De Boer's chapter addresses the main methodological issue raised by Buridan's questions on Aristotle's first book: the place of the De anima in the commentary tradition on Aristotle's natural philosophy. The chapter argues that the fourteenth-century tendency to raise issues concerning the relationship of the mixture of the Aristotelian elements and the substantial form (or substantial forms) of the resulting mixed body in the context of discussions of nature of the soul culminated in Buridan's unified natural philosophy in which the themes from De generatione and De anima supplement and reinforce each other.