In Chapter 5, I show how Aristotle appeals to his demonstrative heuristic in criticising the second head of the Platonic Academy, Xenocrates, for his inability to give a plausible theory of how the soul, defined as a ‘self-moving number’, can be instantiated in the body so as to move it. What Aristotle gains from this obscure set of criticisms is the idea that some kinds of formal entities, namely, mathematical forms, such as points and numbers, are not explanatory of the soul’s motive powers or affections. This places a negative constraint on his theory of soul that leads him to affirm the Hylomorphic Thesis.