Mujtahid

A mujtahid is a jurist qualified to derive rulings directly from the sources rather than following the settled positions of a school, and the office is far more consequential in Twelver Shia usage than in Sunni.

Independent reasoning

The mujtahid is competent to exercise ijtihad, the independent effort of deriving legal rulings from the Quran, the practice of the Prophet, consensus and reasoning, rather than merely applying the established rulings of a school, which is the ordinary work of most jurists. This requires the highest level of legal training and mastery of the sources and the methods, and the classical qualifications for it are demanding, so that the mujtahid stands above the ordinary jurist who follows an inherited position.

The Shia difference

In Sunni Islam the scope for fresh ijtihad narrowed as the schools of law matured, and most Sunnis follow the settled positions of a school, so the living mujtahid is less central. In Twelver Shia Islam, by contrast, the office is fundamental: every believer not himself qualified must follow a living mujtahid of the highest rank in matters of practice, and the most senior mujtahids, recognised as sources of emulation, hold great religious authority and, in the modern Iranian doctrine of the guardianship of the jurist, potentially political authority as well. The office of the mujtahid is thus the point at which the Sunni and Shia understandings of religious authority diverge most sharply, the one resting on the schools and their accumulated law, the other on the living jurist qualified to reason from the sources.

A once-standard narrative held that Sunni Islam formally closed the gate of ijtihad around the tenth century, after which jurists were expected only to follow the accumulated rulings of their school rather than reason independently from the sources, explaining why the living mujtahid of Sunni jurisprudence became a rarer and less consequential figure than the mujtahids of Twelver Shia Islam. Modern scholarship, notably the work of the legal historian Wael Hallaq, has challenged that narrative as an oversimplification, arguing that qualified independent reasoning continued in Sunni legal practice well past the supposed closing date and that no single formal declaration actually closed anything, a genuine and ongoing disagreement among historians of Islamic law about how to characterise the discipline's own development.

Whichever account is right about the historical mechanism, the practical outcome by the modern period is not seriously disputed: mainstream Sunni legal practice today centres on qualified scholars working within an inherited school rather than on individuals claiming the full, unmediated authority to reason from the sources that a Shia mujtahid claims, even as reform-minded Sunni thinkers in the nineteenth, twentieth and twenty-first centuries have repeatedly called for a renewed exercise of ijtihad to address questions the classical schools never faced.