Zooming in on a single city, Chapter 4 focuses on Paris in the twelfth and thirteenth centuries. As a prime example of the social changes brought about by urbanization, Paris was a commercial hub, the seat of royal administration, and a centre for advanced learning and education through its new university. We explore how this environment fed into the cultivation of polyphonic music at the Cathedral of Notre-Dame, looking into the genres of organum, conductus, and motet, and examining the system of rhythmic modes that was developed to notate this music for posterity. Music theorists, writing a generation or more later, provide us with the names of some of those musicians responsible for the musical innovations at Notre-Dame, and thus we can identify the composers of liturgical polyphony for the first time. We learn how Léonin compiled his Magnus Liber Organi, and his successor Pérotin edited and supplemented it, giving us a unique insight into the ways in which medieval musicians preserved and reinvented the music of earlier generations.

This chapter begins with an etymological and historical elucidation of the terms conscientia and synderesis. Philosophical and theological reflection on these terms, beginning with St. Jerome and proceeding through thinkers such as Peter Lombard and Philip the Chancellor, constitutes the background against which St. Thomas Aquinas develops his understanding, not only of conscientia and synderesis, but also of objective right (ius) or rights. Much of the debate regarding synderesis, the infallible basis of conscientia, concerns whether it is a power or habitus. Aquinas settles on understanding synderesis as a ‘habit’ of the potential intellect – which, following Aristotle, he understands as a sort of ‘blank slate’ upon which things can be written. One of the things written on the habit of synderesis is the practical version of the principle of non-contradiction: ‘good is to be done and pursued, evil avoided’. This allows him to develop a theory according to which objective rights are primary, although not to the exclusion of subjective rights. It also allows those in agreement with Aquinas to exclude subjective rights that contradict established objective rights.

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.

Chapters 8–10 constitute Part III, entitled “Angelic Sin,” which raises the issue of how rational agents can do evil under ideal psychological conditions. Chapter 8 is about intellectualist accounts of angelic sin. Since according to these accounts, the will acts as the intellect judges best, evil acts presuppose some cognitive deficiency: either an outright error, or some occurrent nonconsideration that keeps the intellect from making the correct judgment. Thus one difficulty faced by intellectualist thinkers is how the cognitive deficiency can come about – especially since most thinkers here discussed assume that angels are infallible prior to making an evil choice. Another difficulty concerns control of the act. It is assumed that while the angels’ good or evil choice was up to them, the content of their knowledge was not up to them. Aquinas’s solution is that knowledge does not predetermine the use of that knowledge, which is up to the will. By contrast, Godfrey of Fontaines argues that the choice of the angels is caused by the cognized object; he fails to explain, however, how his theory avoids cognitive determinism.