Like many scions of scholarly families in Mamluk Damascus, the jurist, physician, and mystic Ibn Ayyūb al-Qādirī (c. 1380–1464 CE) moved to Cairo as a young adult to pursue a career in law. He ultimately failed to establish this career, but Ibn Ayyūb nevertheless impressed the Cairene scholarly elite with his wit, candor, and writings on medicine and natural philosophy. Though he remains relatively unknown to modern historians, at least three notable Mamluk chroniclers––Ibn Taghrī Birdī, al-Biqāʿī, and al-Sakhāwī––wrote obituaries recounting his professional accomplishments and outstanding moral qualities, uniformly praising him as a circumspect and valuable colleague. This paper will bring this important scholarly figure to the further attention of historians of the Mamluk era by analyzing passages of Ibn Ayyūb’s most important surviving treatise on natural philosophy. Entitled “Blocking the Means of Harm Caused by Teaching the Causal Efficacy of Natures,” the treatise remains in only partially-studied Arabic manuscript. It nonetheless indicates the varied subjects that Ibn Ayyūb’s readership believed to be important areas of scholarly inquiry, offering vivid, idiosyncratic explanations for human morphology, psychology, and sexuality. To this end Ibn Ayyūb frames “Blocking the Means” as a didactic as well as homiletic treatise, serving to introduce novice students to the wide-ranging implications of natural causal efficacy––or the doctrine that natures are themselves sufficient causes for their effects, and as such do not require the intervention of God to produce them. Chief among the concepts Ibn Ayyūb invokes to explain this topic as it pertains to human bodies is innate heat, an elemental power that varies in its strength from person to person. Ibn Ayyūb argues that this power is responsible for, inter alia, the mechanics of erection and ejaculation, the high incidence of lesbianism in the scholarly families of Egypt, secondary sex characteristics specific to Arabs more generally, and the capacity for some human beings but not all to be converted to Islam. In light of Ibn Ayyūb’s reputation among his peers as a model scholar and moral exemplar, documenting the strong explanatory power he ascribed to innate heat will illuminate the diverse repertoire of analytic concepts available to the medieval Muslim scholars as they sought to understand bodies in the world around them. Moreover, it will demonstrate the continuing relevance of natural philosophy to the Islamic scholarly elite in the era of the discourse’s alleged decline.