This book examines the role and influence of Greek philosophy in the final days of the Roman republic. It focuses primarily (although not exclusively) on the works and views Cicero, premier politician and Roman philosopher of the day, and Lucretius, foremost among the representatives and supporters of Epicureanism at the time.

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.

We understand Aristotle’s soul–body hylomorphism better if we first understand the critical discussions of his predecessors which occupy most of the first book of his De Anima.  Given that he regards his view as preferable to all earlier approaches, he must also think that his alternative, hylomorphism, avoids the pitfalls he identifies in those positions. In some cases, it is easy to see why he might think hylomorphism is defensible where they are not: for instance, he regards the reductively materialistic views of the earliest natural philosophers as explanatorily impoverished. In other cases, however, this is far from clear. Aristotle highlights for special consideration the view that the soul is a harmonia (attunement) of the body, a view which, as was noted in antiquity, bears more than a passing resemblance to his own hylomorphism. It proves both difficult and instructive to determine, then, how he supposes hylomorphism avoids the problems he identifies in the doctrine of the soul as a harmonia. The core difference, it emerges, turns on Aristotle’s thoroughgoing teleology: the soul, he thinks, unlike a harmonia, has an intrinsic good toward which the body is orchestrated.

I argue that Aristotle thinks of perception as veridical, and that phantasia – as a secondary motion consequent on perception – is responsible for all sensory error. I neutralize passages where Aristotle seems to countenance misperception by defending what I call an “object-oriented reading,” which holds that though Aristotle says we can make errors about the objects of perception, he is not committed to thinking that we can perceive them erroneously, as there are faculties besides perception (including phantasia) that engage with the objects of perception. According to the object-oriented reading, apparent misperception results when a false phantasia is mistaken for a perception, something that is possible due to the similarities between perception and phantasia. Nonetheless, since the faculties are distinct, perception remains veridical. I also address how this conception of phantasia can explain Aristotle’s appeals to phantasia in contexts like memory, thought, and animal motion.

In De Anima II.6, Aristotle divides perceptibles into three kinds: “special” perceptibles such as colors, sounds, and flavors, which can be perceived in their own right by only one sense; “common” perceptibles such as shapes, sizes, and movements, which can be perceived in their own right by multiple senses; and “incidental” perceptibles, such as the son of Diares, which can be perceived only “incidentally.” In this chapter, I explain what this division amounts to. First, I argue Aristotle’s distinction between perceiving something in its own right and perceiving it incidentally marks a causal distinction: what is perceived in its own right causes perception as such, while what is perceived incidentally coincides with what is perceived in its own right. Second, I argue that, for Aristotle, special perceptibles, unlike common ones, belong to homogeneous bodies on account of their chemical composition and affect sense organs along a range between contrary extremes. Finally, I explain the primacy Aristotle assigns to special perceptibles and his claim that perception of them alone is free from error. I conclude with some brief reflections on the primary/secondary quality distinction.

Aristotle's De Anima discusses the psychological causes of what he calls locomotion – i.e, roughly, purpose-driven behavior. One cause is desire. The other is cognition, which falls into two kinds: thought (nous) and imagination (phantasia). Aristotle’s discussion is dense and confusing, but I argue that we can extract from it an account that is coherent, compelling, and that in many ways closely anticipates modern psychological theories, in particular Dual Processing theory. Animals and humans are driven to pursue objects that attract us. Objects take on that power when we cognize them as valuable. If we rely on imagistic, automatic, uncontrolled processing mechanisms – Aristotle’s phantasia, which closely anticipates the modern notion of Type 1 processing – our resulting desires and actions will be impulsive. If we rely instead on rational, critical, deliberative capacities – Aristotle’s thought, which closely anticipates the modern notion of Type 2 processing – our resulting desires and actions will be reflective. Animals are capable only of the first kind of behavior; the human psyche is constituted of an animal psyche united with an intellectual one, so we are capable of both.

This chapter addresses an interpretive question about why Aristotle identifies generation, growth, and nourishment as the three distinct functions or activities of nutritive soul. Scholars typically try to explain this by appealing to the shared goal of these activities, though there is no consensus about what that goal is: Does Aristotle think that generation is a way of keeping oneself alive (and thus that the shared goal is self-maintenance), or is nourishment really a quasi-generative activity (and thus that the shared goal is “form (re)production”)? Rather than taking that approach, Gelber offers a different but complementary way of accounting for the unity of these activities, by focusing on the continuity of their shared physiological basis. As it is argued here, the fact that these biological processes form a continuous cycle stems from Aristotle’s adherence, in his biological theory, to principles from his hylomorphic metaphysics. Attending to the details in works such as Generation of Animals that focus on the mechanisms underlying generation, growth, and nourishment, it is shown how we can construct a coherent account of the unity of the three nutritive soul activities.

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.

I examine the status of Aristotle’s science of soul and argue that it is trans-generic in the way that Aristotle's universal mathematics is. For just as the branches of the latter differ considerably, so too do the sciences of life: botany, zoology, psychology, and (in Aristotle’s view) astronomy and theology. Discovering the correct definition of soul, which is their starting point or first principle, as with other scientific starting points, involves both induction and dialectic. Induction uses scientific observation of living things to move toward this starting point. Dialectic enables the scientist to assemble endoxa, or reputable beliefs, that allow us to solve each puzzle (aporia) that clouds our understanding (nous) of the starting point that induction enables us to reach.