The Lagoon: How Aristotle Invented Science

by Armand Marie Leroi

First, an Aristotelian soul is not made of matter. That’s clear from his objection to Democritus’ crude materialism, but it also follows from his definition of the soul as the ‘form in a body’. Second, the soul is associated with the presence of organs, which means that it is a functional property of living things. Third, the soul is responsible for change in living things. By this he means that it regulates the body’s processes: growth, maintenance, ageing, locomotion, sensation, ...

Aristotle’s use of entelekheia to describe the soul tells us how important he thought this was, for the word is partly derived from telos, an end or goal. This conception, too, runs deep – into his metaphysics. The soul, he says, is ‘an entity [ousia] in the sense of a definition [logos]’. By this he means that a living thing’s soul is the sum of its functional features. If an eye were a living creature, he says, then its soul would be vision. He is so committed to the idea that functional features define a creature (or an organ), rather than the stuff it’s made of, that he even says that if an eye can’t see (because it’s damaged) then it really isn’t an eye at all. ...

SOULS, THEN, BEAR a heavy burden. They embrace no fewer than three of Aristotle’s four explanatory causes – the formal, moving and final – leaving only the material cause for the stuff of which it is made. But for all their evident importance souls remain mysterious. What, after all, can move the stuff of which living things are made, contain its goals, yet be immaterial itself? Confronted with these demanding criteria, scholars have sometimes concluded that, when Aristotle speaks of the soul, he is invoking some sort of spiritual force. This ‘spiritual soul’ interpretation comes in different flavours that draw on two disparate intellectual traditions, biology and the philosophy of mind, though the end result is much the same – an unnecessary mystification of what Aristotle means. In the ‘philosophy of mind’ version Aristotle is a Cartesian mind–body dualist who believes that mental states are independent of the body; when he speaks of soul he is, in Gilbert Ryle’s phrase, invoking a ‘ghost in the machine’. Now, when Aristotle discusses the ‘active’ intellect, there are some passages that do, indeed, lend themselves to this interpretation, but they are the despair of scholars for they are so very inconsistent with everything else that he writes about the relationship of souls to mental states.

Aristotle denies that souls are agents. He’s particularly clear about this when talking about emotions. He points out that any emotion that we might attribute to our souls (joy, despair) is evident in our bodies as a physiological response (laughter, tears). But then he goes further and argues that our tendency to see these responses as a consequence of the soul’s condition is wrong; rather, they are the soul: To say that the soul is angry is as though one were to say that the soul weaves or builds. For it is, perhaps, better to say not that the soul pities, learns or thinks, but that humans do these things. And: Thinking, loving or hating affect not the mind but what has the mind, to the extent it has it. Actually, it is when this decays that memory and love stop existing since they belonged not to the mind but to that composite thing which has perished.

Aristotle is trying to root out the ‘homunculus’. He is attacking the notion that there is within us all a small, disembodied person – an I – who is thinking our thoughts, hating our hates, loving our loves and controlling, in some mysterious fashion, our bodily machines. He does not ...

the biological version of the ‘spiritual soul’, Aristotle is a vitalist. To be a vitalist is to suppose that living things have some property that cannot be found in, or derived from, inanimate matter; to deny that living things are really just very complicated machines; to believe in the autonomy of life. In the eighteenth and nineteenth centuries a battle raged – particularly in Germany – between biologists and philosophers who thought that living things are just machines and those who did not. The latter were invariably impressed by the very thing that so impressed Aristotle: the goal-directness of living things. Teleology was an invitation to fill the explanatory vacuum with resounding, if empty, phrases: nisus formatus, Bildungsreib, Lebenskraft, vis vitalis, vis essentialis and the like. The last scientist of any repute to wear the vitalist badge proudly was Hans Driesch (1867–1941). A brilliant experimentalist in his youth, one of the founders of Entwicklungsmechanik – experimental embryology – in middle age he abandoned mechanistic theory and became a committed vitalist, arguing that no machine could even in principle construct a living thing. ‘But this may mean no more ...

The goal-directness of living things has been explained by natural selection, which tells us why living things have goals and what those goals are, by physiology and biochemistry which tells us how they achieve those goals and by genetics which tells us where those goals are stored and how they are transmitted from parent to child. Aristotle’s final, moving and formal causes – all the work that he attributed to the soul – have been absorbed by, and divided among, the branches of biology. The question, then, is this: did Aristotle, ignorant of this seamless hierarchy of explanation, succumb to Kantian despair and press a traditional word, ‘soul’, into service as a placeholder for the gap between inanimate material and all the things that living things do? If so, then he is a vitalist. Or did he use ‘soul’ as a term to embrace the processes by which living things develop and function; processes that he thought – rightly or wrongly – were perfectly explicable in terms of the physical science of his day?

It is sometimes said that all modern biologists are ‘materialists’ insofar as all our explanations take account of the brute properties of matter – chemistry and physics. But no biologist is a naive, Democritean, materialist, for all agree that the distinctive properties of living things depend on the arrangement of matter. The elements, though necessary, are not sufficient for life. An ordering principle – information – is needed as well. We are, to coin a term, ‘informed materialists’. That was Aristotle’s view as well. It is why he identifies psychē as the actualization of eidos. It is only a beginning. Soul appears in his book on developmental biology and heredity – The Generation of Animals; in his book on animal locomotion – The Movement of Animals; in his functional anatomy – The Parts of Animals; and, most of all, in his physiology – The Length and Shortness of Life and Youth & Old Age, Life & Death. Soul, in short, pervades Aristotle’s biology. When we survey all that he says about its workings it becomes apparent that he has given a detailed and coherent account, embracing many levels of biological organization, of how animals abstract matter from the environment, transform it, distribute the transformed matter throughout their bodies and use it to grow, maintain themselves, reproduce, perceive the world and respond to it – and that the form, structure or organization of all this activity is the soul. The result is a vision that is at once disturbingly alien and ...

ARISTOTLE DIVIDES THE soul among its functions. All living things have a ‘nutritive’ soul responsible for nourishment – trophē – and all that flows from it, but only animals (and humans) have a ‘sensitive’ soul that controls perception, appetite and locomotion. (He thinks that plants can neither sense their environment nor respond to it.) Humans also have a ‘rational’ soul. These sub-souls are components of a larger whole; sub-systems of the soul tout court. The nutritive soul is the first soul to appear in an animal’s development. Its powers are wide. It reigns over the acquisition, transformation and distribution of nutrition and hence the growth and maintenance of living things, their destruction by ageing and the perpetuation of their forms by reproduction. It holds them together and stops them from crumbling into dust. To put it more succinctly, when Aristotle talks about a creature’s nutritive soul, he’s talking about the structure and control of its metabolism. Metabolism: from the Greek metabolē – literally ‘transformation’. It’s a very Aristotelian word. A metabolism is the system by which an organism acquires matter from the world, transforms it into the stuff that it needs, and then redistributes to the places where it needs it. It is an open chemical system – which is what Schrödinger meant when he spoke of life feeding on negative entropy. Aristotle also perceived that living things were open systems: We must understand this [a growing uniform part] in terms o...

The belief that living things transform food into uniform parts hardly seems like a stunning insight, yet it seems to have been original to him. He says that his predecessors had two views about how the uniform parts grow. Some held that creatures make more of x (flesh, bone, whatever) simply by eating x. Call it the ‘additive’ model of nutrition. Others were more subtle: they held that creatures make more of x by eating its opposite. This ‘anti-matter’ theory is hard to understand and Aristotle’s illustration of it, that ‘water may be said to feed fire’, isn’t very helpful, but it does contain – albeit in peculiar form – the idea of transformation. Aristotle acknowledges, then, that the ‘antimatter’ theory contains a germ of truth. Even so, he’s clear that a much more general theory of transformation is needed. Some chemical notation shows what an advance his theory was over his predecessors’. If the ‘additive’ theory is written x → x, and the ‘anti-matter’ theory is anti-x → x, then Aristotle’s general theory is x → y. Or, to use his words, one kind of matter, x, ‘passes away’ even as another, y, ‘comes to be’. But actually that’s too simple, for Aristotle thinks that uniform parts arise from a system of serial and parallel transformations: x → y → z etc. or x → y + x → z etc. That is, he thinks that metabolic transformations are ordered as a chain or even a network. From this simple, yet pregnant, idea Aristotle constructs an entire system.

THAT MATTER FLOWS through living things, is transformed by them into the various uniform parts that they need to live and is distributed among those various uniform parts in a way constrained by economic laws – these are the elements of any metabolic theory. But any such theory must be underwritten by chemistry. Aristotle has a chemistry, but it is a poor one. It begins with the traditional four elements. Food and all its derivatives – the uniform parts – are compounds composed of these elements in particular proportions. Aristotle credits Empedocles with this idea, and Empedocles actually gives a formula for bone: E2W2AoF4, where E is earth, W is water, A is air and F is fire. Aristotle, by contrast, is very vague about the formulas of the uniform parts and generally speaks of them as being composed of just earth and water: hard uniform parts (bone, nails, hooves, horns, etc.) have lots of earth but little water; soft uniform parts (fat, semen, menstrual fluid) have little earth but lots of water; flesh is something in between. Aristotle suggests that such a formula is a step towards the definition of any uniform part. That makes sense since the recipe dictates its functional properties. All this is intuitive enough. But probe deeper, and difficulties appear. Aristotle berates Empedocles for merely thinking of the uniform parts as mixtures – agglomerated heaps of elements. They’re not, he says; they’re compounds – genuinely new substances. Very well, but how are such comp...

have been talking about the soul all along. The system that I have described – the structure of the metabolic network – is the nutritive soul, or at least a part of it. The ends to which a creature puts its nutrients, how much of each kind of uniform part it will make and when and where it will do so, its growth, its reproduction and its death – all of these depend on the organization of metabolism and all depend, Aristotle tells us, on the nutritive soul. Yet there is more to the nutritive soul than this: Some think the fire itself is the main cause of nutrition and growth. It’s not – the soul is; though it may be a contributory factor. Fires will always grow so long as there’s fuel but the size and growth of all naturally composed (i.e. living) things is limited and defined: this is the job of the soul not the fire, of defining characteristics not matter. We must imagine Aristotle sitting in front of a hearth (as Heraclitus was said to do), staring into the fire, occasionally poking it, thinking about the fire that rages inside him, that keeps him alive, that permits his thoughts to flow apparently without cease, devouring the world. ‘Fire is always coming into being and flowing like a river’ – how very true. But no fire can rage unchecked for ever lest it consume itself. All fires must be fuelled, stoked, damped – regulated – if the tenuous flame of existence is to be maintained.

Where is the soul located? The Aristotelian answer is ‘everywhere’ and ‘nowhere’. A creature’s soul is, after all, not a physical object but the sum of its functional features. That truism, however, does not preclude the possibility that some organ or other is particularly important in its regulatory functions. In blooded animals – vertebrates – Aristotle supposes that organ is the heart. This may strike us as an odd choice: why not the brain? But that’s easily answered: the soul’s first job is nutrition, and that’s obviously no job for the brain. Very well – but what has the heart to do with nutrition? Everything, replies Aristotle. This is where his physiology becomes strange. To be sure, insofar as nutrition is carried in the blood, the cardio vascular system must somehow be involved, but Aristotle puts the heart front and centre in his nutritional physiology. He thinks that it is the main site of concoction; in fact the ‘boiling’ action of concoction is what keeps the heart in motion. The heart is the main site of concoction because it is also the site of the ‘internal fire’. We think that it’s a pump; he thinks it’s a chemical reactor. He calls it the ‘citadel of the body’ and says it has ‘supreme control’.

When Aristotle explains how the internal fire is controlled, he lays the workings of the nutritive soul bare. This is another dimension of his teleology. He claims that the soul is responsible for the formal, motive and final causes and then shows all three at work. He sets a goal for the body and then shows how it is achieved. Many scholars, struggling to convey what Aristotle means by the soul, have described it as a ‘cybernetic system’. The metaphor is consciously anachronistic, but plausible. In the 1860s Claude Bernard showed that mammals regulate their body temperatures by altering the circulation of their blood in response to signals from the nervous system. Bernard’s slogan that ‘It is the fixity of the milieu intérieur that is the condition of a free and independent life’ inspired Walter Cannon to popularize, in 1932, the term ‘homeostasis’. In the 1940s Norbert Wiener formalized homeostasis as the product of regulatory systems that contain negative feedback circuits. Coining the term ‘cybernetics’ for the science of such self-regulating systems,* Wiener argued that they solved the problem of teleology: how torpedoes (the weapon, not the fish) can have goal-seeking behaviours. If machines can have goal-seeking behaviours then so can living creatures. Vitalism was expunged from its last redoubt: ‘Many of the characteristics of organismic systems, often considered vitalistic or mystical, can be derived from the system concept and the characteristics of certain, rath...

But is the soul a cybernetic system? If the metaphor has any power, then it should illuminate Aristotle at his most concrete – that is, when he describes the heart–lung thermoregulatory cycle. Aristotle claims that he’s described how the heart beats and the lungs pump. Has he? The answer is not obvious from his verbal account. If, however, we grant his physics, chemistry and anatomy, and diagram his model using the block-and-arrow formalism of cybernetics, the structure of the mechanism becomes clear.* The diagram, which is isomorphic with his text, shows that his model works, but not as he thinks it does. He thinks he’s described an oscillator that will make the lungs expand and contract rhythmically; in fact he’s described a thermostat. He’s worked out how to keep the heart at a steady boil. But that is no mean accomplishment. For his system contains the essence of any homeostatic device, a negative feedback circuit. It truly is a cybernetic system. Credit for first inventing, or at least applying, negative feedback control usually goes to the Alexandrian scientist Ctesibius (fl. 250 BC), who incorporated it into the design of a water clock. Perhaps credit should also be given to Aristotle who, two centuries earlier, saw the need for such a device in living things and sketched, however fancifully, how it might work.

Cybernetics and von Bertalanffy’s General Systems Theory became, in turn, the progenitors of modern systems biology, that quintessentially twenty-first-century science concerned with networks that depict the flow of matter and information among the parts of which living things are composed. The systems biologist B.Ø. Palsson puts it like this: ‘components come and go, therefore a key feature of living systems is how their components are connected together. The interconnections between cells and cellular components define the essence of a living process.’ Remove the reference to cells and that’s pure Aristotle. Of course, the point is not to make Aristotle seem terribly modern. Rather, it is to better understand his answers to some of biology’s deepest questions. What gives living things their goal-directedness? Souls do – by which he meant control systems of a complexity sufficient to show goal-directed behaviour. What holds living things together? Souls do – by which he meant the functional interconnections of their parts. How should we study living things? We have to take them apart, reduce them down to their individual bits and pieces. But, having done so, we also have to put them back together again for it is only then that we really understand how they work.

WHEN ARISTOTLE WANTS, as he so often does, to convince us that living things have an end – a goal – and that they cannot therefore be purely explained by the workings of matter alone, he appeals not simply to the beauty of animal design, the devices by which they keep themselves alive in the face of the world’s vicissitudes, but rather to the fact that they develop in a regular way. In the Physics Aristotle tackles the claim that he attributes – rightly or wrongly it’s hard to say – to Empedocles that order can just ‘spontaneously’ emerge in the womb. Hence his argument that a child’s teeth require some goal-oriented process, underpinned by a formal nature, if they are to appear in the right place at the right time. In The Parts of Animals he returns to the attack. Now it’s the backbone that worries him. Empedocles apparently also claimed that vertebrae are distinct because the backbone just happens to twist and break during development. That, says Aristotle, can’t be right. The seed from which the embryo developed must already have had the potential to produce the vertebrae. That is why ‘a human being gives rise to a human being’ and not a horse. It’s one of his favourite sayings. It is a very deep truth. It is also not a compelling argument for it merely restates the obvious. How, exactly, does a human being give rise to a human being? It is one thing to say that Empedocles is wrong, quite another to show it. In the recesses of the womb, where no one can see, theories ar...

BUT ENOUGH OF anatomy. What are Aristotle’s views on the female orgasm? He thinks that women want sex – lots of it. Sexual intercourse is ta aphrodisia. He describes highly sexed women as aphrodisiazomenai. Adolescent girls have to be watched since they have a natural impulse to use their developing sexual faculties. They may even contract bad habits (a veiled warning against masturbation?), but usually settle down after having had a few children. Some women are, however, as wanton as mares. Nymphomaniacs are literally ‘stallion-mad’ (hippomanousi). The Greeks lacked a technical term for orgasm so Aristotle simply speaks of the ‘pleasure’ or ‘intense pleasure’ of sex. But he certainly thinks that women typically have them; his models of male and female sexuality are very similar. In women ‘the pleasure of intercourse is caused by touch in the same region of the female as the male’ – which suggests that by ‘pleasure’ he means orgasm and by ‘the same region’ he means the glans and the clitoris. True, he has a name for the former, balanos, but not for the latter, but, to give credit where credit is due, he seems to have found it. Some women, he says, when experiencing pleasure ‘in a way comparable to a man’, produce a saliva-like fluid which is different from the menstrual fluid. That must be vaginal lubrication. Sometimes there’s a lot of it, more than a man’s emission – apparently a reference to female ejaculation. When women have pleasure, and secrete this sexual fluid, it...

the question is not whether women experience pleasure during sex, for Aristotle thinks they should and do; rather, it is do they need to climax to conceive?* Aristotle disagrees with himself. In The Generation of Animals he argues that, although a woman usually experiences pleasure during sex, she can conceive without it and, conversely, can fail to conceive even when she ‘keeps the same pace’ as her male partner. The female orgasm is nice but not necessary. In Book X of Historia animalium, however, the orgasm seems much more important, for there he argues that during sex the menstrual fluid is secreted into an area ‘in front of the uterus’ (presumably the cervix or vagina), where it mingles with the semen. This secretion apparently happens at orgasm, for both partners have to ‘keep the same pace’ if conception is to be successful. In fact, infertility is usually due to men ‘completing quickly’ while their partners have hardly begun to do so (‘for in most things women are slower’). To determine whether premature ejaculation is indeed the cause of infertility he suggests that the man in question should have intercourse with other women to see if he can generate children – which shows an admirably empirical spirit. To solve the problem of unequal timing he also suggests that the woman should excite herself with ‘appropriate thoughts’ even as her lover dwells on his troubles to cool his ardour. It is hard to say whether the ‘orgasm nice’ or ‘orgasm necessary’ theory represent...

Aristotle once again invokes that mysterious stuff, pneuma. It’s not only an instrument of the sensitive soul, but also a component of the inheritance system. Aristotle searches semen for the signs of activity. He finds it in the fact that semen resembles foam – or does so immediately after ejaculation. The foam is due to a charge of pneuma introduced by concoction of the semen during sex. Pneuma does not, however, need to be carried in semen since in those insects with strange sex it gets injected directly into the female. The upshot is a theory for how an animal’s soul is reproduced in the embryo. The structure of the father’s soul is, in effect, encoded in his semen by pneuma-tic action.* We must not think of pneuma as the carrier of genetic information itself: it’s not Aristotelian DNA. Rather, Aristotle’s units of inheritance are much more abstract; they’re the movements that pneuma induces in the semen. When he describes the motive principle in semen, the movement that is the future soul incarnate, he chose a word both apt and elegant: aphros or foam; and he meant by this both literally the foam visible in semen, and the foam visible in the wash of waves receding from a shore. Yet, as the passage makes clear, in choosing this word he was also thinking of something else. It is also for aphros, he says, that the Goddess of Love was named.

WHEN THE MENSES contact semen they congeal into an embryo or an egg. Aristotle uses homespun analogies to explain how this works: ‘The case resembles that of fig-juice which curdles milk, for this too changes without becoming any part of the curdling masses.’ Or, elsewhere, ‘this acts in the same way as rennet acts upon milk’. This is all about making cheese. When rennet, a substance derived from the stomachs of unweaned calves, is mixed with milk it causes them to separate into solid and liquid parts: curds and whey. Aristotle supposes that seminal pneuma does the same thing to menstrual fluid, coagulating the earthy stuff out of it, leaving a fluid behind. He probably thought the analogy particularly apt. The active ingredients (semen, rennet, fig-juice) all draw their power from being charged with vital heat; their substrates (menstrual fluid, milk) are both very closely related derivatives of blood.* The result of all this cheese-making is an embryo enveloped by a membrane floating in a fluid. And now the real business of pneuma begins: the manufacture of the embryo’s parts. Aristotle claims, and repeats the claim with the vigour of a man who thinks he’s made a major discovery, that the heart is the first organ to appear in the embryo. It’s a reasonable one if we allow that he means the first visible functioning organ and so exclude the somites and notochord that form well before the heart does. It isn’t just a fact for Aristotle: it fits with theory too. The heart mus...

In the final stage the judicious application of heat transforms the nutrition into the flesh, sinews, bones and all the other tissues of which a growing embryo is built.

It’s no surprise, then, that Aristotle’s data on inherited variation are poor. Even so, it’s surprising just how poor they are. True, he mentions a few cases of inherited variation, but they’re just confused hearsay and he misses much of what he could have seen. No Darwin, he ignores domesticated animals. Of course, he doesn’t cross anything (though there are some intriguing passages on hybrids). He devotes pages to variation in human eye and hair colour, but gives no indication that they can be inherited. He’s fascinated by teratology – dwarfism, hermaphroditism, conjoined twinning, anomalous genitals, extra appendages – and says that such deformities are often inherited, but sometimes not,

Aristotle’s grasp of the facts of inheritance is only slightly more sophisticated than the musings of any newly minted father: Some children resemble their parents, others do not. Some resemble their fathers, others their mothers, some in the whole body, some in each individual part, some their parents, some their ancestors, some just a general person. Males may resemble the father, females the mother. Some, though, resemble no relative but do resemble a human being. Some do not even resemble a human being in form but, actually, a monster. Mendelian ratios are not even a distant dream.

Much of science is about navigating between the general and the particular, between unifying the phenomena and dividing them, and sometimes you just get it wrong.

Honeybees delight Aristotle. More pages in Historia animalium are given to them than to any other animal, bar man himself. He describes their foods, predators, diseases, the various products they collect or make, their uncanny industry and the complexity of their social lives. He says they are divine.

‘NATURE PROCEEDS . . . by such small steps.’ Or, to express the same thought obversely and in Latin, Natura non facit saltum – nature does not jump. The tag is familiar. It was one of Darwin’s favourite slogans; in the Origin alone it appears seven times. Huxley, famously, thought it a needless weakness of the theory.* It is a recurring motif in Aristotle too: explicitly when he speaks of plant-like sponges or how, in some animals, bone seems to blend into fishbone; implicitly when he says that snakes are elongate lizards (are, indeed, their ‘kindred’), that seals are ‘deformed’ tetrapods and that apes seem so nearly human.

Indeed, reading Aristotle, it’s easy to suppose that he is struggling towards, or even has, a theory of evolution. He isn’t and hasn’t. Nowhere in his works does he claim, as Darwin did, that all animals are descended from some remote common ancestor. Nowhere does he suggest that one kind of animal can transform into another. Nowhere does he lament some kind that has gone extinct. Genos, he says, is a word that can be used in several different senses – but there’s no hint that, in the biology, he’s using the genealogical one. When he says that ‘nature makes small steps’ he means it in a static sense – that one can observe fine gradations between forms. Darwin means it dynamically – that species can transform, but do so gradually. Nowhere does Aristotle appeal to anything resembling natural selection as a force for either stasis or change.

1859 remains year zero. ‘Ever since Darwin . . .’ – it is our story, our origin myth. It is not one that I would, or could, destroy. But I do ask this. Should you come across an apparently Darwinian thought in Aristotle, pause and reflect that you may be recollecting an Aristotelian thought in Darwin.

It may seem, then, that my suggestion that Darwin’s works are infused with Aristotle is no more than wishful thinking, but that is not so. When Darwin said that his ‘two gods’ – Linnaeus and Cuvier – were mere schoolboys compared to Aristotle, he was insufficiently precise. He should have said that old Aristotle taught them.

Around 1260 Albert Magnus, the first modern European to study Aristotle’s zoology, expressed Aristotle’s claim that ‘nature proceeds . . . by such small steps’ in much the same terms that he did: ‘nature does not make [animal] kinds separate without making something intermediate between them; for nature does not pass from extreme to extreme nisi per medium’. By the early seventeenth century the obverse version, Natura non facit saltum (or saltus – plural), was a commonplace. In his Philosophia botanica, 1751, Linnaeus elevated it to methodological principle. ‘This is first and foremost what is required in Botany – Nature does not make jumps.’ Perhaps that is what Darwin was remembering when he quoted it.

idea that nature does not make jumps is closely allied to another: that nature is organized into a linear scale that runs from rocks to God via plants, animals and humans. The scala naturae – the Ladder of Nature – as it came to be known, appears in the cosmic structure of The Timaeus which is nothing if not hierarchical. It is also one of Aristotle’s themes. Every natural thing may be, for him, a form and matter – eidos and hylē – compound, but the relative importance of the components varies. In rocks hylē predominates; in living things eidos does. Among living things there’s a ladder of increasing complexity too, plants to humans, running successively on uni-, bi- and tripartite souls. In The Generation of Animals Aristotle elaborates this ladder of life within the animals and underwrites it with embryology and physiology. He begins by linking his scale of progeny ‘perfection’ (how advanced they are at birth) to their parents: ‘Nature’s rule is that perfect offspring will tend to be produced by a more perfect sort of parent.’ Parental perfection depends on intrinsic heat, hot being better than cold. Heat is reflected in the composition of their uniform parts, hot animals being more fluid and less earthy than cold ones. Heat also reflects anatomy since hot animals have lungs and more elaborate thermoregulatory devices than cold ones. Hot animals also tend to be larger, live longer and be more intelligent than cold animals. The result is a ladder of perfection that reache...

The Ladder of Nature was adopted by Neoplatonists, Christian theologians and early modern philosophers. It underpinned Leibniz’s cosmology. Vastly expanded and much transformed from its Attic origins, it reached the apogee of its influence in the eighteenth century, which is when it appears in Systema naturae.* Linnaeus’ version of the Ladder of Nature is quite Aristotelian. Biologists forget that he classified not just plants and animals, but all Earth’s natural products – Per regna tria naturae runs the subtitle: there’s a taxonomy of stones too. The three great Kingdoms of Nature’s E...

Thus the Platonic–Aristotelian vision of nature as a ladder of perfection and its influence. Yet there is another vision of nature’s order that can also be found in Aristotle. Throughout much of his biology he does not speak of a Ladder of Nature, but only of his great, natural groups of creatures, all of which do much the same kinds of things – eat, sense, move, reproduce – but do them in very different ways using very different devices. Just as both these visions appear at different places in Aristotle’s texts, both also appear in post-seventeenth-century zoology. Sometimes they even coexist, albeit uneasily. Even as the Ladder of Nature triumphed, naturalists such as Pallas were protesting that animals, in all their diversity, could not, should not, be forced into a linear scale. In 1812 Cuvier divided the animals into four great groups that he called embranchements: Vertebrata, Articulata, Mollusca, Radiata. ‘It formed no part of my design to arrange the animated tribes according to perceived superiority,’ he wrote, ‘nor do I conceive such a plan to be practical.’ He elaborated his scheme in Le règne animal, 1817. Bold, clear, detailed and synoptic, it made Cuvier famous. It is here that he celebrated Aristotle as his great precursor who, he said, had left hardly anything for his successors to do. Cuvier’s classification, however, doesn’t look very Aristotelian at all. The great division between the bloodless and blooded animals (reformulated by Lamarck as animaux sans...

When Aristotle speaks of the divine he is not – the point must be made again – invoking a divine craftsman for none exists; rather, he is telling us that immortality is a property of divine things and that reproduction makes animals a little bit divine. We are beginning to touch on Aristotle’s theology, his ultimate explanation for why the cosmos is arranged the way it is and its relationship to an immortal God. Why should animal kinds be immortal? This is where we come to the end of explanation, to one of those indemonstrable axioms that lie at the bottom of every Aristotelian science, and from which all else flows, and it is simply this: it is better to exist than not to exist.

It seems that Aristotle has, in general, quite a dim view of female character, that he thinks that women are less perfect than men. Actually, that’s putting it rather gently for, in The Generation of Animals, he says that females are ‘immature’, ‘deficient’, ‘deformed’ and even ‘monstrous’. Feminist scholars have made much of this.

‘MAN IS, BY nature, a political animal’ – it’s his most frequently quoted apophthegm. It appears in Book I of the Politics. It is sometimes said to be Aristotle’s definition of our species, but it isn’t. If anything, his point is that we have quite a lot in common with some other animals. Aristotle’s politikē epistēmē – political science – is very sociobiological. Both sciences are rooted in animal behaviour; and both assume a strong view of human nature – that is, assume innate desires and capacities. Aristotle would agree with E. O. Wilson and Steven Pinker: humans are not born blank slates; they have an innate desire to co-operate.*

At first the villages were dispersed (‘as was the manner in ancient times’), but then they formed denser associations for the sake of complete self-sufficiency. The city-state – the polis – was born. The ability, desire and need to live in a state are among the marks of humanity. Any man who, by nature, cannot live in the state is either a ‘tribeless, lawless, hearthless’ monster – he quotes Homer on the Cyclopes – or a god.

That is why, for all its flaws, Aristotle loved the polis. Correctly constructed, it could be the home of happiness itself. And yet the Politics is an essentially nostalgic work. By the time he wrote it, the age of the independent Greek city-state was past, and the age of empire had arrived. The conquerors were his friends; he was practically one of them. When Macedon made of proud Athens a vassal, Aristotle was still teaching Alexander at Mieza. The ironies haunt the book.

HE TAUGHT FOR TWELVE years or so. But then Alexander died in Babylon. The Athenians celebrated once again. The anti-Macedonian party turned nasty. They accused Aristotle of praying to Hermias, his friend, dead so long ago, and charged him with impiety. Politics were certainly at play. Had they wanted to get him for heresy they should have attended his lectures on astrotheology. The Delphians had honoured him (and Callisthenes) for recording the victors of the Pythian Games. Now they revoked the honours and smashed the tablet on which they had been proclaimed. He decided to leave. ‘I will not allow the Athenians to commit a second crime against philosophy.’ He was thinking of Socrates.

He was, indeed, re-enacting, for a new generation, the origin myth of modern science; the myth in which Aristotle was the giant who had to be slain so that we could pass through the straits of philosophy to reach the open sea of scientific truth that lay beyond; the myth in which Aristotle is little more than an endlessly fecund source of empirical, theoretical and methodological error; the myth that explains his absence from the scientific pantheon next to Linnaeus, Darwin and Pasteur; the myth that explains why not one scientist in a thousand can name, much less articulate a single result from, his scientific works. I say it is a myth and, insofar as history matters at all, it is certainly a pernicious one for it omits all that we owe him. But it is a myth that has this much truth: that Aristotle’s science was the principle casualty of the Scientific Revolution. It may even be said that modern science was built on its ruins.

Thomas Aquinas, began to construct his equally ambitious synthesis of Aristotelian metaphysics and Christian theology. Thomas abolished Aristotle’s division between first and second philosophy – easily done since it was blurred by Aristotle himself – and turned natural philosophy into a branch of theology. Thomas’s God, the primum movens immobile, is Aristotle’s unmoved mover; the teleology of his ethics is Aristotle’s too.*

In this book I have sketched Aristotle’s account of five interlocked biological processes: (i) the nutritional system by which an animal takes up complex matter from its environment, alters its qualities and redistributes it to its various tissues so that it can grow, thrive and reproduce; (ii) the thermoregulatory cycle by which it maintains itself and which, as it ages, falls apart; (iii) the CIOM system by which an animal perceives and responds to its environment; (iv) the epigenetic processes of embryonic development and its related spontaneous-generator version; (v) the inheritance system. All of these processes are underpinned by Aristotle’s physical theory and are, as such, mechanistic. That the physical theory is wrong is irrelevant; in the long run, all physical theories are.

‘It has been said’, wrote Borges, ‘that all men are born either Aristotelians or Platonists.’ Philosophers may wince at the opposition, but I suspect it to be literally true. Plato invites us to the world of abstractions, Aristotle to the world of tangible things. You begin with particulars, a box of seashells, say; gather them together and rearrange them endlessly in order to apprehend their logic and order. This apprehension, Aristotle says, is the gift of reason and the beginning of science. It is also where true beauty lies. This was inarticulately obvious to me when I was ten.

This is the world that Aristotle gave us: the vividly perceptible world of living things, whole and at home; the world that he enjoins us to love and understand. Aristotle wrote thousands of sentences, but one, the first of his Metaphysics, defines him: ‘All men, by nature, desire to know.’ Not all forms of knowledge, however, are equal – the best is the pure and disinterested search for the causes of the things. And, he has no doubt, searching for them is the best way to spend a life. It is a claim for the beauty and worth of science.