Perhaps the best place to begin a consideration of Purgatorio is not its beginning but its middle. In cantos 16-18, the central three of this the central canticle, we learn about love and free will, perhaps the two principles most important to an understanding of the whole of the Comedy. Because our modern novelistic tradition of structure has led us to expect our plots to be arranged climactically, we tend to find this kind of geometric construction artificial and surprising, even though the practice was fairly common in medieval literature. Dante had himself already experimented with this kind of structure in La Vita Nuova. La Chanson of Roland, to cite another well-known example, seems by our standards to drag on surprisingly beyond the hero's death; the plot has been carefully arranged, however, so that this event of central importance occurs at the very center of the poem.
The first of these three central cantos of Purgatorio, canto 16, deals with the problem of human freedom. To Dante's question of whether the world's evil is imposed by stellar influence, Marco Lombardo, one of the souls in Purgatory, responds that through right reason people can control the impulses that admittedly do originate in the stars. An individual's fate is not, therefore, determined by uncontrollable impersonal forces. Rather, the world has turned to evil through poor leadership. Souls are born as lovers of pleasure, and they will continue to cling to childish self-indulgence unless laws and leaders curb this selfishness and guide them to a higher love. People, however, see their leaders, most notably Boniface VIII, scoffing at the law and indulging themselves, and so they behave similarly.
In canto 17, Virgil asserts that all actions, the virtuous and the sinful, both those performed by the Creator and those by creatures, are motivated exclusively by love. For this to be comprehensible, we must understand that Dante considers instinct a form of love, "natural" or "animal" love, which can never be sinful. A second kind of love, however, "mind-directed" love, can fail in one of three ways and so be sinful, and in explaining this Virgil also explains the way the central portion of Purgatorio is structured around the concept of the seven deadly sins. One can go wrong by loving things one should not, (pride, envy, and anger), by loving what one should love, but with insufficient intensity (sloth) or by loving as ends in themselves things that one should love only in proper relationship to primary ends. In indulging these impulses, therefore, and so committing sins, one is motivated by a species of love.
In canto 18, however, Dante pursues the relationship between free will and love one step further. If love is a powerful force innate in each individual, "what merit is there in loving good or blame in loving ill?" The answer is that "Reason must surely guard the threshold of consent," for only with full consent of the will can a soul be held guilty...