It is clear from his extensive biological writings that Aristotle was deeply interested in life, including a vast range of living things, their parts, lifestyles, life processes, and environments. How are life and living beings, extensively described and explained in the biological writings, reflected in Aristotle’s ontology, his understanding of being? My question is prompted in part by the fact that some of Aristotle’s most important metaphysical concepts apply equally to living beings (animals) and to non-living beings (artifacts). In this chapter I develop an account of the theoretical significance of life and living beings that focuses on Aristotle’s distinction between two ways of being developed in Metaphysics Book 9 – being potentially and being actively.

Aristotle identifies perception as central to all animals, enabling them to fulfill their ends. His biological works clarify his hylomorphic account of perception as a key activity of the soul by providing detailed overviews of types of perception and perceptual organs. Like other bodily organs, these have complex structures comprised of physical components, often in layers, all ultimately involving the four basic elements. I defend a compromise position on scholarly controversies about whether Aristotle can successfully provide a physicalist account of perception. Briefly, the answer is “yes and no.” His biological works, along with “chemical” works, do give physical accounts of perceptible features like colors and tastes, as well as of the organs (and parts) capable of registering them. However, because of his teleological views about nature, such accounts must be “top-down” and are never purely reductive or translatable into structural accounts like those of the atomists. Finally, we must remember that perception is crucial to the behavioral success of the animal as a whole within its environment. Perceptual “experience” in our modern sense does not occur in any organ but rather in the body as a whole, and more centrally in the heart and blood vessels.

The activities undergone by living things are paradigmatically end-directed, and so this chapter examines Aristotle’s invocation of teleological notions (as well as their contrast with non-teleological notions) in his scientific investigation of life. In particular, the chapter looks at how Aristotle explains why various processes occur, why some kinds of organisms have (or lack) certain parts or features, and why those parts or features vary in their sizes and shapes. Aristotle’s biological explanations are complex and rich in detail, thus providing valuable resources for making headway into some of the interpretive challenges facing our understanding of his distinctive form of natural teleology – one that countenances purposes in the absence of intentions and volitions, and one that finds the occurrence of necessity compatible with goal-directedness.

Aristotle wrote extensively about the character and behavior of non-human animals in his Historia Animalium. One aspect of character is cognitive abilities. The chapter sets out Aristotle’s views on the cognitive abilities of animals, evidenced also in other works such as the Metaphysics and De Anima. All animals perceive but many also have imagination, memory, and practical intelligence. For Aristotle nonhuman animals have a sort of practical intelligence suited to their particular ways of life. The considerable overlap in cognitive abilities between human and nonhuman animals allows Aristotle to establish a biological basis for many human traits. Many nonhuman animals not only manage to organize their lives and negotiate new challenges but also maintain relationships with each other over extended periods. Social relationships require complex communication and involve a very important type of intelligence which is perfected in the most political of animals, human beings. The chapter ends with an account of how human cognition differs from that which occurs in other animals.

The chapter offers a brief discussion of Aristotle’s theory of animal self-motion and the conception of animal agency this theory implies. I start with a description of the philosophical problem Aristotle faces in accounting for animal self-motion. His solution to that problem, I argue, lies in a biological conception of the soul as the unmoved mover of the animal’s self-motions. His theory, I further argue, includes a biological account of desire as a process that may be described as a homeostatic mechanism of self-preservation on the level of perceivers. I then turn to the resulting conception of animal agency. Here I argue that Aristotle regards animals as self-movers insofar as they appropriate, and redirect, the energy they receive from the environment for their own purposes and in accordance with how they perceive things in the world. An Aristotelian account of the causation of an episode of animal self-motion will thus have to include reference to how things appear to the animal. I end the chapter with a brief discussion of the relation of Aristotle’s biological account of animal self-motion to his account of (rational) human self-motion.

Aristotle’s writings on animals comprise approximately a quarter of his surviving works. There are three lengthy treatises entitled Historia Animalium, On the Parts of Animals, and On the Generation of Animals. Other works on animals include On the Movement of Animals and On the Progression of Animals. In addition to these, a number of short discussions, collectively entitled the Parva Naturalia, focus on the capacities of living beings such as perception, breathing, and sleep. These works form what has been referred to by scholars in the last fifty years as the “biological corpus” of Aristotle. In them we find rich and varied discussions about anything from keenness of sight to egg laying, from parenting skills to dreaming. Much of the content of these works has been consistently marginalized in the history of philosophy.1 Bringing to light Aristotle’s biology as part of his philosophy is the main focus of this collection. This introduction will touch on the history, content, and methodology of these works and Aristotle’s key ideas on the science of living beings.

As the publication of this Cambridge Companion indicates, Aristotle’s biological inquiries are now accepted as an integral part of Aristotelian studies. The publication of Philosophical Issues in Aristotle’s Biology in 1987 is generally acknowledged to have been an important factor in bringing this change about. In this afterword, I briefly outline the events that led to that publication, and then describe, from a personal perspective, the remarkable growth of scholarly activity focused on Aristotle’s biology from 1987 up to the publication of the Cambridge Companion to Aristotle’s Biology.

The basis for terrestrial life in Aristotle’s biology is the nutritive process by which living things (plants and animals) produce and maintain their uniform parts and the organs made of these uniform parts. The nutritive process is thus extremely general, across all kinds. But it is also general in being present in all stages of the life cycle. Thus, it starts with the beginning of life, increases as the living thing grows, and subsides and is extinguished with the end of life. This variation in quantity is possible because there are two sides to the process, one is the heat necessary for “cooking” food into the parts of the living thing, and the other is the soul which informs this cooking. While the heat can be more or less, the soul is either there or not. The process of feeding (trephein) is shown to be Aristotle’s single sufficient and necessary condition for all natural life. It is the assimilation of food (trophê) to the living thing in question, an activity which the soul performs, thus producing and maintaining the living body, using the body’s heat as an instrument to work on food.

This chapter provides an overview of the theories of generation and hereditary resemblance found in Aristotle’s work On the Generation of Animals. This treatise completes the project of explaining the development of the perfected living being, which the epigenetic process of embryological development aims for. Aristotle’s explanation of how a new animal comes into being fits to his four-causal scheme, by adding in the more specific principles, male and female. The opponent, thinks Aristotle, is wrong to think that the generative contributions of the parents (seed, sperma) derive from all parts of the body. Instead, what male and female contribute is the most refined nourishment that their bodies produce, which is ready to become all the parts of the body. Male and female roles are then differentiated: the female provides this blood-derived product to serve as that material body (material cause) while the male’s seed is further refined so as to initiate and direct that development as the efficient cause. Aristotle also explains how it is that particular animals end up as male or female and come to resemble their blood relatives. The chapter ends by reflecting on Aristotle’s sexism in his theory of generation.

This chapter examines the reception of Aristotle’s biological work from his immediate successors to Roman intellectuals in the late Republic and early Empire. The Peripatetics, notably Theophrastus and Eudemus, endorsed many hallmarks of Aristotelian biology (e.g. classification by differentiae in Theophrastus’ Researches into Plants), and their works on animals focused mainly on areas that were relatively underexplored by Aristotle, such as animal behavior and “character.” Readers and users of Aristotle’s biological works outside philosophical circles were mainly interested in the wealth of facts collected in the Historia Animalium especially, and much less in Aristotle’s causal investigations. The main product of this scholarly engagement with Aristotle’s biology was the Epitome by Aristophanes of Byzantium, librarian at Alexandria around 200 BCE: it does aim to collect facts arranged by individual animals, but it also shows an interest in the main problems raised in GA. In Rome, Lucretius and Cicero were able to draw on Aristotelian biology to bolster arguments for Epicurean materialism and Stoic providentialism respectively. Finally, it is noteworthy that the now-lost Dissections played an important role in the early reception of Aristotle’s biology, at least until Apuleius in the second century CE.

The chapter sketches a broad picture of some ideas, antecedent to Aristotle’s work, about the origin and development of living beings. Against the background of the new cosmological and metaphysical framework of Aristotle’s biological enterprise, it emphasizes what distinguishes Aristotle from the Presocratics and Plato: his rejection of a shared causal story that would account for both the origin of the universe and the birth of animals and plants. This shift helps to make intelligible Aristotle’s rejection of hylozoism and of the opposite view that life arises, mysteriously, from inanimate material ingredients. To demonstrate that Aristotle discusses the biological views of his predecessors without directly using them to build his own theory, the chapter first turns to Presocratic fragments, mostly of Anaximander and Empedocles, which connect biological matters and cosmogony. Second, the chapter takes a fresh look at how Plato reshapes this connection in his Timaeus, offering a new account of the nature of the universe and the nature of human beings. This account then enables us to evaluate, in the chapter’s final section, the changes that Aristotle brings to the study of living beings, including his rejection of the notion of the latter’s progressive formation.