Ikhwān al-ṣafā’ are one of the most significant groups of authors of Islamic thought. Their teachings became influential where philosophy and Sufism and reason and intuition came together. Ikhwān al-ṣafā’ adopted Neoplatonic philosophy to Islamic thought and affected the course of events and developments of Islamic philosophy and Sufism. Nevertheless, some researchers claim to have Ikhwān al-ṣafā’ evaluated issues superficially and not had original ideas and approaches. However, the fact that Ikhwān al-ṣafā’ throughout their thoughts constituted a consistent relationship between cosmology and epistemology is a valuable success. This article aims to examine how Ikhwān al-ṣafā’ approached their concept of reason (al-ʿaql). It is possible to evaluate the concept, reason, in the contexts of cosmology, morality, and epistemology. Ikhwān al-ṣafā’ also used this concept carefully and consistently by taking those contexts into consideration. Reason, which refers to an important communication channel between God and human beings, is the most human side of the soul. At the end of this research, we reached the conclusion that Ikhwān al-ṣafā’ accept even the intuition and revelation as rational activities.