Each sense organ perceives certain kinds of changes in the world; this specificity depends on the elemental composition of their uniform parts. To perceive colour and shape, eyeballs must be transparent and so are made of water; to perceive touch, flesh must be made of something solid and so is made of earthy stuff. His account of what actually happens in the eyeball when we see some object is quite opaque. He probably believes that the eyeball undergoes a physical transformation of some sort. Certainly he supposes that contact with sense organs initiates a chain of physical consequences that reach into the body.
The target of this chain is the central sensorium, the locus of perception itself. By ancient anatomical tradition, Plato thought the central sensorium was the brain and, for once, Plato was right. Aristotle, of course, supposes that it is the heart. His main argument for this is that there should be a single, central principle of all the soul’s functions. To make his theory work, he obviously needs a physical connection between the heart and the peripheral sense organs. You might expect him to appeal to nerves, but he doesn’t know about them. (He uses the term neuron but applies them to sinew; Herophilus would identify nerves as a distinct tissue in the following century.) Aristotle therefore supposes that sensory transmission operates via the network of blood vessels as well as various ‘channels’. Most of these ‘channels’ do not obviously correspond to anything in modern anatomy, but one of them is probably, to use its modern name, the optic nerve. His argument rests on the fact that if it is severed by a blow to the head, blindness follows as if a lantern had been snuffed out. It’s unclear whether he thinks that sensory information is transmitted by the vessels/channels themselves, the blood or something else; in any event, physical continuity between the peripheral sense organ and the central sensorium is essential.
The core functions of the sensitive soul take place in the heart. It’s where raw perceptions are translated into mental representations which, when added to desires, become actions. Aristotle assumes that the sensitive soul’s function is to maintain the animal’s wellbeing by ensuring, inter alia, that it gets enough to eat and that it doesn’t get eaten itself. Animals therefore experience the world in terms of pleasure or pain, as defined by the goal of self-maintenance. The eagle perceives the tortoise with pleasure; the tortoise perceives the eagle with pain. However, a given perception may be either pleasant or painful depending on the animal’s internal state: an eagle sated on tortoises may disdain another one.
The term that Aristotle uses for the mental representation of some object is phantasia. To explain, he personifies: ‘“I have to drink,” says desire. “Here’s drink,” says sense-perception or phantasia or thought.’ He does not, of course, give a mechanistic account of phantasia or any other higher cognitive process, but he recognizes the difficulty. After explaining the physiology of smell, he says, ‘but smelling is more than such an affection by what is smelly – what more? Is not the answer that, while the air owing to the momentary duration of the action upon it of the smelly thing does itself become perceptible to the sense of smell, smelling is an observing of the result produced?’ Indeed.
Phantasia and desire may be black boxes, but when he explains how they effect action, he becomes very physiological again. Heating and cooling of the heart accompany both of these mental events. These thermal changes initiate motion that is then transmitted to the limbs. To explain how he invokes devices that he calls ‘automatic puppets’:
The movement of animals is like that of automatic puppets, which are set moving when a small motion occurs: the cables are released and the pegs strike against one another . . . for they [animals] have functioning parts that are of the same kind: the sinews and bones. The latter are like the pegs and the iron in our example, the sinews like the cables. When these are released and slackened the animal moves. Now in the puppets . . . no qualitative change takes place . . . But in animals the same part has the capacity to become both larger and smaller and to change its shape; the parts change qualitatively when they expand because of heat and contract again because of cold.
The important point about these puppets is that they are automatic (automata). They seem to have been mechanical dolls of some kind. He’s careful to note that their motions aren’t exactly like those of animals since animal motions also involve qualitative change such as expansion and contraction of the kind found in the heart. This gives him another anatomical problem. He has to translate qualitative changes in the heart into mechanical changes and then distribute those mechanical impulses to the limbs, and he has to do this not only without nerves but also without muscles.
It’s not that the Greeks were oblivious to muscles. Classical statues of athletes and heroes display them in enviable relief. Hippocratic texts refer to muscles as myes – ‘mice’ – but are vague about what they do. Aristotle avoids the term altogether and calls them sarx – ‘flesh’ – which he supposes has mostly a sensory function. His effectors of local motion are, then, sinews and a substance called symphyton pneuma.
Variously rendered ‘connate pneuma’, ‘hot breath’, ‘spirit’ or simply ΣP, pneuma is one of the most mysterious, yet powerful, substances in Aristotle’s chemistry. It is something like hot air, but its heat is of a special kind, not the heat of conventional fire. It is analogous to the divine ‘first element’ (aithēr) of which stars are made. More mundanely, it gives organic materials special properties: olive oil is shiny, floats on water and does not freeze thanks to its high pneuma content.
Pneuma is also a proximate instrument of the soul. Pneuma, heated or cooled by the heart, expands and contracts and so moves the heart’s minute sinews. These mechanical movements are, in turn, transmitted to the rest of the body. How they do so isn’t very clear since he knows that the network of sinews, unlike that of bones and blood vessels, is discontinuous. It’s another connectivity problem, not unlike that of getting sensory information from the sense organs to the heart. However it works, he does see that a small change in the heart’s motions can become amplified to move the entire animal. This is his motivation for automaton-causality. Switching similes, he also compares the way in which animals move to the way in which a great change in a ship’s course is brought about by the smallest movement in its rudder.
FARNESE HERCULES. AFTER LYSIPPOS, C. 330 BC
At the end of The Movements of Animals, Aristotle summarizes his account of animal motion in a simple geometrical diagram:
It is reasonable that motions run from the parts to the ‘origin’ [archē] and from the ‘origin’ to the parts and to each other. Then the motions from each letter in the diagram we have drawn arrive at the origin and from the origin, as it moves and changes, being potentially many, the motions of B goes to B, that of Γ to Γ, that of both to both. But from B to Γ it goes by going first from B to A as to a principle, then from A to Γ as from a principle.
This is a long-winded way of explaining that movements are initiated and effected at peripheral organs (B and Γ), but that, whatever happens, they are always mediated by A – the origin; the heart; the seat of the soul. It is the essence of the CIOM model; all we have been doing is putting flesh on it.* Aristotle allows that animals can have action without phantasma (involuntary movements such as the heart’s beating); and phantasmata that initiate action without actual perception (dreams, hallucinations); and that humans have a whole separate level of cognition, nous – reason – that regulates their actions. Our eagle, however, is a much less complex creature. Soaring above Sicily’s stony hills, it perceives the glint of Aeschylus’ head, constructs an (erroneous) phantasma of it as a rock, responds to its ravenous appetites, fires its pneuma, loosens its joints, slacks its sinews, opens its talons, drops the tortoise, stoops to kill and simply satisfies its desires.
LX
WHEN ARISTOTLE SAYS that the heart has ‘supreme control’ he does not just mean that it’s in the middle of the sensory and metabolic network – he means ‘control’ in a very literal sense. He compares the organization o
f animals to that of a well-governed city. A central organizing principle, the soul, sets things in motion and the rest just follows.
This is most obvious for the workings of the sensitive soul. But it’s true for the nutritive soul as well. Deeply impressed by the fragility of life, Aristotle worries that the heart’s internal fire will rage unchecked, consume all its fuel and so precipitate a metabolic crisis. He therefore argues that animals must have a variety of devices that keep their fires under control. The most important of these involves air.
Fires, says Aristotle, are regulated by altering the flow of air around them. In the same way air from the lungs regulates the heart’s fire. This is how: (i) the lungs expand and draw in cool air as a smithy’s bellows do; (ii) the cool air flows to the heart and damps the internal fire; (iii) the heart contracts; (iv) the lungs contract; (v) the newly heated air is expelled; (vi) the heart heats up once again; (vii) the heart expands; (viii) the lungs expand; (ix) the cycle repeats.
It’s an ingenious mechanism. Of course, it’s all wrong.* And it only works for mammals, birds and reptiles; other animals, he says, must cool their internal fires in some other way. Bees, cockchafers, wasps and cicadas breathe through their skins;* fish do not breathe at all. They don’t gulp air, indeed die when exposed to it, so they are cooled by the water that they take in over their gills. But many small divisibles (insects etc.) and soft-shells (lobsters, crabs and the like) don’t really need much cooling at all since their internal fires are simply not very intense.
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, rather general, systems equations’ – so von Bertalanffy in 1968.
The Aristotelian soul certainly has many properties of a system. It is a set of interacting units (organs) that form an integrated whole (a body). It has modules (the nutritive, sensitive and rational souls); and these modules have specialized functions and are hierarchically arranged. In some cases (humans) it is centralized; in others (centipedes) it is distributed. It has a purpose: to regulate the functions of life.
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.
This is, of course, an Aristotle for our times. 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.
FOAM
FORMATION OF A HUMAN EMBRYO BASED ON AN ARISTOTELIAN MODEL
LXI
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 are free to breed.
Aristotle’s solution is to mount a research programme to find out what’s going on in there. He studies a forty-day-old human embryo:
Place a male embryo, detached at forty days, in anything but cold water, and it dissolves and disappears. In cold water, it coheres somewhat inside a membrane. If that is pulled apart, the embryo is revealed, as big as one of the large ants, its parts clearly visible, including the penis and the eyes. These, as in other animals, are very large.
He doesn’t say where he got it from. He seems to have studied more than one. The passage, which is just a description, appears in Historia animalium. Its explanation appears elsewhere. This second work contains a mec
hanistic account of how animals develop, one deeply integrated with his physiological theory; an explanation for why creatures have two sexes when they do, and why, sometimes, they don’t; a mechanistic account of the transmission of form from parent to embryo, and a theory of inherited variation, that is to say, a genetics. It also has an analysis of life-history variation and a discussion of environmental influences. The Generation of Animals is, in short, a general account of how a human being gives rise to another human being or a fish a fish. Historia animalium aside, it is his longest book. It is also his most luminous.
LXII
THE REPRODUCTIVE BIOLOGY is orgiastic in content but clinical in tone. During the mating season, Aristotle says, ‘all animals are excited by desire and the pleasure derived from copulation’. Male frogs, rams and boars call to female frogs, ewes and sows; pigeons kiss. Some females show desire by coming into heat. Mares are wanton and female cats wheedle toms for sex, but hinds are reluctant for they find the stags too stiff.
Males fight, of course. Stallions, stags, boars, bulls, camels, bears, wolves, lions, elephants, quail and partridges all have a go at each other in a display of sexual mayhem. Stags round up females, dig holes in the ground and bellow at rivals. Gregarious creatures tend to be combative, solitary ones less so. And if partridge cocks are ‘lecherous’ and break the eggs of hens, pigeons are much gentler and pair for life – though occasionally females will go off with another male.
All this is but a preamble to the act itself. Aristotle defines a male as an animal that ‘reproduces inside of another animal’.* To get inside another animal, most males mount her from behind. Sharks and rays, however, mate belly to belly, dolphins copulate side by side, lions, lynxes and hares copulate back to back and snakes intertwine. He also says that during sex hedgehogs stand on their hind legs and face each other so that their spines don’t get in the way, that bears adopt the missionary position and that camels take all day about it.*
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