In DA I.1, Aristotle asks whether nous (understanding or reason) is chōristē (separable) and presents a separability condition: the soul is separable if it has some activity proper to it that is not shared with the body. I argue that Aristotle is speaking here of separability in being, not separability in account or taxonomical separation. In the case of the soul, this sort of separability would allow the soul to exist apart from the body. Met. Λ.3, GA II.3, and DA III.4 suggest that Aristotle introduces the separability condition because understanding meets it. Reason is independent of the body in a way that no other power of the soul is. Nous alone is divine and separable. DA III.5 then situates this claim: there is an aspect of understanding that can only be active in connection with the body, but understanding is what it is and continues to be active apart from the body. This raises further questions about the life and ontological status of the sort of separated human nous Aristotle envisages. While figuring out its precise contours is difficult, Aristotle is, in fact, committed to the possibility of human intellectual activity continuing apart from the body.

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.