Aql: Reason Stands Beside Quran, Sunna and Consensus as a Source of Law

Sunni jurisprudence settled, after Shafii's systematic work, on four roots of law: Quran, the Prophet's Sunna, the consensus of the community, and analogical reasoning, qiyas, extending a known ruling to a new case that shares its underlying cause. Twelver Shia jurisprudence kept the first three roots and replaced the fourth with something structurally different, aql, reason. Moojan Momen's history of Twelver doctrine explains that this was never an invitation for a jurist to rule however seemed sensible to him. Classical Usuli theory holds a specific principle, al mulazama, that whatever reason judges to be necessarily entailed by revelation carries the same authority as revelation itself; aql functions as a tool for uncovering what the divine law logically requires where the texts and the Imams' own transmitted teaching are silent, not as an independent source competing with them. The practical difference from qiyas is real. Analogy reasons from one specific textual case to a similar one; rational judgment in Twelver theory reasons from premises the tradition already holds to be certain toward their necessary consequences, a narrower and, its defenders argue, a more disciplined operation than extending a single verse or hadith by resemblance.

Twelver jurisprudence did not arrive at its reliance on reason without a fight fought entirely within its own tradition. From roughly the seventeenth century, a movement called the Akhbaris argued that Shia law should be drawn only from the transmitted reports, akhbar, of the Prophet and the twelve Imams, and rejected independent juristic reasoning, ijtihad, and its use of aql as an illegitimate import from Sunni method that had no place in a school built on infallible, directly transmitted guidance. Their opponents, the Usulis, held that the Imams themselves had authorized rational inference within fixed bounds, and that abandoning it would leave believers with no way to resolve cases the transmitted reports never addressed. Moojan Momen's account of the dispute traces its resolution to the work of Muhammad Baqir Bihbahani in the eighteenth century, whose forceful campaign against Akhbari teaching in the Shia shrine cities left Usuli method dominant across Twelver Islam by the following century. The institution of the marja al taqlid, the living jurist ordinary believers are required to follow, is itself a product of that Usuli victory: it presumes a jurist qualified to exercise reasoned ijtihad, which is exactly the authority Akhbari teaching had denied anyone but the Imams themselves.