Chapter 2 examines the reception of Aristotle’s action theory from the 1220s to 1277 and its influence on novel theories of free will developed in this period. It shows that the reception of Aristotle led to a “psychological turn”: instead of assuming the existence of free will, theologians began to argue for it by clarifying the nature of intellect and will, in which free will is grounded. The chapter canvasses the theories of free will in broad strokes from William of Auxerre to Bonaventure, and in more detail regarding Thomas Aquinas and Siger of Brabant, whose views will provoke strong reactions. Following Aristotle closely, Aquinas understands choices as determined by the practical deliberation that precedes them; one chooses as one judges worth choosing, and one can choose otherwise only because deliberation allows one to judge otherwise. Appealing to the authority of Avicenna, Siger argues that what causes the will’s acts does so necessarily.

Chapter 7 examines medieval theories of the first cause of evil. Although Augustine denied that something good is the cause of evil, medieval thinkers routinely attributed to him the view that something good – the will understood as a power of the soul – causes something evil. The majority of the thinkers considered in this chapter use Aristotle’s notion of accidental causality to argue that the will can cause evil not per se – that is, not intending evil as evil – but incidentally. Nevertheless, if the will causes evil, they face the dilemma that a good will cannot cause evil at all, and an evil will cannot cause the first evil will. Medieval thinkers deal with this dilemma in different ways. For example, Bonaventure and others hold that the created will is naturally defectible, and hence not entirely unflawed, and so it can do evil. Aquinas argues that the will causing evil for the first time presupposes a momentary nonculpable deficiency, which becomes culpable at the moment of the evil choice. For Scotus, the will is so free that it can do evil even if it is unflawed. Although all these views trace evil to the will as its cause, they hold that evil is ultimately unexplainable.