Up until the 6th century AH, ijtihad, as a concomitant of ra’y and qiyas, failed to gain acceptance among the Imami fiqhi community. However, in the 7th century AH, a new development in the school of Hillah served as a watershed in the history of Imami approach to ijtihad. There, Muhaqqiq Hilli succeeded in formulating a new definition of ijtihad free from reliance on qiyas, and thus one that was acceptable to a wider community of jurists. He went as far as considering the Shi`ites as the Partisans of Ijtihad (ashab al-ijtihad). Here, it should be borne in mind that what took place in Hillah went far beyond a lexical correction, for the revival of ijtihad among the circles in Hillah was reflected both in the area of fiqhi theorization and in the elaboration of practical issues.
Though Hilli scholars are mainly viewed as heirs to the legacy of the theological schools of Baghdad as well as Shaykh Tusi, their works reflect much that is new in terms of fiqhi theories. The concept of the “consensus of the community”, which in the earlier fiqhi school of Baghdad served as a guiding principle and as a factor that through bolstering the interpretations of the Qur’an and akhbar al-ahad (sing. khabar al-wahid) elevated the fiqhi ruling from the level of a mere personal opinion (zann), lost its prominence as an effective method of deduction. Thus, in the Hilli fiqhi school, ijma` lost its status as mediator between the mufti and the legal ruling, and given the inevitable uncertainty (zann) that accompanies many fiqhi sources, the deduction of secondary rules from less than definitive sources became a practice of increasing importance, so much so that the type of ijtihad carried out in Hillah may be defined as “an endeavor (the literal meaning of ijtihad) to the best of one’s ability to reach an opinion with regard to a shar`i matters of less than certain status (zanni)”.
An increased emphasis on the authority of the literal aspect (zahir) of the Qur’an, which was one of the consequences flowing from the new Hilli approach to zanni sources of reasoning – a factor that resulted in the compilation of a large body of works by Hilli scholars on ayat al-ahkam (the Qur’anic verses whose contents are conducive to fiqhi rulings), may be viewed as a return to a textual-based method of reasoning. However, the distinctive aspect of the ijtihad of Hilli faqihs is reflected less in their application of textual methods than in their employment of formal methods of reasoning. Among the most significant manifestations of this formal approach in `Allamah Hilli is his method of treatment of akhbar al-ahad. By making use of the hadith criticisms (al-jarh wa ’l-ta`dil, lit. disparaging and declaring trustworthy; in the science of hadith, a technical phrase used regarding the reliability or otherwise of traditionists) contained in ancient Imami works of rijal (lit. men, the science of determining the trustworthiness of the transmitters figuring in isnads), together with the common classifications of hadiths in Sunnni works, he devised a four-grade system for the rating of the hadiths used in fiqhi deliberations: sahih (sound), hasan (fair), muwaththaq (trustworthy), and da`if (weak). Based on this quadripartite system Hilli carried out a critical analysis of the traditional methods of Man la Yahzarahu ’l-faqih, al-Tahdhib and al-Istibsar, based on a formal and quasi-mathematical approach.