetfulness in the souls of the deceased. When a soul drinks from the river Lethe, they forget their earthly experiences and past lives, which allows them to be reincarnated without the burdens of their previous existence. This concept emphasizes a cycle of life and rebirth, reflecting the belief in the importance of forgetting one's past to begin anew.

In contrast, the other rivers serve different symbolic purposes. The Styx is primarily known as the boundary between the living world and the underworld, while Acheron is often associated with the river of pain or sorrow. Cocytus is referred to as the river of wailing, representing the deep grief and sorrow found in the underworld. Each river has its own significance, but it is Lethe that specifically embodies the theme of forgetfulness.