The soul is an explanatory principle of Aristotle’s natural science, accounting both for the fact that living things are alive as well as for the diverse natural attributes that belong to them by virtue of being alive. I argue that the explanatory role of the soul in Aristotle’s natural science must be understood in light of his view, stated in a controversial passage from Parts of Animals (645b14–20), that the soul of a living thing is a “complex activity” of its organic body. This paper explores the role of this “complex activity” model of soul in Aristotle’s study of soul in De Anima. I argue, first, that the model has its origins in De Anima II.4, where Aristotle argues that living things do all they do by nature for the sake of a single, teleologically primary end. I argue further that Aristotle uses this model to account for the psychological attributes naturally present in living things, including their capacities for vital activities like nutrition, reproduction, and perception, and that this is the task to which Aristotle devotes the obscure final chapters of De Anima III.