The Lagoon: How Aristotle Invented Science

by Armand Marie Leroi

Among the isles of Greece there is a certain island, insula nobilis et amoena, which Aristotle knew well. It lies on the Asian side, between the Troad and the Mysian coast, and far into its bosom, by the little town of Pyrrha, runs a broad and sheltered lagoon. D’Arcy Wentworth Thompson, On Aristotle as a biologist (1913)

It was said that Aristotle used to walk the Lyceum’s shady paths and that, as he did so, he talked. He talked about the proper constitution of the city: the dangers of tyranny – and of democracy too. And of how Tragedy purifies through pity and fear. He analysed the meaning of the Good, to agathon, and spoke of how humans should spend their lives. He set his students logical puzzles and then demanded that they reconsider the nature of fundamental reality. He spoke in terse syllogisms and then illustrated his meaning with endless lists of things. He began his lectures with the most abstract principles and followed their consequences for hours till yet another part of the world lay before them dissected and explained. He examined his predecessors’ thought – the names of Empedocles, Democritus, Socrates and Plato were forever on his lips – sometimes with grudging recognition, often with scorn. He reduced the chaos of the world to order, for Aristotle was, if nothing else, a systems man.

His students would have regarded him with awe and, perhaps, a little fear. Some of his sayings suggest an acid tongue: ‘The roots of education are bitter, but the fruit is sweet.’ ‘Educated men are as superior to uneducated as the living are to the dead.’ Of a rival philosopher he said: ‘It would be a shame for me to keep quiet if Xenocrates is still talking.’ There is a description too, and it isn’t an attractive one. It’s of a dandy who wore lots of rings, dressed rather too well and fussed about with his hair. Asked why people seek beauty in others he replied: ‘That’s a question only a blind man would ask.’ It is said that he had thin legs and small eyes.

Aristotle was an intellectual omnivore, a glutton for information and ideas. But the subject he loved most was biology. In his works the ‘study of nature’ springs to life for he turns to describing and explaining the plants and animals that, in all their variety, fill our world.* To be sure, some philosophers and physicians had dabbled in biology before him, but Aristotle gave much of his life to it. He was the first to do so. He mapped the territory. He invented the science. You could argue that invented science itself.

But most of Aristotle’s science isn’t descriptive at all: it’s answers to questions, hundreds of them.

Diogenes Laertius, the gossiping biographer who recorded Aristotle’s looks (five centuries after his death), said, ‘In the sphere of natural science he surpassed all other philosophers in the investigation of causes, so that even the most insignificant phenomena were explained by him.’ His explanations penetrate his philosophy. There is a sense in which his philosophy is biology – in which he devised his ontology and epistemology just to explain how animals work. Ask Aristotle: what, fundamentally, exists? He would not say – as a modern biologist might – ‘go ask a physicist’; he’d point to a cuttlefish and say – that.

The science that Aristotle began has grown great, but his descendants have all but forgotten him. Throw a stone in some boroughs of London, Paris, New York or San Francisco and you’ll be sure to hit a molecular biologist on the head. But, having felled your biologist, ask her – what did Aristotle do? You will be met with – at best – a puzzled frown. Yet Gesner, Aldrovandi, Vesalius, Fabricius, Redi, Leeuwenhoek, Harvey, Ray, Linnaeus, Geoffroy Saint-Hilaire père et fils and Cuvier – to name just a few of many – read him. They absorbed the very structure of his thought. And so his thought became our thought, even when we do not know it. His ideas flow like a subterranean river through the history of our science, surfacing now and then as a spring, with ideas that are apparently new but are, in fact, very old.* This book is an exploration of the source: the beautiful scientific works that Aristotle wrote, and taught, at the Lyceum. Beautiful, but enigmatic too, for the very terms of his thought are so remote from us that they are hard to understand. He requires translation: not merely into English, but into the language of modern science. That, of course, is a perilous enterprise: the risk of mistranslating him, of attributing to him ideas that he could not possibly have had, is always there.

I like to think that a scientist may, just occasionally, see in Aristotle’s writings something that the philologists and philosophers have missed. For sometimes he speaks directly to any biologist’s heart, as when he tells us why we should study living things. We must imagine him in the marble colonnades of the Lyceum, addressing a group of truculent students. He gestures towards a mound of ink-stained cuttlefish decomposing in the Attic sun. Pick one, he says, cut, open, look. ‘. . .?’ Exasperated, he tries to make them understand: So we should not, like children, react with disgust to the investigation of less elevated animals. There is something awesome in all natural things. Some strangers, so the story goes, wanted to meet Heraclitus. They approached him but saw he was warming himself by the stove. ‘Don’t worry!’ he said. ‘Come on in! There are gods here too.’ One should, similarly, approach research on animals of whatever type without hesitation. For inherent in each of them there is something natural and beautiful. Nothing is accidental in the works of nature: everything is, absolutely, for the sake of something else. The purpose for which each has come together, or come into being, deserves its place among what is beautiful. Scholars call this ‘The Invitation to Biology’.

Perhaps, then, what marks the physiologoi as early scientists is not so much the use of naturalistic explanations for the mysteries that the world presents, as rational ones. They believed that wisdom did not merely have to be received, but that ideas were worth debating and, if need be, discarding. They argued with each other and those who came before them; they were ambitious for their ideas. Here is Heraclitus (fl. 500 BC) evaluating some of his predecessors: ‘great learning does not teach sense: for otherwise it would have taught Hesiod and Pythagoras and again Xenophanes and Hecataeus’. Nasty – and unmistakably the sound of an intellectual at work.

Empedocles (c. 492–432 BC) was an exception. A Sicilian of noble birth, he was an orator, poet, politician, healer and charismatic seer. In the opening lines of his religious poem Purifications, he presents himself as an immortal god and describes how, when he enters a city, thousands flock to him requesting cures and oracles – requests which he satisfied, on at least on one occasion, by raising the dead. Jesus with an ego, then, or Zarathustra with attitude, but he was also an immensely influential natural philosopher who wrote On Nature, several thousand lines of verse containing, among other things, a cosmogony, a zoogony, a mechanistic, if implausible, theory of respiration and a four-element chemistry that Aristotle would adopt as his own.

Hippocrates (fl. 450 BC?) was going to school. In the plateia of Kos town there is a plane, ancient and gnarled, that is – so the label claims – the very tree under which the adult Hippocrates once sat dispensing cures and wisdom. It can’t be the same tree, but then the medical writings that are attributed to Hippocrates probably aren’t his either. Parts of the Corpus Hippocraticum, a medley of some sixty works, are old enough to have been written by him or his pupils, but others date from around the first century AD. Most of them are sober, professional texts that give naturalistic explanations for disease. Some are simple case studies, but others are more intellectually ambitious. The author of Fleshes says he wants to ‘explain how man and the other animals are formed, that is, come about, what the soul is, what health and sickness are, what is bad and good in man, and what causes death’. Profound or banal, they’re very different from Empedocles’ effusions. Here’s ‘Hippocrates’ on curing acute diseases: Often, in such cases, you will find ‘oxymel’, as a drink, extremely useful. It helps bring up sputum and promotes breathing. It is best employed under the following conditions. When strongly acidic, it is particularly effective in cases where there is difficulty in bringing up sputum. By lubricating the sputum, it facilitates expectoration thus clearing the windpipe as if with a feather. This is soothing to the lung and brings relief. And if, in combination, it achieves t...

He was born in 384 BC in Stagira, a coastal town not far from modern Thessalonika. His father, Nicomachus, was an Asclepiad – part priest, part physician. No common quack, he was physician royal to Amyntas III of Macedon. This is less impressive than it sounds. Macedon was a semi-barbaric backwoods state with a court to match. At the age of seventeen Aristotle was sent to Plato’s Academy in Athens. He remained there, as student and teacher, for nearly twenty years. By the time the teenaged Aristotle arrived at Athens to sit at Plato’s feet, the tradition of natural philosophy, no more than two centuries old, was dead. Literally so: Democritus of Abdera, the last and greatest of the physiologoi, had died just a few years earlier. Years later, Aristotle would see in Democritus a formidable adversary, a foil against which to test the mettle of his own system. Democritus, Aristotle says, made advances. ‘But [even] at this time men gave up inquiring into nature, and philosophers diverted their attention to political science and to practical goodness.’ He was talking about Socrates. Socrates (469–399 BC) was a stonemason with a taste for speculative thought. As a young man, he loved natural philosophy. At least that is what Plato makes him say in The Phaedo. He puzzled over the origin of life, the physical basis of thought and the motions of the heavens. His efforts were for naught. He followed, or tried to follow, the arguments of the physiologoi, now this one, now that, but he...

‘Socrates called philosophy down from the heavens, placed it in cities, introduced it into families, and obliged it to examine into life and morals, good and evil.’ That was Cicero’s judgement – and he meant it as praise.

Plato shared the Pythagoreans’ fascination with geometry, and The Timaeus contains one of the first attempts to use mathematics to describe the natural world. ‘Let no one ignorant of geometry enter here’ is said to have been inscribed on the lintel of the Academy’s entrance; the same phrase is written above the swipecard-sealed doors of any Department of Physics, even if you can’t see it. Then, too, if Plato’s science is barely distinguishable from theology so, to judge by the pronouncements of some physicists, is modern science: ‘If we discover a complete theory, it would be the ultimate triumph of human reason – for then we should know the mind of God.’ Plato? No, Hawking.

Aristotle would turn his back on his teacher’s idealism and see the world, our world, for what it is: a thing that is beautiful and so worth studying in its own right. He would approach it with the humility and seriousness that it deserves. He would observe it with care and be unafraid to dirty his hands doing so. He would become the first true scientist. That he made of himself this after having been taught by one of the most persuasive intellects of all time – that is the mystery of Aristotle. All he ever said by way of explanation is: ‘piety requires us to honour truth above our friends’.

Yet, if D’Arcy Thompson is right, it was on Lesbos that Aristotle began the great work of charting, and understanding, the world of living things. It may have been a conversation; a chance comment that prompted an excited reply. And then more talk, and yet more, until a vision of the whole enormous, daunting, thrilling thing emerged. It’s an appealing thought – that biology began so. And it’s not an implausible one. For when Aristotle went to Lesbos it seems that he had at least one other philosopher to talk to: a man who would become one of his closest friends and who would inherit his intellectual wealth.

It is a charming conceit to think of the two philosophers strolling in an olive grove, not too far from the Lagoon, dividing up the natural world between them; agreeing, as any two scientists might, to collaborate rather than compete: ‘You do the plants, I’ll do the animals – and together we’ll lay the foundations of biology.’ Charming, but too simple. Theophrastus wrote books about animals and Aristotle wrote at least one about plants; but in both cases they have been lost. That botanists look to one as the founder of their science and zoologists the other is, it seems, largely due to the vicissitudes of history – which texts the monks chose to save.

This is how Thompson put it: ‘He will be a lucky naturalist who shall go some day and spend a quiet summer by that calm lagoon, find there all the natural wealth, * and have around his feet the creatures that Aristotle knew and loved.’ I have done so. He is right.

TO ASSERT THAT ARISTOTLE was a scientist is to suppose that we can recognize one. Sociologists and philosophers have long tried to get the creature in their sights, with indifferent results, for so diverse are their activities and preoccupations that it is hard to find a definition that will embrace them all yet exclude astrologers. Scientists, who are much less exercised about definitions, simply recognize their kin but, if pressed, might offer something like ‘A scientist is someone who seeks, by systematic investigation, to understand experienced reality.’ This definition, a generous one, allows room for theoretical physicists and coleopterists and some sociologists too; and, though we may quibble about the edges, it narrows the field of human activity considerably, excluding gardeners and physicians (no systematic investigation), literary critics and philosophers (no experienced reality), as well as homeopaths and creation-‘scientists’ who fail on both counts. It includes Aristotle, whose investigations were nothing if not systematic and who was deeply committed to understanding experienced reality. To be sure, Aristotle never called himself a ‘scientist’, but he did have a term for ‘natural science’ – physikē epistēmē, literally the ‘study of nature’. And he called himself not merely a physiologos – ‘one who gives an account of nature’ – but a physikos – ‘one who understands nature’.

All men, by nature, desire to know. An indication of this is the delight that we take in our senses; for even apart from their usefulness they are loved for themselves; and, above all others, the sense of sight . . . The reason is that this, most of all the senses, acquaints us with, and brings to light, many differences between things. Aristotle does not mean ‘know’ just in the sense of ‘understand’; he also means ‘perceive’. Thus in the first instance we should read his words as the claim that men take pleasure in the exercise of their senses, and the reason why they do so is because it allows them to perceive all the different things of which the world is composed. This is merely an opening gambit. For Aristotle goes on to argue that ‘knowing’ in the sense of ‘perceiving’ is the foundation of ‘knowing’ in the sense of ‘understanding’ – indeed, is a requirement for wisdom. The reason, then, that this statement comes at the very start of the Metaphysics is plain. Aristotle is raising his battle standard and declaring war on the Academy’s idealism. His project is not Plato’s, for it concerns this world – and he wants us to know it.

To get from perception to wisdom, Aristotle gives us a hierarchy of understanding. When we perceive something, he says, we acquire a memory of it. And many memories of a given kind of thing allow us to generalize about it. Memories of Socrates and Plato, say, allow us to generalize about ‘men’. This is Aristotle opening another front contra Plato who held that we are born with all the knowledge that we have – indeed all the knowledge that we could have, that is, all the knowledge in the world. It’s just that, unfortunately, we have forgotten it; our task, then, is to retrieve that knowledge. Such an epistemology is, of course, a call to empirical quietism. If we already know everything, then we need not actually investigate the world; perhaps if we talk about it enough it will all come back to us. It is no accident that Plato wrote dialogues.

We have to begin, he says, with the phainomena –whence comes our ‘phenomena’, but perhaps the best translation is ‘appearances’, for he means by this not only what he sees with his own eyes, but also what other people have seen, and their opinions about it. He favours reports from ‘wise’ and ‘reputable’ people. He’s conscious that one man can’t see everything; sometimes you just have to trust what other people tell you (the Greeks inherited huge astronomical catalogues from Babylon and Egypt).

The way in which Aristotle deals with the last of these questions – the talking heads – is instructive. Many people, he says, believe that a struck-off head can talk, and they cite Homer in support. Also, he says, there is an apparently credible description of just such a case. In Caria (Anatolia) a priest belonging to the cult of Zeus Hoplosmios was decapitated. The grounded head named its murderer as one Cerides. A Cerides was accordingly found and put on trial. Aristotle does not comment on the fate of the man, nor even on the possible miscarriage of justice, but he dismisses the story on the grounds that: (i) when barbarians chop people’s heads off the heads don’t speak; (ii) when animals get their heads chopped off, their heads don’t make any sounds, and given that, why should human heads be able to do so?; (iii) speech requires breath from the lungs via the windpipe, which it can hardly supply to a severed head. All of this is admirably sane. We should never take such sanity for granted.

IN HIS ZOOLOGICAL WORKS Aristotle mentions the following mammals: ailouros (cat), alōpēx (fox), arktos (bear), aspalax (Mediterranean mole), arouraios mys (field mouse), bous/tauros (oxen), dasypous/lagos (hare), ekhinos (hedgehog), elaphos/prox (deer), eleios (dormouse), enydris (otter), galē (beech marten), ginnos (ginny), hinnos (hinny), hippos (horse), hys (pig), hystrix (porcupine), iktis (weasel), kapros (boar), kastōr (beaver), kyōn (dog), leōn (Asian lion), lykos (wolf), lynx (lynx), mys (mouse), mygalē (water shrew), nykteris (bat), oïs/krios/probaton (sheep), onos (ass), oreus (mule), phōkē (seal), thōs (jackal), tragos/aïx/khimera (goat). All of these species are, or were, native to Greece and Asia Minor, so it is natural that he should do so. More surprisingly, the number of species that he mentions, but that are native to the Nile delta, the Libyan desert and the plains of Central Asia, is not much smaller: alōpēx (here the Egyptian fruit bat), boubalis (hartebeest), bonassos (European bison), dorkas (gazelle), elephas (elephant), hyaina/trokhos/glanos (striped hyena), hippelaphos (nilgai), hippos-potamios (hippopotamus), ichneumōn (mongoose), kēbos (monkey), kynokephalos (baboon), onos agrios/hēmionos (wild ass or onager), onos Indikos (Indian rhinoceros), oryx (oryx), panthēr/pardalis (leopard), pardion/hippardion (giraffe?), pithēkos (barbary ape), kamēlos Arabia (dromedary), kamēlos Baktrianē (Bactrian camel) – to which we can add creatures such as the ibi...

Where does Aristotle’s exotic zoology come from? He was hardly ever out of sight of the Aegean Sea, so he could not have collected it himself. The Roman encyclopaedist Pliny the Elder gave an answer. As so often with Pliny’s assertions, it has a fantastical air. He said that Alexander the Great supplied it. King Alexander the Great, inflamed with a desire for discovering the natures of animals, entrusted this task to Aristotle, a man outstanding in every department of knowledge. Several thousand men in the whole region of Asia and Greece were put under his command – all those who made their living from hunting, bird catching and fishing as well as those who had in their care animal collections, herds of cattle, beehives, fish-ponds, aviaries. The idea was that nothing anywhere in the world might be overlooked by him. It was as a result of his thorough questioning of these men that he composed some fifty famous and distinguished volumes about animals.

Aristotle was summoned to the Macedonian court. He had reason enough to go. Macedon was, after all, home, and it was no longer the backwater that he had left behind nearly a quarter of a century previously. Amyntas was long dead; Philip II had succeeded to the Macedonian throne, had raised an army and was flexing his military muscles. In Athens, Demosthenes warned the citizenry, in ever more apocalyptic tones, of the danger brewing on their doorstep. They ignored him – to their cost. Philip wanted a tutor for his son: someone to rub the rough edges off the boy and give him the philosophical education befitting a prince. Did Aristotle make the boy into the man he would become? Or did he try to temper his natural powers? We long to know, but do not. For Aristotle’s teenage pupil was not just any spoilt princeling, but Alexander himself, future King of the oikoumenē, the Known World. It is one of the most remarkable conjunctions in history: one of history’s greatest thinkers has, for a few years, the whip hand over one of her greatest military leaders – and then unleashes him on the world. (Pierre-Simon Laplace merely examined Napoleon for admission to the École Militaire.) Writing...

In 336 Philip was murdered. Alexander became king. He began by reducing Thebes, second among Greece’s cities, to rubble. In a letter Aristotle counsels him to be a leader to the Greeks and to look after them as if they were ‘friends or relatives’, but Alexander sold Thebes’ citizens into slavery. He later crucified all of Gaza’s men. That was a bit more Aristotelian: in the same letter he tells Alexander to be a despot to barbarians and to ‘treat them as if they were beasts or plants’. As the young general rampaged across the known world, he carried with him a copy of the Iliad in Aristotle’s edition. In 335 Aristotle, his work done, returned...

PLINY’S STORY IS charming. Alexander, no mere kohl-eyed sensualist or megalomaniacal conqueror, loves plants and animals too, and, recalling his old tutor’s interests, affectionately lays the biological booty of an empire at his feet. Writing a century or two later, Athenaeus says that Alexander gave Aristotle 800 talents for his research, and so turns the King into a Macedonian National Science Foundation. There is a whiff of romance about these tales. Eight hundred talents was several times Macedon’s annual GDP; and in his biological works Aristotle says nothing about subsidies, a zoo nor even Alexander

It is also clear that Aristotle got some of his exotic zoology from travel books. Ctesias of Cnidus, a fifth-century Greek physician to the Persian court, wrote several books about Persia and India that Aristotle felt he could neither ignore nor trust. None of these kinds [genē] of animals [live-bearing tetrapods, i.e. mammals] has a double row of teeth. Well, there is one, if Ctesias is to be believed. He claims that a beast that the Indians call the martikhōras has a triple row of teeth, resembles a lion in size, is just as shaggy and has the same sort of feet. It has a face and ears like a man’s, blue eyes, vermilion colouring and a tail like a scorpion’s. It has a sting in the tail, shoots spines like arrows, and has a voice halfway between a shepherd’s flute and a trumpet. It runs as fast as a deer, is savage and a man-eater. Behind the thicket of fable that is Ctesias’ martikhōras lurks a tiger (the Persian is martijaqāra, literally ‘man-eater’). Elsewhere, ‘What Ctesias has written about the elephant’s sperm [that it is as hard as amber] is false.’ ‘And in India, so Ctesias claims, there are no wild or tame pigs, but the bloodless and scaly animals are all large.’ This is a reference to Ctesias’ ‘Indian worm’ that lives in trees and devours domestic animals and is obviously a large python.

Most of the recombinants were unfit and perished, so we see only the survivors today. Many early natural philosophers, Simplicius remarks, had this idea. That, if true, is remarkable, for it suggests that in Aristotle’s time the idea of selection as a source of order was a commonplace. Certainly Epicurus, a generation younger than Aristotle, gave an even more elaborate selection-based cosmogeny than did Empedocles – at least he did if Lucretius’ Epicurean verses are to be relied on.

His arguments are fascinating, for some of them have been used against the theory of evolution by natural selection. (1) Spontaneous events are rare, but the signature of genuinely purposeful events is that they are common: teeth always come up in exactly the same way. This is a probabilistic argument for the existence of a purposeful agent and, like all such arguments, it is wrong, for selection can regularly produce order from disorder.* Admittedly, Empedocles helps Aristotle to this conclusion by making his cosmogonies indeterminate: ‘[sometimes] it may happen to run one way, but often it ran otherwise’ – the line is quoted by Aristotle. (2) It’s not just the end of development that has the appearance of purpose; it’s also the process. Every step in development is obviously directed towards a final goal, rather like each step in the construction of a house. These steps must be the product of an intelligence that has the final product in mind. (3) Although development is very regular, mistakes do happen (in The Generation of Animals he has a lot to say about conjoined twins and dwarfs), but they are mistakes – deviations from some existing, purposeful programme that must be already in place. Indeed, even Empedocles’ original recombinant animals could not have come from nothing; they must have sprung from ‘some corruption of some principle corresponding to what is now the seed’. (4) Besides, we simply don’t see that much variation. Granted, monstrous progeny sometimes app...

‘Every part of the body is for some action: so what the body as a composite whole is for is a multifaceted action.’ And although Aristotle’s explorations of these profound truths are delightfully detailed and endlessly ingenious it appears that he has impaled himself on the horns of a dilemma. He sees, as Socrates and Plato did before him, the evidence of purpose writ across the face of the world; sees too that material forces alone cannot explain it, but refuses to yield to the expedient of a cosmic designer. So the question remains: from where do plan and purpose in nature come? Aristotle’s answer to this question is brilliantly subversive. He appropriates another of Plato’s doctrines, one that underpins his, Plato’s, entire ontology and epistemology, one that is indeed the very mainspring of his contempt for the perceptible world, destroys it, rebuilds it and turns it to the service of science. There is much to dislike in Plato: his anti-science, his totalitarianism and the seductive charm of his prose; but to his credit let this be said: he taught Aristotle.

Plato’s theory of Forms is the ancestor of all species of idealism. Modern scientists are generally realists and so will find it incomprehensible or bizarre. So did Aristotle. He wanted to explain the features of the physical world. But, if Forms are eternal and static how then, he asks, can they actually do anything? And what does it mean to say that the physical world ‘participates’ in the world of Forms? And if a Form is merely a mental conception, then does not any one physical object have as many Forms as ways that you can think about it? And if a Form exists for any given physical object, say Socrates, then why should there not be two, three or an infinite number of copies of Socrates walking about? Platonic Forms, he concludes, are merely empty words and poetical metaphors. They annihilate the study of nature.

Aristotle believes that the nature of a living thing, or at least the most important part of it, is in fact its form – if not its Form. The term he uses for ‘form’ is the term that Plato used, eidos. It is one of the most vital organs of his thought. Aristotle holds that any sensible object is a compound of form (eidos) and matter (hylē). One can speak of ‘form’ and ‘matter’ in the abstract, but in practice they’re actually inseparable. To explain what he means Aristotle appeals to various metaphors. If wax is hylē, then eidos is the impression made in it by a signet ring. In its most general sense, eidos is the way in which matter is structured to make the things we see.

However, when he applies the term to the world of living things, he uses it in several distinct, but related, senses. The first biological sense in which Aristotle uses eidos is close to the English meaning of ‘form’ – as the appearance of an animal. His word for a taxon of animals is genos (pl. genē) – which I translate as ‘kind’. Some genē are small – the sparrow kind; others are large – the bird kind. So when he wants to describe the features that make a sparrow a sparrow rather than a crane, or a bird a bird rather than a fish, he speaks of its eidos.

Which brings us to the second sense of eidos – as the fundamental unit of biodiversity, that is, close to what we mean by ‘species’. Indeed, the traditional Latin translation of eidos is precisely species, just as genus is of genos.* One could, then, rewrite the above passage as: ‘There are many species of fishes and birds.’

Indeed, there’s a third sense in which Aristotle uses eidos. It’s related to the other two, but goes much deeper, and is much more surprising. It’s the appearance of an organism, but – if this is not too paradoxical – its appearance when it cannot yet be seen. It is the ‘information’ or the ‘formula’ which was transmitted to it by its parents, from which it built itself in the egg or womb, and which it will, in turn, transmit to its progeny. It is in this sense that Aristotle thinks that the nature of a thing resides primarily in its form. To speak of eidos as ‘information’ risks anachronism. Aristotle certainly does not conceive of information in the general sense that we do. Yet this interpretation is supported by passages in which he draws a parallel between the transmission of animal form and the transmission of knowledge.

Aristotle considers how a woodcarver might explain his art. He clearly wouldn’t just talk about the wood – that’s merely the matter out of which it’s built. Nor would he just talk about his axe and auger – they’re merely tools. Nor would he just talk about the strokes that he makes – that’s mere technique. No, if he is really to convey the origin of the thing he’s making, he has to talk about the idea that he had when he began his work – the process by which it will unfold in his hands, its final design and ultimate purpose – he must talk about its eidos. In the same way, when a scientist seeks to explain why living things have the features they do, he must talk about their eidē. It’s just that the forms of living things are not, as Plato held, located in the mind of some divine craftsman, but rather located in their parents’ seeds. There’s a passage in the Metaphysics where Aristotle gives another metaphor for the relationship between material and formal natures. Rather marvellously, he compares the components of a body to a symbolic system. Some things, he says, are compounds. The syllable ab is a compound of the letters a and b. But putting a and b together is not enough to give you that particular syllable; you need something else: you need to specify the order of the letters (lest you get ba instead) or, as we would now say, you need information. In the same way, flesh is a compound of fire and earth and something else: the way in which they are ordered. And that orde...

The whole problem was that none of his predecessors saw that natures could be, should be, understood in several different ways. Our hearts beat – but not just because of chemistry; nor just to keep us alive; nor just because one grew in our embryonic torsos; nor even just because our parents had hearts too; rather, our hearts beat because of all of these things. All these kinds of causes complement each other, indeed, are deeply intertwined. Or so Aristotle argues in a famous methodological dictum known as the ‘four causes’. But ‘cause’ isn’t quite right: ‘four questions’ or ‘four kinds of causal explanation’ capture his meaning better: There are four basic causal explanations: first, what something is for (i.e. what its goal is); second, the formal cause or ‘definition of the essence’ (these first two should be treated as pretty well the same thing); third, its material basis and, fourth, its efficient cause or origin of movement.

The efficient (or moving) cause is an account of the mechanics of movement and change. It is now the domain of developmental biology and neurophysiology. The material cause is an account of the matter – the stuff – of which animals are made, and their properties. It is now the domain of modern biochemistry and physiology. The formal cause is an account of the information transmitted that any creature received from its parents, and that is responsible for the features that it shares with other members of its species – that is, the subject matter of genetics. The final cause is teleology, the analysis of the parts of animals in terms of their functions. It is now the part of evolutionary biology that studies adaptation. To the degree that function moulds the stuff of which animals are made, the way they develop, how they maintain themselves in the face of the world’s vicissitudes, and how they reproduce and die, the final cause embraces, as Aristotle says, the other three. He gives us the structure of our thought even when we do not know it.

Plato’s fans – he still has some – are particularly prone to viewing Aristotle as his teacher’s epigone. That can be accomplished only by studiously ignoring how Aristotle transformed Plato’s ideas. As a student Darwin read, and enjoyed, Paley’s Natural Theology and may have even acquired from it his keen sense of the design displayed by living things. Yet who would call Darwin a Paleyite? Calling Aristotle a Platonist is like that.

Aristotle not only produced a new system of explanation, but also applied it. His predecessors viewed the world as if from Olympus. It lay far below them blurred by distance or obscured entirely by mist, and speculation filled in what they could not see. Aristotle, however, went down to the shore. He observed, applied his causes to his observations and wove them together in the books that make up his Great Course in Zoology: The Parts of Animals, The Length and Shortness of Life, Youth & Old Age, Life & Death, The Soul, The Generation of Animals, The Movement of Animals and The Progression of Animals. By the time he was done matter, form, purpose and change were no longer the playthings of speculative philosophy but a research programme.

The beauty of living things arises from their endless variety, the sense that they give of unity within diversity, and the intricacy of their relations. In the face of nature’s munificence it’s all too easy to surrender to an inarticulate sense of the numinous connectedness of things, or else let it all surge by in a kaleidoscopic blur. Haeckel looked down upon the coral gardens of Al-Tur and babbled of the Magical Hesperides; Darwin entered the Mata Atlântica at Rio and discovered devotion – faced with a rainforest, anyone can go weak at the knees. If, however, we wish to understand the natural world, we must focus on, isolate and name its components. We must dissect, parse and label. But, as the Bird Hall shows, there are many ways to carve reality, and each cut reveals a different facet. The question that confronts us, then, is this: where did Aristotle make his cuts? What sort of science did he invent?

EVERY SCIENTIST HAS a conception of what constitutes ‘good science’. It is a sense, as firmly held as it is poorly articulated, of which causal claims are sound and which aren’t. It’s not, of course, that scientists necessarily agree on the soundness of any given claim. If you have ever contemplated the reviews of a manuscript, submitted with such hope to Nature or Science (for at the gates to these journals hope truly does spring eternal), you will know that your peers’ notions of what constitute sound causal claims are often very different from your own and really quite confused. Aristotle also faced the problem of securing causal knowledge from observation, but he faced it alone. Behind him lay generations of speculative theories about the causes of the natural world; at his feet stretched the world itself. He saw, and saw as no one before him had, the need for a way to connect them. So he developed one. In Book I of Historia animalium he alludes to it. First, he says, we have to get the facts about the different features of animals, then we have to work out their causes. Doing things in that order, he continues, will make the subject and target of our demonstrations clear. It seems like a rather banal introductory statement. It isn’t. For, when Aristotle talks of ‘demonstration’, he means an intellectual structure of daedal complexity whose foundations are sunk in metaphysical bedrock and whose pillars are constructed of steely formal logic. He means his scientific met...

ORGANON MEANS ‘TOOL’ or ‘instrument’.* It’s the title often given to six of Aristotle’s books. It’s an apt one for they are tools for the production of knowledge. One of these books, the Posterior Analytics, contains his scientific method. Aristotle distinguishes the rules for debating opinions from the rules for constructing scientific explanations. The first he called ‘dialectic’, the latter ‘demonstration’ (the Greek is apodeixis). By ‘demonstration’ he means exactly what a modern scientist means when he says, ‘we have demonstrated that A is the cause of B’ – that is, he and his collaborators have shown that the presence of A is a necessary and sufficient condition for B. He had a high notion of the power of scientific demonstrations: he thought that they delivered truth. That’s because they are the products of logical operations. Aristotle invented the theory of inferential logic known as his syllogistic. It was his greatest technical achievement and dominated the subject for millennia even if it was incomplete and, in parts, wrong. His syllogistic aimed to deduce new conclusions from established premises where the premises are propositions that contain a subject and a predicate, e.g. ‘All octopuses [subject] are eight legged [predicate].’ To analyse such statements he invented a formalism that substituted letters for the terms, e.g. ‘All A are B.’ This formalism allowed him to speak generally of all propositions of a given form, manipulate them and derive the many res...

For all its limitations, Aristotle’s theory of demonstration is a genuine scientific method. It is part of ours. Scientists may quarrel about methodology but they also agree about a lot. They understand the domain of science: the kinds of things it investigates. They understand how it delimits things and problems and investigates them piecemeal rather than trying to study and answer everything. They understand the reciprocal role of theory and evidence and the distinction between hypothesis and fact. They understand that science begins with induction to give generalizations from observations and then deduction to give firm causal claims from generalizations. They understand that a scientific claim must be a logical one – and can recognize a logical argument when they hear one. They understand that some causal claims are strong, others weak – and that the trick is to tell them apart. That they understand all this is because Aristotle told them it was so.

THE TRADITIONAL GREEK conception of soul was Homer’s. Patroclus falls at Troy and his disembodied soul takes wing for the House of Hades. Perhaps this explains why the Greek name for a butterfly is the same as that for ‘soul’ – psychē – for, as the soul flees a corpse at death, so a butterfly clambers from its chrysalis. In The Phaedo Plato elaborates the traditional theory. The soul is no longer merely something lost when we die; it reasons and regulates the body’s desires while we are alive. Now it is Socrates who is dying. His soul, too, will leave the body in which it is trapped and travel to Hades, but, where Patroclus’ soul will live, at best, a kind of feeble afterlife, Socrates’ soul can look forward to the prospect of perpetual reincarnation – or so he optimistically argues. In The Republic the soul gains further complexities. It becomes the seat of moral virtue. Plato describes the human soul as being mutilated by evil rather as the sea monster Glaucus – evidently modelled on a species of spider crab – is weighed down by the shells and seaweeds that encrust it.

Aristotle wrote a whole book about the soul, de Anima, or The Soul. Devoid of Platonic moralizing, it is resolutely scientific in tone: Some types of knowledge may be especially fine and worthwhile for their precision or because their objects have greater value and elicit greater wonder. It is for both these reasons that we should treat the study of the soul as one of extreme importance. However its investigation seems to be of special importance for truth as a whole and the study of nature in particular. For souls are the principle of animal life. This, to us, is a very strange, even suspect, claim. ‘Soul’ is a word burdened with many meanings but none in modern science. Perhaps we would do better to abandon translation, but mere transliteration hardly helps matters at all. For us, ‘psyche’ refers to mental states – in particular, consciousness. To be sure, Aristotle does treat mental states in his book, but he treats them as physiology: the Cartesian problem of consciousness hardly arises. Indeed, The Soul is not a psychological treatise at all, but his most general statement about the systems of command and control that enable living things to do what they do. Aristotle asserts, with fairly little argument, two propositions: that all living things – plants, animals and humans – have souls; and that, when a living thing dies, its soul ceases to exist. These were probably commonplace among fourth-century Greek intellectuals. Plato clearly believes the first and has to arg...

I have argued that, when Aristotle speaks of the ‘form’ or ‘formal nature’ of a creature, he often means the information required to order matter into a creature of a given kind. This interpretation is based not only on the various analogies he gives (imprints in wax; letters and syllables), but also on the fact that forms are present even when they are invisible. They are somehow present in an animal’s seed and are responsible for the development of the embryo and the appearance and functions of the adult. So an animal’s soul is its form, albeit under particular circumstances: If we must say something general about all types of soul, it would be the first actuality of a natural body with organs.

‘actuality’ – entelekheia. It is this word, a bit of Aristotelian jargon, that is most distinctive about his theory of the soul. He often uses it in opposition to ‘potentiality’ – dynamis. The opposition runs deep into his physical theory. Any change, in Aristotle’s view, is the actualization of a potential. Thus when he says that the soul is an actuality, he’s stressing the fact that it’s something that previously existed only potentially; that it’s something that comes into being from something else. When combined with the claim that the soul of a living thing is ‘its form in its body’, it becomes clear that he means that the forms of unfertilized seed are mere potentials; and that those forms when realized in growing embryos and functioning adults are souls. This is still irritatingly abstract. But Aristotle tells us much about the properties of souls and they, in turn, tell us what he’s getting at. Some properties are quite general and apply to all that souls do in all creatures; others are more specific and apply just to humans.