Different attempts to address the question of the definition of the human being (insān) reveal the tensions between various scholarly disciplines and intellectual trends in classical Islam. This article investigates the various definitions of the human being proposed by classical Muslim theologians, particularly Imāmīs who were influenced by Muʿtazilism. It focuses primarily on the works of al-Shaykh al-Ṣadūq (d. 381/991-2), al-Shaykh al-Mufīd (d. 413/1022) and al-Sharīf al-Murtaḍā (d. 436/1044). Though seemingly a question of subtle theology (laṭīf al-kalām; daqīq al-kalām), the essence of the human being is in fact intimately related to the main premise of grand theology (jalīl al-kalām), that is, the question of moral obligation (taklīf). For the concern of theologians in the discussion is to identify the subject of moral obligation, themukallaf, in every human being, to which they refer as the living and/or active being (al-ḥayy al-faʿʿāl; al-dhāt al-faʿʿāla). In their attempts to produce a satisfying answer, theologians relied on the contributions of thephilosophers, jurists, mystics and traditionalists, thus arriving at different answers that betray the extent of the influence of other scholarly disciplines on various theological schools at different points in time.