As the title indicates, the main purpose of this paper is to consider how the early mystics dealt with the essential concept of reason. To this end, we have examined forty works either written on or including the ideas of the mystics living in the first five centuries of the Islamic era. The findings are then placed in ten main categories: Reason and one’s knowledge of god; Reason and the Koran; Reason and Riligion; Reason and Ethics; Reason, this world, and hereafter; Reason and Mystical Hierarchies; Reason and Mystical states; Reason and Love; Reason and soul. Each category has then received its due attention. The paper intends to delineate how these mystics discuss divergent issues related to knowledge, this world, the hereafter and mysticism with reference to reason. Also, attempts are made to shed lights on the relationship between them. It must be admitted that these mystics had a limited view of reason; however, the very reference to reason in their arguments points to their concern about this concept. In fact, in their delineating of the limitations of reason in different realms of knowledge, they have resorted to the same notion (i.e reason). Although our main concern here is issues related to reason in one way or another, one can note the extent and diversity of knowledge theses mystics have touched upon in their   works.