Inferno speaks of sin and justice: of how each sin receives the punishment that describes it, and of what a man learns to feel before the sin of others. Dante's descent through the other world serves as the frame for these two questions.
The first is in the order of the circles. Virgil explains, in Canto XI, that all malice aims at injustice, and that injustice is done by force or by fraud; fraud, which is man's own evil, displeases God more, and for that reason it lies below. The criterion is not the pain caused, it is the use of intelligence. The punishment, for its part, is not imposed from outside: those carried away by desire are swept by a windstorm, the soothsayers walk with their heads turned backward, and the thieves lose the body, which was theirs.
The second is the thread of the poem. Dante is afraid in the wood, swoons with pity before Francesca, wishes to see Filippo Argenti sunk in the mud, listens with too much relish to the quarrel of two forgers and, in the ice, treats traitors harshly. The poem does not say at the outset whether it approves what the pilgrim feels.
Inferno also speaks of Florence. Dante, exiled when he wrote it, places living and dead Florentines within the poem, and the dead announce what will happen to him. The themes that appear most are sin, justice and law, desire, politics, knowledge and wisdom, loyalty and betrayal, honor and reputation.