Canto 4 of Dante's Inferno describes the first circle of hell, otherwise known as Limbo. The inhabitants of Limbo are unbaptized infants and virtuous pagans. There is much more focus on the virtuous pagans, and they are further subdivided into two groups: those excluded from heaven on temporal grounds and those excluded on geographical grounds.
The virtuous pagans excluded on temporal grounds are those who lived before Christ and, therefore, through no fault of their own, never had the opportunity to hear his message. They include great poets such as Homer, Horace, Ovid and Lucan, and great philosophers such as Socrates, Plato and Aristotle. Virgil, Dante's guide, belongs in the former group and turns aside to talk to them.
The virtuous pagans excluded on geographical grounds are Muslims, specifically Saladin, the Kurdish warrior prince, and two philosophers, Avicenna and Averroes. Although they all lived after the birth of Christ, the fact that these men were not born in Christian countries has excluded them from heaven. Dante shows that he appreciates their work and character, as he does with all the virtuous pagans. In particular, he speaks of "Averoìs, che ’l gran comento feo" (Averroes, who made the great commentary), referring to Averroes's commentary on Aristotle, which not only explained the great philosopher's works to Muslims, but also re-introduced him to a European audience.