In the next (fourth) trench, Dante sees souls weeping quietly, their heads turned completely around so that they have to walk backwards. At this sight of such bodily disfigurement (Dante describes the sinners' tears welling up and streaming down their backs), Dante begins to weep. Virgil chides him for this, and tells him he can choose between pity and piety. Since this punishment is ordered by God as justice, he rhetorically asks Dante, "Who's wickeder than one / That's agonized by God's high equity?" (20.30)
Despite Dante's apparent move away from pity in the last canto, the bodily disfigurement of the souls in this fourth trench causes Dante to weep with pity. This gives Virgil the chance to re-emphasize how all of the suffering in hell is part of God's divine justice. To weep at what God has rightly ordered against the wicked is itself wicked, as he teaches Dante.
Virgil tells Dante to look at one of the backwards-facing souls, Amphiaraus (a seer of Greek mythology). Virgil explains that because Amphiaraus tried to see too far ahead, he now can only see behind him. He then points out Tiresias, another famous mythological seer, as well as Manto, a female seer who settled in Mantua, in northern Italy (Virgil's birthplace). After she died, people gathered and built a city around her burial place, naming the city Mantua after her.
Virgil points out to Dante three famous seers of classical myth. As an illustration of God's divine justice, their punishment fits their sin. For trying to see too far ahead, they can now only see behind them. Their punishment thus matches and completes their sin.
Virgil insists that this is the true story of the origins of Mantua, and that other versions of its foundation are false. Dante assures Virgil that he believes him entirely. Virgil points out more seers and witches in this part of hell, before telling Dante that it is time to move on, because the moon is beginning to set.
Virgil insists upon the truth of his speech and Dante readily believes his trustworthy guide.