This chapter explores Aristotle’s criticism of the Platonic idea of a cosmic soul as a first principle of motion, in the theory of animal voluntary motion that heoffers in theDe Motu Animalium. According to Aristotle, animal self-motion and the movement of the heavens are alike in that they both depend on an unmoved mover. But it is not immediately clear how this comparison works in detail, since for Aristotle the unmoved mover in animal motion is not directly an external object of desire but the animal’s thinking about an object of desire. Hence, there must, for Aristotle, be some parallel thinking involved in the movement of celestial bodies. Such an account, however, is missing from the De Motu Animalium. To find one, we need to consider the metaphysical cosmology set forth in Lambda, chapters 6–10 of Metaphysics, which posits a soul for each of the moved heavenly bodies, a soul which thinks of the sole absolutely unmoved mover of the universe, indesiring it through a form of rational desire. Thus Aristotle departs sharply from both Plato in the Timaeus and the subsequent Platonic, Stoic and Neoplatonic traditions, according to which celestial motion is not be explained by individual souls in each of the moved celestial bodies but by a single soul of the cosmos as a whole , located at the outermost sphere of the cosmos.