This chapter highlights the close interconnection between cosmology and human nature in the Timaeus. According to Timaeus, human beings are not merely part of the cosmos; they play a crucial role in explaining how the cosmos came to be. The cosmos must contain three kinds of mortal beings in order to be complete, and all three derive from human beings, as a result of varying degrees of moral and cognitive failure. Recognizing the distinctive role human beings play in completing the cosmos complicates the standard picture of Timaeus’ cosmology, as well as his account of human nature. While in large part the cosmos is a product of divine craft, in some part it is the product of the inevitable disturbance of immortal souls due to mortal embodiment. Human beings have a special status as the first generation of mortal beings, as well as the only ones produced solely by divine craft. However, this distinction does not extend beyond the first generation, nor does it include any women. Ultimately, Timaeus’ account of human nature blurs the lines between humans and gods, as well as between humans and non-human animals.

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.

The chapter sketches a broad picture of some ideas, antecedent to Aristotle’s work, about the origin and development of living beings. Against the background of the new cosmological and metaphysical framework of Aristotle’s biological enterprise, it emphasizes what distinguishes Aristotle from the Presocratics and Plato: his rejection of a shared causal story that would account for both the origin of the universe and the birth of animals and plants. This shift helps to make intelligible Aristotle’s rejection of hylozoism and of the opposite view that life arises, mysteriously, from inanimate material ingredients. To demonstrate that Aristotle discusses the biological views of his predecessors without directly using them to build his own theory, the chapter first turns to Presocratic fragments, mostly of Anaximander and Empedocles, which connect biological matters and cosmogony. Second, the chapter takes a fresh look at how Plato reshapes this connection in his Timaeus, offering a new account of the nature of the universe and the nature of human beings. This account then enables us to evaluate, in the chapter’s final section, the changes that Aristotle brings to the study of living beings, including his rejection of the notion of the latter’s progressive formation.