While many studies of Islamic law have centered on the development of legal theory and substantive law, especially in their formative period of development, Mariam Sheibani instead argues that the rich legal history of the post-formative period and the Islamic legal philosophy that developed in it have been comparatively neglected. This innovative study traces the ethical turn in medieval Islamic legal philosophy through the pioneering work of the prominent jurist and legal philosopher Izz al-Dīn Ibn ʿAbd al-Salām (d. 660/1262). Sheibani demonstrates how Ibn ʿAbd al-Salām advanced a comprehensive analysis of the law's purposive and coherent rationality, articulated in a distinctive genre, with direct bearing on legal doctrine and social praxis. Ibn ʿAbd al-Salām expanded on previous theological and legal reasoning, furthering two ideas developed by Khurasani Shāfiʿīs: maṣlaḥa (human benefit) and qawāʿid (legal maxims). He also sought to embody and deploy the teachings of his legal philosophy for socio-religious reform in Ayyubid Damascus and Cairo, breaking with the dominant formalism of legal practice. The new forms of legal reasoning and writing that Ibn ʿAbd al-Salām developed would influence subsequent jurists from diverse legal schools and across regional traditions until the present day.