Upon arriving in Auschwitz Primo Levi discovered that rational discourse, in which actions are done for reasons, was left lying on the carriage floor together with his human dignity. By responding ‘Here one doesn't ask why’, the camp guard succinctly conveys the insight that evil defies reason. This paper examines two studies of evil that are predicated on that idea: Kant's and Augustine's. It argues that their theories share an underlying formation wherein evil remains incomprehensible, except in negative terms as an absence of the reasonable. This deep similarity in the structure of the concept of evil is exceptionally striking if we bear in mind the radical difference in the scope of its application: whereas for Augustine ‘evil’ is a general metaphysical concept that concerns everything that goes wrong in creation, for Kant it is limited to agents and what they do. My argument is that the privative structure which underlies their understanding of evil stems from a shared belief in the central role of rationality in ethical discourse, and in an absolute dependence of moral responsibility on free will.