This article describes the group of ninth-century Zoroastrian philosophers I call the ‘Dēnkard School’ and sketches the way they do philosophy. It presents their argument against substance dualism, which the Zoroastrians argue is in tension with the belief in repentance. From an analysis of this polemic, there follows a reconstruction of the Dēnkard School's own doctrine of the consubstantiality of body and soul. To understand these arguments, I describe some background eschatological and ontological beliefs upheld by the Dēnkard School and their specific conception of substance, which includes the notions of ownership and responsibility. Overall, the argument can be seen as a new position on a traditional problem, and so increasing the scope of philosophy in a more global perspective.