I present an overview of On the Soul, Aristotle’s investigation into how psuchē (soul) explains biological phenomena in a unified way. This principle serves as a final, formal, and efficient cause of living activities. Soul needs specific consideration because it is a unique sort of form. It is responsible not just for giving living things their capacities, but also for when and how they exercise these capacities. Soul orders the ways in which living things grow, reproduce, move, and cognize the world. It accounts for all the more specific capacities and activities of the living thing. Studying soul thus gives Aristotle the opportunity to make some of his most subtle distinctions about kinds of capacity and activity. Aristotle’s discussion of soul as cause also prepares the way for considering how it works together with body, as Aristotle does in the Parva Naturalia and biological works. I then present synopses of the chapters in this guide and discuss how they relate to one another.