In the opening lines of the Qānūn fī l-ṭibb (Canon of Medicine) Avicenna outlines the epistemological status of medicine: it is a derivative natural science, whose epistemological underpinnings are given in natural philosophy - the theoretical science to which medicine is said to be subordinated -, and their investigation is declared off-limits to the physician. In providing the theoretical setting of the medical investigation in the first book of the Qānūn, Avicenna lists the things that the physician must accept on authority because their existence has been already ascertained elsewhere (i. e. in natural philosophy). Among those things there are the psychic faculties, their existence, their number, and their location. Consequently, in dealing with the diseases related to and affecting the psychic faculties, Avicenna has to assume their ascertainment provided in natural philosophy and, notably, in psychology. Nutrition and the nutritive soul seem not to escape this paradigm: Avicenna provides a formal account of nutrition in the Kitāb al-Nafs (Book of the Soul), i. e. the psychological section of the Kitāb al-Šifāʾ (Book of the Cure or the Healing), and its physiological account in the first book of the Qānūn. However, is it really indisputable that the physiology of nutrition provided in medicine supplements its formal account provided in psychology, and is subordinated to it? A close inspection of the texts devoted to nutrition and the nutritive soul reveals that, with respect to the psychic faculties, medicine is not entirely subordinated to the conclusions of natural philosophy, but it integrates them with another theoretical framework inherited by the previous medical tradition.