This article aims to explain the authority of reason from the perspective of Imam Sadiq (a.s.), and answers the question: According to Imam Sadiq (a.s.), what teachings in religion should be based on reason? By reflecting on the Quran, the Prophetic Hadiths, and the life of the Ahl al-Bayt (a.s.), it becomes clear that reason is one of the most important sources of knowledge in Islam. Imam Ja’far al-Sadiq (a.s.) is not only the main authority of Shiite teachings, but also a symbol of rationality, knowledge, and faith among other Islamic sects. With a religious study approach and based on the analysis of narrative sources, the Imam’s meaning of reason is an internal force and a level of the soul that is the source of perception and the means of recognizing good and evil. Reason, if followed, paves the way for the service of God and divine pleasure. According to Imam Sadiq (a.s.), principles such as the proof of God, divine attributes, the possibility and necessity of the resurrection, and prophecy are based on reason and evidence. His teachings emphasize the authority of theoretical and practical reason (in recognizing good and evil), although reason, due to its inherent limitations, is unable to fully encompass all areas.