The pioneer in this field is The Splitter of Knowledge, Imam Abu Jafar Muhammad al–Baqir and his son Abu Abdillah al–Sadiq. They dictated the principles of this discipline and other relevant issues to a group of their students who collected and passed them over to the later generations who compiled and arranged them according to their topics into books, such as Kitab Usul Al al–Rasul, Kitab al–Fusul al–Muhimmah fi Usul al–A’immah and Kitab al–Usul al–Asilah. All these books consist of authentic narrations related by reliable scholars from the Ahl al–Bayt through continuous chains of transmission.
The first person to devote time for writing about specific topics on usul was Hisham ibn al–Hakam, the master of theologians and a student of Abu Abdillah al–Sadiq (‘a). He wrote Kitab al–Alfa wa Mabahithiha. It is the most important work written in this field. Then comes Yunus ibn Abdurrahman, a client of Al Yaqtin and a student of Imam al–Kazim, Musa ibn Ja’far (‘a). He wrote Kitab Ikhtilaf al–Hadith which deals with the contradiction (ta’arud) between evidences, the equivalence (ta’adul) between them and the preference (tarjih) of one over the other.
Al–Suyuti recorded in Kitab al–Awa’il that “All the scholars of the Sunni school agree that Al–Shafi’i is the first to write about the principles of jurisprudence.” It is similar in size and method of treatment is Kitab Usul al–Fiqh by Sheikh al–Mufid, Muhammad ibn Muhammad ibn al–Nu’man known as Ibn al–Mu’allim. Both works have been printed.
The most extensive book on the principles of jurisprudence of the earlier period is the comprehensive two–volume Kitab al–Dhari’ah fi Ilm usul al–Shari’ah by Sharif al–Murtada. He wrote numerous books on this subject, the best and most extensive of which is his Al–Dhari’ah. Better still, is Sheikh al–Tusi’s Al–Uddah, an unprecedented magnificent work characterised by accuracy of exposition and depth of treatment. It is worth mentioning that the Shi’ah have reached the peak of rigorous examination in this discipline and the issues related to it through the continuous efforts of successive generations. Some scholars were able to compile extensive treatises on particular issues in addition to the comprehensive works. The masters of this science are innumerable and thus, it is difficult to list even a single category of them.