Lethe is the water of Eden that takes from whoever drinks it the memory of sin. It rises from a spring that opens into two channels, and Dante is dipped into it by Matelda, after the confession. She uses the forgetfulness the water causes as proof of guilt: if he does not remember having strayed from her, it is because he drank from Lethe.
Where It Appears
Quotes
On this side it descends with the virtue of taking the memory of sin from someone; from the other, it gives back the memory of every good deed. From here, Lethe; so, on the other side, it is called Eunoe, and it does not work unless it has first been tasted on one side and on the other.
Cantos XXVI to XXIX: The fire and Eden, Matelda, to Dante, explaining the two waters of Eden
And if you cannot remember, she answered, smiling, remember now how today you drank from Lethe; and if from smoke one infers fire, this forgetfulness clearly concludes guilt in your will, which attended to the other part.
Cantos XXX to XXXIII: Beatrice, Beatrice, to Dante, using Lethe as proof