This research examines the relationship between the Divine Law (Sharī‘ah) and practical reason in the thought of Khwajah Naṣīr al-Dīn Ṭūsī. Employing a descriptive-analytical method, it first elucidates the nature of the juridical-religious decree (ḥukm-e shar‘ī) and the criteria of good (ḥusn) and evil (qubḥ). It then investigates the role of practical reason as an independent source in ethical cognition. The findings indicate that while Khwajah Naṣīr acknowledges the capacity of practical reason to apprehend universal ethical principles, he emphasizes the necessity of prophetic guidance to complete human knowledge and to reveal decrees beyond the reach of reason. From this perspective, the Divine Law (Sharī‘ah) and practical reason are two complementary elements on the path to the actualization of human felicity (sa‘ādah). Ultimately, this study analyzes the status of practical reason as an independent source that nevertheless requires the guidance of the Sharī‘ah, and it evaluates the implications of this viewpoint in the domain of inferring juridical-religious decrees (istinbāṭ al-aḥkām).