As the rational soul is characterized by understanding, which is oriented towards knowledge, it is also characterized by will, which is oriented towards free choice. Augustine considers Cicero's reasoning against God's foreknowledge, "If there is free will, all things do not happen according to fate; if all things do not happen according to fate, there is not a certain order of causes; and if there is not a certain order of causes, neither is there a certain order of things foreknown by God." Against this argument, Augustine maintains both human freedom of the will and divine foreknowledge of all future events. Even if there is free will and an absence of any all-encompassing deterministic fate, there can still be "for God a certain order of all causes," among which causes are our freely choosing wills.

Evodius asks, "why God gave human beings free choice of the will," given that it enables us to sin. Augustine's answer is that it is a necessary condition for virtuous action: "The fact that human beings could not live rightly without it was sufficient reason for God to give it. "No action would be either a sin or a good deed if it were not performed by the will, and so both punishment and reward would be unjust if human beings had no free will."