The chapter argues for a reading of Parts of Animals I.1, 639b11–640a9 as a continuous argument, divided into 3 main sections. Aristotle’s point in the first section is that teleological explanations should precede non-teleological explanations in the order of exposition. His reasoning is that the ends cited in teleological explanations are definitions, and definitions – which are not subject to further explanation – are appropriate starting points, insofar as they prevent explanations from going on ad infinitum. Aristotle proceeds in the following two sections to criticize certain non-teleological accounts offered by his predecessors on the grounds that they are explanatorily defective: those accounts – unlike teleological explanations – neither begin from appropriate starting points nor entail the phenomena that they purport to explain. Along the way, the chapter proposes an alternative way to understand what “hypothetical necessity” refers to, for Aristotle.

In Parts of Animals II.10, Aristotle introduces an approach to studying the nonuniform parts of animals: “to speak about the human kind first” (656a10). This chapter asks why Aristotle adopts this strategy and how he goes about implementing it. I argue that he selects it because he holds that human bodies offer particularly clear illustrations of some of his scientific concepts, including the relationship between parts and the ends they are for the sake of. As a result, he thinks that beginning with the causal explanations of human parts helps us to develop such explanations for the parts of other animals, especially when it is difficult to do so.

The analogies Aristotle employs in Parts of Animals (PA) are indispensable to the scientific investigation he undertakes in that work. This is because many analogies in PA express relations strong enough to ground a unique variety of unity. What is analogical unity? What sort of relationship must an analogy capture to ground such a unity? What role does analogy play in the scientific study of animals and their parts? I first contrast analogical unity with two different varieties of unity: formal unity and generic unity. I then examine the analogies in PA to discern which of the proportional relationships they express yield analogical unities. The most promising interpretations of these passages risk analogical unity’s collapse into one of the other varieties of unity Aristotle accepts. I argue that Aristotle employs the same concept of analogy in PA and in the Metaphysics and that this consonance allows us to preserve analogical unity’s unique explanatory role.

This chapter explores the three levels of material constitution presented in the Part of Animals: (a) elemental powers, (b) uniform and (c) nonuniform animate parts, in order to answer the question whether uniform materials that go into the constitution of animal bodies are produced for the sake of the organism. My answer is negative. At the bottom level, we encounter inanimate mixtures, not just elemental powers, that possess all the non-elemental material properties that may then be used by an animal nature. Aristotle’s chemistry (in the GC and Meteorology) exploits non-teleological processes that explain the dispositions of uniform material bodies. Such materials are then used by animal natures, through the teleological process of concoction, for the constitution of uniform and nonuniform parts of their bodies: the animal kind works within the confines set by the non-elemental material properties. This helps locate the difference between mixis and pepsis (concoction) and to understand why these conceptual tools are used in different contexts by Aristotle.

On the Parts of Animals (PA) is our main source for Aristotle’s explanations of animal character. This he locates in the qualities of an animal’s blood (or it’s analogue), whether it is hot, cold, thick, then, turbid, or pure (PA II.2, 651a16). This chapter sets out the main debate about character in Aristotle’s biological writings, whether it is formal or material, and argues that it is part of an animals’ material nature. While the materials existing in the blood vessels are not put there for the purpose of underlying character, they are often utilised for this end, displaying a complex coordination of material and formal natures. The chapter ends with a detailed analysis of which fluid elements in the body are responsible for underlying character and at which point they emerge in the digestive process. This further clarifies the relationship between animal bodies, nutritive processes, and the character potentials animals possess.

In Parts of Animals IV.10 Aristotle compares the human anatomy to the anatomy of other blooded, live-bearing animals with regard to their external, nonuniform parts. Of all these animals, Aristotle states, human beings alone have hands and arms instead of front-legs due to their erect posture. He associates the erect posture with human beings’ alleged divine nature, exhibited in their intellectual capacities. This poses two challenges that Aristotle addresses in the remainder of PA IV.10: first to show how most distinctive features of the human body (e.g. broad chests, fleshy buttocks, big feet, hands) can ultimately be traced back to the erect posture and second to account for the assumed connection between upright posture and intellectual capacities. Regarding the latter point the present chapter shows why, according to Aristotle, unimpaired thinking requires the upright posture and why the upright posture again requires a certain proportion between the upper and the lower bodily part.

Is mind a proper topic of investigation in Aristotle’s science of nature? The question is surprisingly vexed. Although some evidence suggests that mind should be studied by natural philosophy as well as first philosophy (metaphysics), Parts of Animals I.1 (641a32−b23) presents a series of arguments often construed as decisive evidence that he excludes mind from natural philosophy. This chapter goes through the relevant text and argues that Aristotle presents three arguments to exclude mind from nature but all in the voice of an opponent. Then in a final argument (641b23−642a1) he responds directly to the third argument, with indirect implications for the second argument as well.