The conversation on love continues. Dante presses: if love is always given from without, and the soul naturally follows what attracts it, how can we be responsible for our loves? Virgil responds with the doctrine of free will: the soul has an innate power to judge and assent or withhold assent. This power is what Beatrice calls "free will" (and its exercise constitutes merit or demerit). Reason can look at the first impressions of desire and approve or reject them. We are not compelled by our appetites; we can reflect. This freedom is the foundation of moral responsibility — and the reason that Purgatory exists at all.
Dante, lulled by the discourse, drifts into the shallow sleep of thought — his mind is drowsy but still running. The moon rises. Midnight. And then: a sudden thunder of feet and voices. A great throng of souls comes rushing past, running with tremendous speed, crying out examples of zeal and urgency. The first: "Mary made haste to the hill country" (Luke 1 — the Visitation, where Mary ran to Elizabeth after the Annunciation). The second: "Caesar made haste to subdue Spain" — even Caesar, the embodiment of military ambition, serves here as an example of energetic will. The running souls shout these examples as they sprint. Their penance is perpetual motion against their former inertia.
One soul briefly slows — the Abbot of San Zeno in Verona, who managed a monastery in the time of Barbarossa. He mentions without lingering (he cannot linger) that Alberto della Scala has corruptly installed his crippled, illegitimate son as abbot there. Another pair of voices trails behind, crying out counter-examples: the Israelites who died in the wilderness because of their faithlessness and sloth, and the Trojans who turned back from following Aeneas. Dante falls asleep. Purgatory's most dynamic terrace — all movement and noise — sends him to rest. The irony is gentle and rich.