The function, domain and boundaries of reason are important in Islamic legal thought. Reason and many issues related to it were discussed in the jurisprudential works of the earlier generations (muteḳaddim) in connection with various other issues. We find a lot of information regarding the function and boundary of reason in the works written during the formative period of religious law in the discussions on the condition of actions in societies with no existing shari‘a law.  In the debates which is the most concrete results of the discussions about the problem of good and evil, the views of various sects about the function of reason become more visible. Apart from the discussions about whether the time before the coming of sharʿī is imaginary (hypothetical) or real, in the light of the views of the sects on this issue, we get some information as well about the source of judgment in Islamic legal thought, the beginning of law (or obligation), and the relationship between law and ownership. With this debate between the Ash‘arites who lay down the existence of Shari‘a as a condition for fundamental rights such as security of life, right of property and freedom to become legal obligations with binding otherworldly consequences and the Mu‘tazilites who assert that reason by its nature can attain some religious and legal truths with its own capacity, we also find an opportunity to express some opinions about the existence of notion in Islam regarding natural law. Our discussion which contains both jurisprudential and theological aspects is limited only to this period in which the views of the sects are more clearly visible.