"Master, where shall be found Lethe and Phlegethon"

(Canto 14, lines 130-131)

Dante and Virgil have already passed Acheron and Styx (while Cocytus, the frozen lake in the ninth circle, will appear in a later Canto), and the boiling river in the seventh circle is now revealed to be Phlegethon. Lethe, the fifth river of the underworld, was the river of forgetfulness, which souls drank of to forget their earthly life. Dante puts Lethe at the summit of Purgatory mountain, where the purged souls finally wash off the last fragments of sin and are allowed into Paradise.