The monograph is primarily concerned with the deep structure of the study of perishable life as envisioned by Aristotle and Theophrastus. This entails giving full attention to a few transitional passages where Aristotle and Theophrastus offer their own account of what they are trying to accomplish in their own works. The subtitle Aristotle and Theophrastus on Animals and Plants has been chosen for three reasons. The first and most obvious is that two separate corpora of writings have been transmitted to us: one by Aristotle on animals and another by Theophrastus on plants. The second is that Aristotle and Theophrastus engaged in a study of perishable life via separate studies of animals and plants. What we know about the biological discourse before Aristotle and Theophrastus suggests that this approach to the study of perishable living beings was an important innovation. There is a third and final reason for the subtitle, which is also the least obvious: there is a precise order in which animals and plants were studied in the early Peripatos—namely, first animals and then plants. In his monograph, Falcon takes this order of study very seriously. It explores the reasons that may have led Aristotle and Theophrastus to adopt it.

There are a few questions prompted by this idiosyncratic approach to the phenomenon of perishable life. The first, and most pressing, is what licenses this strategy. The monograph argues that the Peripatetic decision to approach the study of perishable life via separate studies of animals and plants can be traced back to Aristotle’s claim that perishable life manifests itself in the form of plant and animal life (with human life as a special kind of animal life). The second is whether separate studies of animals and plants really exhaust the study of perishable life. Falcon answers this question in the negative. He shows that there is room in the Peripatetic scientific enterprise for a study of what is common to animals and plants in addition to separate studies of animals and plants. In the end, the Peripatetic study of perishable living beings as envisioned by Aristotle and Theophrastus turns out to be a complex scientific endeavor consisting of three distinct but related components: a study of what is common to animals and plants followed by separate but coordinated studies of animals and plants. However, what they are able, or willing, to say in common for animals and plants qua perishable living beings is limited by their own overall strategy. This is an interesting conclusion and is also a conclusion that prompts at least another question: Why is there a single science of living beings rather than two separate studies of animals and plants? In his book, Falcon shows it is analogy that allows us not only to bridge this gap but also to speak of a single science of perishable living beings.

On the basis of TIDA’s insights concerning the fundamental role of the De anima for Aristotle’s science of living things, the book offers a novel reading of one of the most difficult stretches of text in the whole Aristotelian corpus, De anima III 4–8. The argument of the book shows that this stretch of text contains a single and coherent account of the essence of the human capacity for thought. Aristotle gives this capacity the name of nous. Nous is the first principle, and ultimate explanans, of the phenomena of human thinking. What is new and distinctive about the interpretation advanced in this book can be introduced with the help of four key ideas. They apply to nous understood as the first principle, and ultimate explanans, of human thinking. They are: (i) separatism, (ii) direct realism, (iii) embeddedness in the cognitive soul, and (iv) rationalism. Let us briefly review these ideas.

1. Separatism. Human nous turns out to be a very special kind of power to the extent that it is featureless apart from being capable of receiving the essences of things: It is unmixed with the body, has no dedicated bodily organ, and is, in a precise way, separable from the body. As a result, human nous cannot be assimilated to any of the other parts of the soul. While the definition of the latter always requires a reference to motion and to a certain kind of body, nous does not: it is separate from motion and body in account, and so it cannot be a part of nature. Separatism implies that nous, while belonging to our essence as human beings, is not part of the natural world, which is a world of material extension and motion; in particular, human nous is not to be equated with the rest of the soul as the first actuality of an organic body that is potentially alive. Furthermore, the fact that nous is separate from the body and nature, on Aristotle’s account, is explanatory of the cognitive function that Aristotle assigns to it. Nous can think all things, in a completely unrestricted way, as they objectively are, including nous itself. An. III 4–8, we argue, offers an explanatory account of the characteristic features of human thinking. It does so by appeal to the immaterial nature of human nous and the existence of a separate and self-thinking immaterial substance.
2. Direct realism. Aristotle’s account of acts of human thinking is as direct a realist account as one can be. The immaterial essences of things come to be literally present in the thinking soul. Human nous, due to its featurelessness, is the power of receiving the essences of all things. This means that human nous is not restricted to any particular category of being, as is the case in perception, which is only perception of qualities. Instead. human nous can receive essences present in all categories.
3. Embeddedness in the cognitive soul. Nous as the principle and ultimate explanans of human thought is always embedded in a cognitive soul. But a cognitive soul is an embodied cognitive system given that the soul is the form of a living body. Among other things, this means that the distinctive activity of human nous—thinking—can only take place in the context of a larger set of activities which are common to the body and the soul. While Aristotle sharply distinguishes the non-bodily activity of nous from those other activities, he also seems to think that our ability to engage in both practical and theoretical thinking crucially depends on their support. But if our capacity to engage in practical and non-practical thinking requires the full functioning of the cognitive soul, and the cognitive soul is by definition an embodied cognitive system, it follows that nous, while definitionally separate from the human body, is ontologically inseparable from it. In line with Aristotle’s methodology, this inseparability seems to be grounded in the dependence of human thinking on phantasia. And this dependence is, in turn, due to the fact that the objects of human thought are such that they can only be thought “in phantasmata.” Even though separatism and embeddedness in the cognitive soul may seem to pull us in opposite directions, Aristotle develops an account of nous in which both ingredients feature in a conspicuous and coherent way.
4. Rationalism. A clear, and indeed central, message of Aristotle’s De anima is that we come to know the world around us via two fundamentally different cognitive powers, namely nous and perception: Nous and perception are fundamentally different cognitive powers because the nature of their corresponding objects is fundamentally different. This way of drawing the distinction between nous and perception is at the very heart of Aristotle’s theory of cognition. They remain fundamentally separate ways to know the world around us even when they are jointly involved in thinking matter-involving essences. Aristotle envisions two such cases: our thinking of the essence of hylomorphic compounds and our grasping of mathematical essences. Fundamentally, however, the world and the things in it can be successfully grasped by human cognition. Aristotle is an epistemological optimist. All things in the world can be known by human rationality (nous). At the same time, Aristotle is not a classical rationalist in that he does not believe in any innate ideas or concepts; he is a rationalist in the sense that the contents of human thought—the essences of things—cannot be reduced either to their perceptual forms (which are qualities) or to any kind of abstraction thereof. It rather requires an additional dimension over and above perception to endow human cognition with a mode of cognition that is explanatory of the perceptual features of things. This is human nous with its distinctive properties of universality, objectivity, and necessity.