The Usuli believe that the Hadith collections contained traditions of very 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).
The dominance of the Usuli over the Akhbari came in last half of the 18th century when Muhammad Baqir Behbahani led Usulis to challenge Akhbari 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."
An important tenant 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 marji 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 the islamic taxes.