Abstract The second chapter traces the genesis and development of both ijtihad (reasoning) and usul al-fiqh (legal theory) in Shi‘i intellectual history. It argues that the concern for knowledge and certitude, which characterized much of Shi‘i juristic literature in its formative period, was displaced with a recognition and acceptance of doubt as an inalienable feature of the law. The chapter also demonstrates that in the thirteenth and fourteenth centuries, Shi‘i scholars elaborated and developed a specific Shi‘i theory of ijtihad. After discussing the Usuli–Akhbari dispute and its impact on legal theory, the chapter shows that Murtada Ansari constructed and methodically explored the epistemological categories of certainty, speculation, and doubt. This was important to Shi‘i legal history because it empowered jurists to issue rulings on many spheres of law that had hitherto remained beyond their realm, broadening, in the process, the scope of Islamic law and the possibilities of its revision.