The Shi'is call Abu Ja'far Muhammad al Tusi (d. 460/1068) the 'Elder of the Denomination' (Shaykh al-ta'ifa), since he is the founder of Shi'i demonstrative jurisprudence (al-fiqh al-istidlali); in other words, he was the first person to give Shi'i law a systematic basis.
Kitab al-tahdhib and Kitab al-istibsar are two masterpieces by Shaykh Muhammad Tusi