Middle Persian translations and interpretations of Avestan texts employ the word īšt in the translation of the Avestan ī̆šti- “capability, capacity, competence”. The word became a vocabulary item in the Middle Persian corpus. It seems to be a calque of its Avestan counterpart. The Avestan ī̆šti- has presented challenges in the Avesta scholarship and is translated with words from different semantic domains. This article discusses the definition of Avestan ī̆šti- and how it is reinterpreted and understood in the Middle Persian translations. It is argued here that Av. ī̆šti- refers to “capability, capacity, and competence”. However, it is understood and interpreted in the MP texts as “wealth, property”, “remuneration”, or “reward”. It is sometimes translated to a verb form from xwāstan “desire, want”.

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.