The nature and purpose of the principles of uṣūl al-fiqh have been the subject of increasing scholarly attention in recent decades. Some of those studies suggested that uṣūl al-fiqh was primarily concerned with the establishment of a formalist model of deduction of legal rules from textual sources, which created the fiction that the legal system operated independently of any subjective considerations. This paper will attempt to problematize this view by presenting a case study in which uṣūl al-fiqh’s epistemological framework structurally relied upon a notion of personal moral improvement. This argument will be made by analyzing the concept of taklīf (loosely translated as “legal capacity”) in the uṣūl al-fiqh work of the influential Mālikī-Ashʿarī judge Abū Bakr b. al-Bāqillānī (d. 1013), and by following the evolution of this concept in the writings of Imām al-Ḥaramayn al-Juwaynī (d. 1085). Unlike the highly formal conditions of legal capacity in most modern systems (such as age), the conditions for taklīf centered on matters internal to the human mind, and largely concerned the legal subject’s state of consciousness. This study will argue that the pronounced concern with the legal subject’s state of mind followed from the premise that the legal system was designed to achieve a pietistic goal, which consisted of the ideal of “rapprochement” to God (qurbā). Between the revelation of the divine moral order in spoken form (khiṭāb), and the ascription of a moral judgment (ḥukm) to actions, taklīf stood as a necessary condition for the articulation of legal knowledge production and compliance. Taklīf was largely defined as God's imposition of a normative bond (kulfa) on the actions of humans. Unlike command (amr), taklīf was a concept that pertained to the attributes of a particular person, rather than the normative state of an action. As a result, the question of taklīf in Ashʿarī jurisprudence centered on each legal subject’s mental state, and the extent to which it allowed him or her to be a bearer (mukallaf) of divinely-imposed duties. It will be shown that “bearing” a duty was understood as the ability to perform the action at a given point in time while fully conscious of its moral implications. On the basis of this analysis, the paper will conclude that the concept of taklīf ensured that the legal system was structured in a manner that aimed to satisfy its central moral purpose.