al-Ashʿarī, Abū al-Ḥasan ʿAlī b. Ismāʿīl b. Isḥāq (ca. 260–324/874–936), scholastic theologian and eponymous founder of the most important theological school in Sunni Islam, Ashʿarism (Ashāʿira) (q.v.). The defining characteristic of this school is its use of rational argument to defend the theological positions of the Ahl al-Sunna wa al-jamāʿa and Aṣḥāb al-ḥadīth against the doctrines of the Muʿtazila, which they regarded as bidʿa (innovation), insufficiently grounded in the Qurʾān and the Sunna, …