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Usulis (Arabic: الاصولية) are the majority Twelver Shi'a Muslim group. They differ from their now much smaller rival Akhbari group in favoring the use of ijtihad i.e. reasoning in the creation of new rules of fiqh; in assessing hadith to exclude traditions they believe unreliable; in considering it obligatory to obey a mujtahid when seeking to determine Islamically correct behavior.
Since the crushing of the Akhbaris in the late 18th century, it has been the dominant school of Twelver Shi'a and now forms an overwhelming majority within the Twelver Shia denomination.
Usuli implies the doctrine of Usul which means the principle of the Jurisprudence, and Ilm al-Usul concerns with establishing the legal standards on the basis of Quran, hadith, Ijma' and Aql. Ijma' is the unanimous consensus. Aql, in Shia Jurisprudence, is applied to four practical principles namely bara'at (immunity), ihtiyat (precaution), takhyir (selection), and istishab (continuity in the previous state) which are applied when other religious proofs are not applicable. 
The Usuli believe that the Hadith collections contained traditions of varying degrees of reliability, and that critical analysis was necessary to assess their authority. In contrast the Akhbari believe that the sole sources of law are the Qur'an and the Hadith, in particular the Four Books accepted by the Shia: everything in these sources is in principle reliable, and outside them there was no authority competent to enact or deduce further legal rules.
In addition to assessing the reliability of the Hadith, Usuli believe the task of the legal scholar is to establish intellectual principles of general application (Usul al-fiqh), 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 (see Ijtihad).
An important tenet of Usuli doctrine is Taqlid or "imitation", i.e. the acceptance of a religious ruling in matters of worship and personal affairs from someone regarded as a higher religious authority (e.g. an 'ālim) without necessarily asking for the technical proof. These higher religious authorities can be known as a "source of imitation" (Arabic marja taqlid مرجع تقليد, Persian marja) or less exaltedly as an "imitated one" (Arabic مقلَد muqallad). However, his verdicts are not to be taken as the only source of religious information and he can be always corrected by other muqalladeen (the plural of muqallad) which come after him. Obeying a deceased taqlid is forbidden in Usuli.
Taqlid has been introduced by scholars who felt that Quranic verses and traditions were not enough and that ulama were needed not only to interpret the Quran and Sunna but to make "new rulings to respond to new challenges and push the boundaries of Shia law in new directions." Critics also say a major motive behind introducing this was to collect Islamic taxes.
By their debates and books, Al-Mufid, Sayyid-al Murtada, and Shaykh al-Tusi in Iraq, were the first to introduce the Usul of the Jurisprudence under the influence of the Shafe'i and Mu'tazili doctrines. Al- Kulayni and al-Sadduq, in Qom and Ray, were concerned with a traditionalist approach. The second wave of the Usulies was shaped in the Mongol period when al-Hilli used the term Mujtahid, the one who deduces the ordinance on the basis of the authentic arguments of the religion. By Ijtihad, al-Hilli meant disciplined reasoning on the basis of the shari'ah. By developing the principles of the Usul, he introduced more legal and logical norms which extended the meaning of the Usul beyond the four principle sources of Shari'ah. Amili was the first who fully formulated the principles of the Ijtihad. Bihbahani, as the founder of a new stage in Shia Jurisprudence, took a new practical method. He attacked the Ikhbaries and their method was abandoned by Shia. The dominance of the Usuli over the Akhbari came in the last half of the 18th century when Muhammad Baqir Behbahani led Usulis to dominance and "completely routed the Akhbaris at Karbala and Najaf," so that "only a handful of Shi'i ulama have remained Akhbari to the present day."
- Nasr, Dabashi & Nasr 1989, p. 284-285
- Momen, Moojan (1985). An introduction to Shi’i Islam : the history and doctrines of Twelver Shi’ism. Oxford: G. Ronald. p. 225. ISBN 0-85398-201-5.
- Nasr, Vali (2006). The Shia Revival: How Conflicts within Islam will shape the Future. Norton. p. 69. ISBN 0-393-06211-2.
- Nasr, Dabashi & Nasr 1989, p. 284-285
- Nasr, Dabashi & Nasr 1988, p. 230
- Momen, Moojan (1985). An introduction to Shi’i Islam : the history and doctrines of Twelver Shi’ism. Oxford: G. Ronald. p. 127. ISBN 0-85398-201-5.
- Twelvers / Ithna Ashari Islamic Schools of Thought
- Newman, Andrew J. (1992). "The Nature of the Akhbārī/Uṣūlī Dispute in Late Ṣafawid Iran. Part 1: 'Abdallāh al-Samāhijī's "Munyat al-Mumārisīn". Bulletin of the School of Oriental and African Studies (University of London) 55 (1): 22–51. JSTOR 620475.