In Chapter 6, I argue that Aristotle suggests that the definition of soul as a harmony of bodily opposites nominally passes the requirement of providing a principle that explains the soul-body relation. However, by appealing to his demonstrative heuristic, he is able to show that no bodily, arithmetical, or geometrical harmonic properties are explanatory of any of the soul’s per se attributes. The results of this criticism, I argue, show that the soul’s power to act as an efficient and formal cause of the body, like in the case of Xenocrates, cannot be explained by identifying soul with mathematical form or geometrical organisation, nor with an emergent entity whose existence is derived from the mixed elemental powers. What Aristotle takes over from the harmony theory, however, is the idea that soul is plausibly a kind of non-mathematical form or determiner (λόγος), and the idea that (some kinds of) soul and living body are likely to be existentially interdependent. By this criticism, Aristotle is negatively constrained to affirm the Hylomorphic Thesis.