Most of us in the academic world have written and received more administrative memos than we care to recall, and we tend to think of the memo as an ephemeral genre. It would require a powerful act of imagination to think that a memo written in the early twenty-first century could enjoy a vigorous afterlife and serve to inspire not just legal but theological (and perhaps indirectly also medical) theory four centuries later, far out of proportion to any concrete relevance it might ever have had. While the analogy is not perfect, this is more or less what happened to an opinion rendered in the ninth century by Hincmar of Rheims. It was not Hincmar's comment on the highly publicised divorce (857–69) of Lothar and Theutberga, but the somewhat less high-profile case of the Aquitanian count Stephen (860) that had this afterlife, because the latter document focused more specifically on the question of impotence, as distinct from magic for love and hate. Impotence was of crucial importance because churchmen were called upon to judge whether and when it was a valid ground for annulling a marriage. In medical discourse, the work of Constantine the African had importance parallel to that of Hincmar in canon law and theology. In pursuing the significance of this issue, Rider has produced a model of thematically organised historical writing. She gives a thorough account of developments in thinking about magically-induced impotence among the three relevant academic faculties (the canon lawyers, the theologians and the physicians) and also among writers contributing to pastoral reform. The last of these areas turns out to have special significance, helping to turn attention towards the actual practice of magic, although this had always been in varying measure an interest of writers on the topic. Rider traces a complex pattern of influences on three main levels: over time, among genres, and between popular practice and learned theory. The interplay of these variables is particularly important: popular practice was more often examined at times of pastoral reform and less in the forefront at times (such as the twelfth century) when theoretical issues most needed to be resolved; when attention turned to the use of magical cures for impotence, theologians tend to be most restrictive and physicians most permissive, but in the fifteenth century the theologians' concern about demonic agency spread to other writers and led to a general hardening of attitudes. Rider's book is important mainly for what it says about learned commentary on the proper response to magic, but it does shed light on magical practices as well, including one especially important passage (p. 174) on the magical use of the hoopoe. This is in every respect a thorough, clear and admirable work.