Reason has nonpareil role both for making the components of cognition and obtaining divine knowledge in Shia schools of thought. In epistemology, we would have very few certain prepositions which could be comprehended without mediation of intellect. It is intellect with which foundation of cognition will be formed and the constituents of cognition will be built. In obtaining divine knowledge, it also has a unique function alongside the holy book, tradition, intuition and consensus. Because of the crucial role of reason in Shia school of thought, scientific currents are more rational comparing to Sunni. Schools of philosophy, rational theology, exegeses and principles of jurisprudence are advent in Shia cultural context. We can even say: moderate rationality is the main stream of Shia current that is far from Akhbarits and Tafkiki School, which are exceptional in Shia school of thought. Therefore, religious beliefs, moral values and practical commandments in Shia thinking are more rational. Although reason and rational thoughts were not so important at the time of the Infallibles because they were considered as manifestation of truth, at the occultation age, reason obtained its necessity alongside the text. Thus, it can be said: moderate rationality is an outstanding feature of Shia in Islamic history.