This article analyses Avicenna's Ḥayawān III, 1, which deals with the well-known disagreement between physicians and philosophers on the origination of blood vessels (arteries and veins) and nerves. However, the proposed analysis is not limited to this chapter and its main topic. The more general purpose of this article is to reconstruct the psycho-medical context in which Avicenna's exposition lies, that is, the soul's oneness and the consequent conditions for body ensoulment (i. e. the soul's need for a primary, unitary attachment to the body through the heart and the cardiac pneuma). The article then outlines the strategy through which Avicenna presents medical positions (heart, brain, and liver are all on an equal footing) that challenge his (and Aristotle's) anatomical model, which is coherent with his theory of the soul. In this connection, firstly, the article shows how Avicenna takes physicians’ arguments apart in a philosophical context (he usually points at their logical shortcomings). Then, it clarifies the contribution of anatomy to determine the conditions of body ensoulment and, ultimately, how to reconcile medical practice with philosophical truths, if need be.