The first part of this article presents al-Mustamlī al-Buḫārī and his work, a voluminous commentary on al-Kalābāḏī’s compendium of Sufi doctrines, al-Ta‘rruf limaḏhab al-taṣawwuf. Both al-Kalābāḏī and al-Mustamlī had strong tendencies to the discussions of kalām, and of the two al-Mustamlī wrote extensively on theological issues in his commentary, Šarḥ al-Ta‘arruf. In light of the presence of topics of kalām in al-Mustamlī’s book, this article will demonstrate that despite his geographical proximity to Abū Manṣūr al-Māturīdī’s theological school, al-Mustamlī was probably (...) a follower of Abū l-Ḥasan al-Aš‘arī. The second part of the article concerns al-Mustamlī’s discussion of the nature of spirit. It will be argued that in dealing with the nature of the spirit, al-Mustamlī goes beyond the physicalist anthropology of the kalām and presents an argument for the existence of the spirit that has some affinities with Avicenna's proof of the existence of human rational soul, as it appears in the context of his famous thought experiment, the “flying man”. In both parts, the main claim of this article is that al-Mustamlī’s book is a valuable source for understanding the intellectual history of Transoxiana, and the intricate interactions of different disciplines, kalām, falsafa, taṣawwuf with each other. (shrink)