The article deals with philosophic and religious study of the heritage of Mutazili Ibrāhīm ibn Sayyār al-Naẓẓām (d. about 845), a prominent scholar of Baṣran School of Kalām (Arab-Jewish Rational Theology). The study is based on two newly discovered treatises: A Study in Arguments by Mutazili thinker ʾAbū al-Ḥusayn al-Baṣrī (d. 1085; published by S. Schmidtke and W. Madelung in 2006) and an outstanding encyclopedical work The Commentary on ‘Sources of Questions and Answers’ (pt. IV) by Zaydi philosopher and publicist al-Ḥākim al-Ǧiššamī (d. 1101; published by F. Nofal in 2021). The author specifies a number of provisions of the doctrine of the medieval Arab thinker, known to specialists for the late doxographic works of the classical era (10-17 A.) to complement his fundamental study published in 2015. The first part of the issue represents the bibliographical history of sources the study is based on. The second part is devoted to the most important elements of al-Naẓẓām’s physical and theological doctrines. In particular, there is theoretical substantiation of the thesis of absolute divine justice developed by him. Additionally, details of his reasoning about being are revealed, a new passage is published, explaining the al-Naẓẓām theory of the leap — the first Arab physical theory, opposed to autochthonous Muslim atomism. Furthermore, for the first time in the history of world oriental studies, the doctrine of al-Naẓẓām about cognition and sensory perception is being reconstructed. The nature of existence, according to al-Naẓẓām, is determined by a separate attribute created by God and merged with sustainable thing (ʻayn). On the other side, all phenomena of created being are available to the person due to the sense of the soul which examines existed things simultaneously and forcefully. Therefore, he believed that perceptive and discursive types of knowledge can be mutually convertible or created by the human or God. Particular attention in the article is paid to the physical and anthropological views of the philosopher. The author concludes: al-Naẓẓām rejected an abstract paradigm of his predecessors (al-ʿAllāf's, in particular) and tried to construct a totally physic-based anthropology, pronounced in material union between the slim substance of the soul and the rough substance of the body — two necessary elements of human being. The connection between early Muslim philosophical schools is also considered: the given material enables us to discuss real relations between al-Naẓẓām’s theories and Ghaylanite doctrine, as well as assume its influence on al-Fārābī and al-Naǧǧār ontological and gnoseological teachings.