In this article the philosophical and religious heritage of the Zaydi ‘alimof the 13th century, al-Sayyid Humaydan ibn Yahya al-Qasimi, is investigated. For the very first time in the history of philosophical Islamic studies, the theory of knowledge of the thinker is considered: in particular it is proved that gnoseological constructions of as-Sayyid have anti-Mu‘tazilte character and sum up the result of Mutazilite’ philosophy domination in the XI–XIX culture of Yemen. Separately the original Humaydan’s doctrine about prerequisites of reliable speculation, which designating a knowledge limit by borders of a material universe, is described. Besides, the conceptualistic theory belonging to the theologian’s plume is given in this work: according to Humaydan, the subject comprehends the things in its attributes called as ‘truths’ of all created-by-God objects. As for the Mu‘tazilte’ categories, the ‘alimdevotes to it so many interesting passages of his apology, calling it a "blamed innovation", which seems false according to early Muslim thinkers. Also, the anthropology of the outstanding Zaydi philosopher is considered by the author of article: being a consecutive dichotomist, as-Sayyid assigns to soul a number of negative properties (as like as natural tendency to doubt and defect). Contrary to soul the reason, according to the theologian, is a divine virtue and the pure informative ability which is selectively granted by the Creator to the people. The diff erence between people’s reasons truly defines their degree of ‘right duty’.