Professor Devin J. Stewart delves into early Islamic legal thought, tracing how reason (ʿaql) evolved as a recognised source of law among Muʿtazilī and Twelver Shīʿī scholars. Through a historical journey from al-Jaṣṣāṣ to al-Ṭūsī, Prof. Stewart shows how theological debates on rational investigation shaped the very foundations of Islamic jurisprudence, revealing a vibrant intellectual culture negotiating between divine command and human judgment.