[Virgil]: "Philosophy, for one who understands,
points out, and not in just one place," he said,
"how nature follows – as she takes her course –
the Divine Intellect and Divine Art;
and if you read your Physics carefully,
not many pages from the start, you’ll see
that when it can, you art would follow nature,
just as a pupil imitates his master;
so that your art is almost God’s grandchild." (Inf. XI, 97-105)
Virgil explains a central concept in Dante’s vision of Christianity: the Divine is natural, since "nature follows…the Divine Intellect and Divine Art." Man’s instinct is to follow nature and thus follow God. Consequently, anything made by man’s art is usually natural and thus somewhat like "God’s grandchild" (if man is God’s child). As a rule, then, anything that goes against nature inherently goes against God or, in other words, sins.
Quote 2
[Virgil]: "From these two, art and nature, it is fitting,
if you recall how Genesis begins,
for men to make their way, to gain their living;
and since the usurer prefers another
pathway, he scorns both nature in herself
and art, her follower; his hope is elsewhere." (Inf. XI, 106-111)
Following the train of the thought from the last few lines, Virgil arrives at what is natural or good for men to do with their lives: to "make their way, to gain their living." In other words, it is good for man to work and to gain his living by the sweat of his brow, the depth of his mind, the creation of his hands. Usurers violate this natural order by growing fat off man’s greed for money instead of winning their bread through honest work. Thus, usury is a sin against nature.
[Virgil to Dante]: "But fix your eyes below, upon the valley,
for now we near the stream of blood, where those
who injure others violently, boil." (Inf. XII, 46-48)
Because Dante ultimately sees violence as a distortion of nature, the landscapes of the Seventh Circle feature some twisted aspects of nature. Here, the boiling river that tortures the tyrants does not flow with water, but with blood. Thus, the violent are punished by natural forces which have been fundamentally perverted.