During his career as a historian of medieval medicine, John Riddle’s scholarly interests have turned around a question that is both deeply important and singularly dicult: how did premodern societies identify and understand the power of natural substances to alter the body’s workings, and how did they put these powers to use? Riddle is persuaded that the physiological and medical eects of the substances these people used are objectively veriable, and that historians need to take this reality into consideration when analyzing the textual record. is commitment to a certain pharmacological realism, exemplied by his work on contraceptives and abortifacients, was already announced in his article “eory and Practice in Medieval Medicine.”2 In this essay, Riddle argued that early medieval medicine, overwhelmingly practical in character, was a seamless continuation of ancient medicine, and that it was clinically innovative and eective. e theoretical turn of the twelh century, associated with the translations of Arabic medical works and the elaboration of a scholarly style of medical literature oriented toward the construction of abstract models of physiology, pathology, and therapy, was not the dawn of medical progress that historians commonly thought it was. To the contrary, in those branches of medical learning that impinged on practice, notably pharmacology and pharmacy, the eect of theory was at best negligible and at worst pernicious. Scholastic pharmacology’s construction of a theory of staged “degrees” of the primal qualities of heat, cold, moisture, and dryness, especially when applied to compound remedies, was unworkable in its complexity. In fact, it was ignored by

most practitioners, even the most highly educated. Nonetheless, the ascendancy of “theory” had the negative eect of demoting the value of the empirical knowledge recorded in the older medical literature.