In the eighteenth century there was a dispute between the Akhbari and Usuli schools of Shia Islam. Briefly, the Akhbari believed that the sole sources of law were the Qur'an and the Hadith, in particular the Four Books accepted by the Shia: everything in these sources was in principle reliable, and outside them there was no authority competent to enact or deduce further legal rules. The Usuli believed that the Hadith collections contained traditions of very varying degrees of reliability, and that critical analysis was necessary to assess their authority. On this view, the task of the legal scholar is to establish intellectual principles of general application, from which particular rules may be derived by way of deduction: accordingly, legal scholarship has the tools in principle for resolving any situation, whether or not it is specifically addressed in Quran or Hadith. Traditionally these intellectual principles were analysed through the procedures of Aristotelian logic. The Akhbari scholar Muhammad Amin al-Asterabadi criticised this approach, arguing that since the alleged general principles were arrived at by way of generalisation from the existing practical rules, the whole process was circular.
The role of Behbahani
Analysis of probability
Behbahani led the Usuli intellectual challenge to Akhbari dominance in Karbala in the 1760s. With the defeat of the Safavid state following the Afghan invasion in 1722 and the rise of a new generation of Bahraini Akhbari clerics, state-centric Usulism lost its self-confidence. The most influential Bahraini cleric, Yusuf Al Bahrani, was installed as the dean of scholarship in Karbala, where he led an intellectual onslaught on Usuli scholarship in the mid-eighteenth century. Behbahani's role in the 1760s was at first to tentatively challenge Al Bahrani's neo-Akhbarism before building up the supporters, confidence to eventually lead an Usuli revival following Al Bahrani's death in 1772.
Behbahani's main contribution to legal theory was the analysis of probability. Granted that general principles cannot be arrived at with complete syllogistic certainty, there are still presumptions, sufficient in ordinary life, for deciding on the better view. For example, where there is doubt whether a given situation exists but it has certainly existed in the past, one can rely on a presumption that it is more likely to have continued the same than to have changed. Given the probabilistic nature of this approach, and the need for practical certainty, it followed that there must be people with the authority to decide on the application of these principles to particular facts. Accordingly, the role of Islamic jurist becomes one of political and judicial authority, and not merely one of scholarly expertise. Behbahani is blamed, in particular by Akhbaris, for having used physical force to enforce his authority and for having laid the intellectual foundations of Iranian Shi'a theocracy. The principles of probability were further analysed by Shaykh Murtada al-Ansari in the mid-nineteenth century, and the Usuli school remains the dominant force in Shi'ite Islam.
Takfir
According to scholar Moojan Momen, Behbahani played a very important role in Shii Islam by bringing in "the threat of takfir" — i.e. declaring the opponent an apostate, apostasy being a capital crime —
into the central field of theology and jurisprudence where previously only ikhtilaf had existed. Bihbahani was now to exclude by takfir all who disagreed with the principles of reasoning and ijtihad as sources of law. This paved the way for a great increase in the power and influence of the mujtahids in Qajar times and for the evolution of the concept of the marja at-taqlid.
Parallels in other religions
In Roman Catholic moral theology, a distinction is recognised between "direct principles" and "reflex principles". Reflex principles are presumptions to be applied in cases of doubt, such as the rule that where the moral right to something is unclear the law favours the person in possession. Such principles are widely found in Western legal systems.
