The Imams (peace be upon him) had a fundamental role in the emergence of the knowledge of Islamic Jurisprudence, especially in the time of Imam al-Baqir and Imam al-Sadiq (peace be upon them both). They taught their companions the methods of deduction from the Qur’an and Sunnah, and how to refer the Usul (roots) and Fur’oo (branches) of the religion. They used two methods:
The first: Explaining to their companions, the general rules of deduction, which they wrote done. The four hundred Usool (roots) are the outcome of these written records and represent the raw sources of the four books of Usool (origin) of the Shi’a, and, were the hopes of the Imams, peace be upon them.
The second: Teaching their companions the correct methods of elicitation through practice and practical application. On the authority of Imam Al-Sadiq (peace be upon him) who said: “We present you with the Usool (roots), and you establish the Fur’oo (branches).”
Some of the Imams’ companions (peace be upon them) wrote letters on fundamental issues. One example is Hisham ibn al-Hakam, one of Imam al-Sadiq’s companions (peace be upon him) who composed “The book of meanings”. Another is Yunus ibn Abdul Rahman, the student of Imam al-Kadhim (peace be upon him), who compiled the book “The differences of Hadith and their Issues”. A study that teaches the reader about opposing evidence and how to prioritise one evidence above another in matters of religion.