The time period between the third and fifth century AH, of the Twelver Shiite theology has been known as the systematic transformation stage, as this stage has witnessed a qualitative change in the deductive approach in theology. Sheikh al-Mufid and Sayyid al-Murtadha had a big role in establishing this so-called systematic transformation stage with its features and foundations. In this stage, syllogism was a general standard in most of theological studies. From the most important reasons that led to the development in the tools of the theological approach at that stage was the suitable ground that encouraged rational topics and studies in the Twelver Shiite thought, thanks to a good group of Shiite theologians, who had inherited scientific legacy in theological researches from the theologians of the [religious] Text Era. And also because of the common belief among the theologians of that stage that there was no legal prohibition against the benefiting from these cognitive tools in doctrinal fields, as long as they were rational tools acknowledged by human mind. In this article, we shed light on the role of Sheikh al-Mufid and Sayyid al-Murtadha in developing the mechanisms of theological reasoning. In this study, we make use if the descriptive, analytical approach.