The current Shii revival in much of the Muslim heartland finds its origins in early modern Iraq, where Shii scholars redefined clerical authority and their ability to produce binding legal judgments. The most important architect of this transformation was Mortaza Ansari, (1799-1864) whose texts are still considered seminal works in the Shii seminaries of Iran and Iraq. Hailed for his piety and erudition, Ansari became the first scholar ever to be recognized by mainstream Shiis as the single head of the international Shii community. As a result of his accomplishments in the field of Shii jurisprudence, Ansari was already considered by most Shiis to be the most knowledgeable jurist of his time. In fact, Shii historians have proclaimed that doctors of Islamic law after his time will always be operating within his school of thought, indicating that he brought Islamic legal theory to fruition. In his treatise on Shii law, entitled Matchless Principles (Fara'id al-usul), Ansari outlines the proper methodology for legal scholars to derive absolute knowledge from scriptural texts. Acknowledging that truth is not back and white, he divides legal rulings into four categories (certainty, conjecture, doubt, and error) and separates knowledge into two realms (absolute and subjective). Additionally, Ansari argues that all lay Shiis must choose the most knowledgeable living cleric to emulate and delineates specific criteria on which laymen must make their choice. Based on Ansari's Arabic and Persian writings as well as numerous commentaries on his work, my paper outlines the critical innovations of his conception of Shii leadership and legal theory. Further, it argues that these changes continue to be far reaching. Although authority has not often remained in the hands of a single living cleric after Ansari's death, each generation has been defined by no more than a few clerics competing for adherents. Ansari's solidification of a rationalist approach to law has allowed the Shii establishment to present a unified front at critical historical moments during the past two centuries. The establishment of clear lines of authority has also allowed high ranking clerics to collect and distribute charitable donations, which had not been done systematically since the ninth century. Further, Ansari's expansion of Shii law has allowed clerics to issue judgments on any legal question, not just those that can be found in traditional Shii sources.