Affectus and affectio, however we translate these words, became key concepts in Cistercian thought in the twelfth century. The figure most well-known for theorizing affectus—in the sense of inclination or what in German is called Affekt—at least in terms of friendship, is Aelred of Rievaulx (c. 1110–1167), whose ‘affective anthropology’ is the focus of an important monograph by Damien Boquet. 1 Aelred, however, was only extending ideas already developed by Bernard of Clairvaux (1190–1153), who, together with his friend, William of Saint-Thierry (c. 1085–1148), was thinking about what the Song of Songs could reveal about longing and desire in the human soul. 2 Deserving particular attention are two treatises about the soul, the De anima of Isaac of Stella (c. 1100–1169), in which he says he is responding to questions raised by his friend, Alcher, and the De spiritu et anima, a text which expands on Isaac’s Platonizing ideas about the soul with arguments indebted to Augustine. 3 While the De spiritu et anima (hereafter DSA) would be widely circulated in the thirteenth century as a work of Augustine, this attribution was questioned by both Albert the Great and Thomas Aquinas. 4 Their dismissal of its authority has influenced a scholarly tendency to downplay the originality of how its author responds to the ideas of Isaac of Stella about both reason and affectus in the soul. The DSA was much more influential than Isaac’s treatise in the thirteenth century in reformulating Augustinian thought about both reason and affectus prior to the impact of Aristotelian thinking about the intellective soul.