The Controversy between al-Kindī and Yaḥyā b. ʿAdī on the Trinity, Part Two: Gregory of Nyssa’s and Ibn ʿAdī’s Refutations of Eunomius’ and al-Kindī’s ‘Error’

Abstract

This article is the second part of an investigation into the controversy between the Arab-Muslim philosopher Abū Yūsuf Yaʿqūb b. Isḥāq al-Kindī (died after 256/870, ca. 873) and the Christian-Jacobite logician and theologian Yaḥyā b. ʿAdī (d. 363/974). It argues that we can draw a line from Basil of Caesarea’s and Gregory of Nyssa’s refutation of Eunomius of Cyzicus to Yaḥyā b. ʿAdī’s refutation of al-Kindī. According to Gregory’s and Ibn ʿAdī’s reasoning Eunomius’ and al-Kindī’s refutation of the consubstantiality of God-Father and God-Son is grounded in a series of misunderstandings starting from the fundamental error of a false interpretation of the relationship between substance and hypostases. The term ‘hypostases’ in Gregory’s and Ibn ʿAdī’s interpretation does not indicate ‘individual substances’ but rather different subsistences of a nature or essence realised by the properties peculiar to that nature. The nature in turn is knowable by its intelligible properties, and the fact of the subsistence of properties is signified by an appellation or by the predication of a circumscription (πeριγραφή) or characterisation and attribute (ṣifa) respectively.