In canto XVII, Virgil teaches that every act, good or bad, is born of love: neither creator nor creature was ever without love, whether natural or of the soul. Natural love, instinct, never errs; love of the soul, the love of choice, can err through a wrong object, through too much or too little intensity. This is the thesis that organizes the mountain.
The lesson comes from a question of Dante's: what offense is purged on that ledge? Virgil answers with the name of a love, not of a sin. He reaches the harm to one's neighbor by exclusion, as one who demonstrates, and says that this love arises in three ways, which are purged on the three lower ledges. Lukewarm love of the good is that of the ledge where they stand, and excessive love of lesser goods is purged on the three above. Vice ceases to be a separate kind: it is disordered love, and it can be corrected.
In canto XVIII, Virgil completes the lesson with the image of wax and seal: the matter of love is good in everyone, but not every seal is good. Dante then objects that, if love comes from outside and the soul walks only on that foot, walking straight or crooked is not its merit. The objection threatened the possibility of penance, and Virgil answers as far as reason sees: in each person there is the power to hold back love, and that noble virtue, which Beatrice understands as free will. The rest, he says, is a work of faith, and he waits for it.