Jafar al-Sadiq taught in Medina during the decades that saw the Umayyad caliphate fall and the Abbasid caliphate rise, a period in which political upheaval left scholarly circles comparatively freer to develop their own authority. Twelver Shia tradition counts him the sixth Imam, a lineal descendant of Ali ibn Abi Talib and Fatima, and holds that each Imam inherited not only the right to lead the Muslim community but also a special, divinely supported knowledge of the faith's true meaning. His teaching circle in Medina was large and by report included students later claimed as founders by Sunni schools of law as well as by the Shia tradition, evidence of how fluid the boundaries between emerging legal schools still were in the eighth century.
The school that carries his name diverges from the four Sunni schools chiefly in its sources. Where Sunni jurisprudence draws on the Quran, the sunna of the Prophet as preserved in hadith, and reasoning tools such as analogy and consensus, Jafari jurisprudence adds a fourth primary source, the transmitted sayings and rulings of the twelve Imams, treated as an authoritative extension of prophetic teaching. A ruling attributed to Jafar al-Sadiq or to another Imam therefore carries an authority in Jafari law that no scholar's opinion, however learned, carries in the Sunni schools.
Codification continued long after Jafar al-Sadiq's own lifetime. The occultation of the twelfth Imam, whom Twelver Shia Islam holds to be hidden rather than dead, removed a living Imam from direct access from the tenth century onward, and it was this absence that pushed the school toward its enduring reliance on ijtihad, structured legal reasoning exercised by qualified scholars acting in the Imam's place until his return.