This article examines and comparatively analyzes the rational arguments of Sheikh Saduq and Sheikh Mufid regarding the necessity of the existence of an infallible Imam as the successor to the Prophet on earth. The issue of succession has been considered in various fields of religious knowledge, but by comparatively examining different viewpoints on this subject, new results can be achieved. According to Sheikh Saduq and Sheikh Mufid, the existence of a successor on earth is a necessary, rational, and divinely ordained matter, but there are noticeable differences in the rational arguments of these two theologians. Sheikh Saduq, using the rational principles of his school, believes that the existence of an infallible Imam is essential for preserving and continuing divine guidance and preventing religious deviations, and he presents succession as a trust bestowed by God. By referring to verses and using rational arguments, he explains the necessity of the existence of a divine proof at all times and elucidates the role of the infallible Imam in guiding the Islamic community. In contrast, Sheikh Mufid, using rational arguments, proves that the infallible Imam, as the successor to the Prophet, plays a vital role in preserving the Sharia and explaining divine laws, and he introduces the Imam, due to being the Prophet's successor, as possessing the attributes of the Prophet and the previous infallible Imam