Chapters XI and XII of De mixtione have been read as a digression from the main argument of the treatise. In the following, I will show that what takes place in IX–XII is not at all secondary regarding the issue of blending, or, more generally, regarding Alexander’s opposition to Stoic philosophy. In my view, chapters IX–XII aim to produce a more fine-grained account of blending. They set the stage for the first requirement of blending in chapter XIII: that there is blending only of corporeal substances, i.e. of independently subsisting entities. To accomplish this, chapters XI–XII must bring their investigation up to the nature of the Stoic principles and criticize the Stoic notion of body. This is why Alexander must examine the fundamentals of Stoic ontology. It also explains why these chapters, despite being essentially refutative, make explicit some of the main claims of Alexander’s own ontology. In these chapters, Alexander makes us pivot smoothly from a Stoic ontology to an Aristotelian one.