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Aristotle’s Biology (Stanford Encyclopedia of Philosophy)

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Aristotle’s Biology

First published Wed Feb 15, 2006; substantive revision Sun Apr 19, 2026

Aristotle is properly recognized as the originator of the scientific study of life. This remains true despite the fact that many earlier Greek natural philosophers speculated on the origins of living things, and that much of the Hippocratic medical corpus—written before or during Aristotle’s lifetime—displays a serious interest in human anatomy, physiology and pathology, and occassionaly in animals and plants as well. Plato, too, has Timaeus, in the eponymous dialogue, devote a considerable part of his speech to the human body and its functions (and malfunctions), while in the Symposium, he has Diotima explain sexual reproduction in terms of a natural desire for immortality present in all living things. Nevertheless, Aristotle is the first who studies life both systematically and in a manner that is thoroughly empirical. While often building on the findings of his predecessors, Aristotle also critically revises their interpretations of the facts, significantly expands the scope of his own research, and replaces existing accounts with his own explanations that not only better accord with the phenomena but also integrate the study of life within the broader teleological framework of his natural science.

Crucially, Aristotle considered the investigation of living things, and especially of animals, central to the theoretical study of nature. Constituting roughly 25% of the extant corpus, his zoological writings provide a theoretical defense of the proper method for biological investigation; and they provide a record of the first systematic and comprehensive study of animals. There was nothing of similar scope and sophistication again until the 16th century. In the nineteenth century the great anatomist Richard Owen introduced a two lecture survey of Aristotle’s zoological studies by declaring that “Zoological Science sprang from his [Aristotle’s] labours, we may almost say, like Minerva from the Head of Jove, in a state of noble and splendid maturity” (Owen 1992, 91). Before examining this remarkable achievement, a few words about its creator are in order.[1]

  - 1. Life and Work

  - 2. Aristotle’s Philosophy of Science

  - 3. Caveat lector

  - 4. Philosophy of Biology

  - 5. Aristotle’s Biological Practice

	5.1 The History of Animals as a Report of the results of a Hoti-Investigation

	  - 5.2 From Inquiry to Understanding; from hoti to dioti.



  - 6. A Concluding Puzzle

  - Bibliography

	Selected Texts, Translations, Commentaries

	  - Secondary Sources on Aristotle’s Biology



  - Academic Tools

  - Other Internet Resources

  - Related Entries

1. Life and Work

Aristotle was born in Stagira on the northern Aegean coast in 384 BCE. His father Nicomachus was physician to King Amyntas III of Macedon, and his mother Phaestis was of a wealthy family from the island of Euboea. He was sent at the age of 17 to Athens, where he studied in Plato’s Academy for 20 years, until Plato’s death in 347. By then he had developed his own distinctive philosophical ideas, including his passion for the study of nature. He joined a philosophical circle in Assos on the coast of Asia Minor, where he also married Pythias (and had a daughter, also named Pythias), but soon after moved to the nearby island of Lesbos where he met Theophrastus, a young man with similar interests in natural science. Between the two of them they originated the science of biology, with Aristotle carrying out a systematic investigation of animals, and Theophrastus doing the same for plants.

In 343 Aristotle was asked by Philip II of Macedon to tutor his son Alexander. By 335 he had returned to Athens, now under the control of his former student Alexander. With Theophrastus he founded a ‘school’ in a public sanctuary known as the Lyceum. In this period, his wife died, and Aristotle formed a relationship with Herpyllis (also a native from Stagira) and had a son, Nicomachus. He headed the Lyceum until the death of Alexander the Great in 323. With anti-Macedonian feelings running high in Athens, Aristotle retired to Chalcis, his mother’s birthplace. He died there in 322BCE.

The surviving corpus of Aristotle derives from medieval manuscripts based on a 1st century BCE edition (attributed to Andronicus of Rhodes, the eventh successor of Aristotle as the head of the Lyceum). There were no commentaries on the biological works written until they were collectively translated into Arabic. The first appearance of Aristotle’s biological writings in the West are Latin translations, by Michael Scot, of an Arabic edition, which forms the basis of Albertus Magnus’ De animalibus. In the 13th century, William of Moerbeke produced a Latin translation directly from the Greek. The first printed editions and translations date to the late 15th century, the most widely circulated being that of Theodorus Gaza. In addition to the three works traditionally referred to as History of Animals (HA), Parts of Animals (PA) and Generation of Animals (GA), there are a number of briefer ‘essays’ on more specialized topics: On animal motion, On animal locomotion, On respiration, On life and death, On youth and old age, On length and shortness of life, On sleeping and waking, On the senses and their objects (the last six being included in the so-called Parva naturalia). Whether one should consider On the Soul (DA) part of this project or not is a difficult question. What is certainly clear, however, is that there are important connections between the theoretical approach to the relationship between body and soul defended in that work and the distinctive way that Aristotle approaches the investigation of animals.

In order to understand Aristotle’s distinctive approach to the study of living things, it is imperative to situate it within his natural philosophy and his philosophy of science. The first book of Aristotle’s Parts of Animals is, in fact, largely devoted to doing just that, and after a brief discussion of Aristotle’s general views about scientific inquiry and explanation, we will turn to it.

2. Aristotle’s Philosophy of Science

In sections 4, 5 and 6, we will explore the ways in which Aristotle systematically organizes and explains an extraordinary body of information about animal anatomy, physiology, and development. In order to accomplish this, Aristotle devoted a great of thought to the nature of scientific inquiry, ranging from questions about how one ought to collect and organize data to how to explain them and also to how to test the results to make sure they track the truth. For exactly does one progress from the superficial and unorganized state of everyday experience toward organized scientific knowledge? To answer this question, one needs a concept of the goal to be achieved, and Aristotle developed such a concept in his Prior and Posterior Analytics (henceforth abbreviated as APr. and APo., respectively). The goal of scientific inquiry, he argued, was a system of concepts and propositions organized hierarchically, ultimately resting on knowledge of the essential natures of the objects of study and of certain other necessary first principles. These definitions and principles form the basis of causal explanations of all the other universal truths within the domain of study. Those other universal truths should identify attributes belonging to a subject per se, in virtue of that subject’s nature. The examples he uses when he introduces his account of scientific demonstration to illustrate such propositions is from geometry: ‘having interior angles equal to two right angles’ belongs to all and only triangles in virtue of their being triangles (APo. I 4, 5). This attribute belongs to all equilateral triangles as well—not, however, because they are equilateral, but because they are triangles. Thus scientific knowledge of such a proposition, i.e., knowledge that displays the reason why any triangle has this property, must explain why this property belongs to triangles as such. The explanation, of course, will appeal to the essential character of three-sided rectilinear plane figures, i.e., to what it is to be a triangle.

The second book of the Posterior Analytics discusses how to achieve this goal of scientific knowledge, with one central concern being how knowledge of essences, expressed in definitions, is related to explanations expressed in the form of causal demonstrations. Plato had formulated a famous paradox of inquiry in his dialogue Meno: either you know the object of your inquiry, in which case inquiry is unnecessary; or you don’t know the object of your inquiry, in which case inquiry is impossible (Meno 80d5–e5). Aristotle reminds us of this paradox in the first chapter of the Posterior Analytics, but his full solution only emerges in book II. There, he argues that perceptual experience gives us a grasp of the target of inquiry that, though it does not count as scientific knowledge, does serve to direct further inquiry. He begins the discussion by presenting us with a claim about how objects of inquiry are linked to objects of scientific knowledge.

The things about which we inquire are equal in number to the things we know scientifically. We inquire about four things: the fact, the reason why, if something is, what something is. (APo. II 1, 89b23–25)

Aristotle conceives of these four inquiries as paired, and there is a natural sequence in each pair. Knowing that some state of affairs is the case, we can inquire into the reason why it is the case.

When we know the fact we inquire about the reason why (e.g., knowing that it is eclipsed or that the earth moves, we inquire into the reason why it is eclipsed or why the earth moves). (APo. II 1, 89b29–31)

Similarly, if we conclude an inquiry into whether something exists, we can go on to investigate its nature, what it is.

And having come to know that it is, we inquire what it is (e.g., Then what is a god? Or what is a human being?). (APo. II 1, 89b34–35)

The examples reveal a distinction that structures much of the discussion for the next ten chapters. For it looks as if ‘factual’ inquiries concern whether some attribute belongs to some subject (movement to the earth, eclipse to the moon), and that the search for the reason why is a search for the causal explanation of the attribute belonging to that subject. By contrast, it looks like the move from ‘if’ to ‘what’ is a move from establishing the existence of some subject (‘god’, ‘human being’) to establishing what it is.

However, the distinction is not, it turns out, so clear-cut. Having begun to illustrate the distinction between inquiry if or whether something is and what it is with the question ‘whether there is or is not a centaur or a god’, he then characterizes the knowledge achieved as ‘knowing that it is’. And in the second chapter, he begins to link the two sequences of inquiry by means of his syllogistic concept of “middle term,” the term that is common to the two premises in a syllogistic proof.

Thus it results that in all our research we seek either if there is a middle term or what the middle term is. For the middle term is the cause, and this is in every case what is sought. (APo. II 2, 90a7–9)

That is, in any valid syllogistic inference, the middle term shared by the premises is the warrant for the conclusion. In scientific explanation, however, the middle term must also identify the cause of the fact given in the conclusion—what that term identifies is the causal link between the subject and attribute. To use another of his common examples, if we seek to explain the periodic sound of noise in the clouds, the middle term must identify the cause of the connection between that noise and those clouds, which Aristotle thinks is the extinction of fire in there. Moreover, on Aristotle’s account of the relationship between causal demonstration and scientific definition, knowing the cause of thunder is at the same time knowing the essence of thunder, what thunder really is.

There is a difference between saying why it thunders and what thunder is. In the one case you will say: Because the fire is extinguished in the clouds. But: What is thunder?—A noise of fire being extinguished in the clouds. Hence the same account is given in different ways: in one way it is a continuous demonstration, in the other a definition. (APo. II 10, 94a4–8)

In the APo., Aristotle returns regularly to such standard examples of natural phenomena as thunder and eclipses—but he also, and importantly, provides an extended biological example, namely of the seasonal loss of leaves in broad-leafed plants.[2] In chapter 16, Aristotle imagines an inquiry that begins with questions such as ‘Why do fig trees and grape vines lose their leaves?’. The model answer is ‘Because they are both broad-leafed.’ That is, the inquiry seeks some other feature, common to both kinds, related to the target of inquiry, the seasonal loss of leaves. The ‘because’ is, however, preliminary—it is best to see being broad-leafed as a step toward causal explanation. Chapter 17 picks up the example, in the context of arguing that basic scientific inquiry seeks, wherever possible, co-extensive predications, which those between leaf loss and fig trees, or leaf loss and grape vines, are not. The candidate major premise, however—‘Whatever is broad-leafed loses its leaves’—does identify a co-extensive relationship between subject and predicate. It may thus serve as a proper scientific explanandum, and ‘broad-leafed’ can serve to identify a kind, all and only the members of which lose their leaves.[3] Thus, the middle term of the preliminary explanation becomes the subject of a more basic, co-extensive predication. The cause of broad-leafed trees losing their leaves will, then, be something more fundamental about broad-leafed trees, here identified as the solidification of moisture at the leaf juncture, which can thus serve as the middle term in a causal explanation of this fact. But it will also serve as part of a definition of leaf loss.

The middle is the account of the first major term [i.e., the predicate in the conclusion], for which reason all the sciences come about through definition. (APo. II 17, 99a22–23)

That is, we will have, if our research goes well, an account of what loss of leaves is. Along the way a process of identifying the kind, all and only the members of which will lose their leaves due to sap coagulation, is assumed. Yet the Analytics provides no systematic discussion about whether there are general criteria for identifying these basic scientific kinds. As we will see, this is the topic of one of the most interesting sections of Parts of Animals book I.

There are two quite different questions we need to ask about how these ideas about inquiry and explanation in the Analytics are related to Aristotle’s investigations of animals: First, how is the philosophy of biology presented in PA I related to the general account of explanation, definition, and inquiry in the Posterior Analytics?[4] And second, to what extent do the treatises reporting his actual investigations of animals conform either to his general account of scientific knowledge and inquiry in APo., or to the norms regarding the study of animals laid down in PA I? The remainder of this entry will be organized around these two questions.

3. Caveat lector

First some preliminary remarks are in order about what we are—and are not—discussing. It seems obvious, once stated, that the actual activity of studying animals is different from the activity of writing or teaching about animals based on that study. We have access to a number of Aristotle’s systematically organized writings on animals; we do not have direct access to his actual investigations. Some authors studied by historians of science discuss, in their written work, the methods they used to gather the information and work out their theoretical ideas and even provide ‘diaries’ describing their day-to-day studies—Aristotle is not one of them. Nor did anyone else report observing Aristotle carrying out his studies. On the other hand, there are reasonable inferences we can make from his writings. For example it is clear that he regularly consulted other ‘experts’ such as bee-keepers, fishermen, and sponge divers; that he (or someone under his guidance) performed a great many dissections on a wide variety of animals; that there were at least some diagrams produced based on these dissections; that he was in possession of or had access to an impressive library which held a variety of (alleged) facts on all kinds of topics recorded by others; and so on. Moreover, on the question of how he reasoned to specific explanations we can make some reasonable inferences from things he says about proper methods of biological inquiry. But it is important to keep in mind that we are studying texts that present, in a highly structured and theoretical manner, the results of an actual investigation, the details and practicalities of which we know very little.

It is also unclear what is the intent of the texts we do have that report on these investigations. It is sometimes said they are ‘lecture notes’. That seems pretty clearly wrong; they are too carefully written and structured. But it does seem clear, from cross-references, that some of them were to be studied in a certain order, and this order may conform to a course of study in the Lyceum. Interestingly, while all of the other biological treatises refer for further information to ‘the animal inquiries’, that is to something like the History of Animals that has come down to us, often in conjunction with ‘the dissections’, our History of Animals has no references to the other, explanatory studies. This suggests that the ‘inquiries and dissections’ had a different function in such a course of study, perhaps something akin to ‘reference works’.

Finally, it is also worth recalling that the series of treatises we have was likely compiled hundreds of years after Aristotle’s death from whatever he left. It seems unlikely we will ever be able to determine the exact relationship between the so-called ‘Andronican’ edition, the likely source of our texts, and what was produced during Aristotle’s lifetime.

These three caveats place constraints on what we can reasonably claim to be doing. We will be discussing the treatises that report the results of Aristotle’s investigations of animals. We will assume that the texts that have been passed down to us reflect what he wrote on this subject, and that the cross-references in those texts are his and reflect his own views about how these various studies are related to each other.

4. Philosophy of Biology

Parts of Animals book I (PA I) begins by outlining its purpose, which is to establish a set of standards for judging natural investigations (639a15). Its five chapters pursue this purpose, discussing the appropriate level of generality for such studies, the modes of causality and of necessity to be used in biological explanations, the relation of form to matter in living things, the proper method of logical division for this subject matter, the means of identifying kinds and their activities at the proper level of abstraction, and much more. Two sorts of evidence support the conclusion that this book is intended to deal with problems and questions that arise in the application of Aristotle’s general philosophy of science, found in the Posterior Analytics, to his theoretical investigation of living nature.

The first kind of evidence consists of passages introducing his biological investigations that appear to make explicit reference to the account of scientific knowledge in APo. The following passage from the History of Animals (a better though less familiar translation would be Animal Inquiries), for example, suggests that the entire biological project is organized in accordance with the theory of inquiry developed in APo. II. This passage comes near the end of chapter six in the first book of HA. After five chapters in which Aristotle lays out the kinds of similarities and differences among animals to be studied and sketches the ways in which they are to be investigated, he makes the following programmatic statement about the investigation to come, and where it fits in the entire scientific study of animals:

These things, then, have now been said by way of outline to provide a taste of what things need to be studied, and what it is about them that needs to be studied, in order that we may first grasp the differences and the attributes belonging to all animals. After we do this, we must attempt to discover the causes. For it is natural to carry out the investigation in this way, beginning with the inquiry into each thing; for from these inquiries it becomes clear both about which things (peri hôn) the demonstration (tên apodeixin) should be and from which things (ex hôn) it should proceed. (HA I 6, 491a7–14)

The natural way to proceed, then, is to begin with inquiry (historia) into the facts of each living thing, with the aim of grasping the differences between, and attributes of, all the animals; and then to attempt to discover their causes. This is natural because, given that our goal is demonstrative understanding, we want to end up with a clear distinction between the facts to be explained (the peri hôn) and their explanation (the ex hôn). This statement echoes the summary, in APo. I 10, of the components of demonstrative knowledge:

Nevertheless there are by nature these three [components of demonstrative knowledge]: that about which (peri ho) it proves, what it proves, and those things from which (ex hôn) it proves. (76b21–22)

The History of Animals characterizes itself as establishing the attributes and the differences that belong to all animals, and claims that by carrying out this inquiry we are prepared to go on to search for the causes. Indeed, Aristotle 8000