Animal

Plato's account of animals in the Timaeus is mythological in form but philosophical in intent. The Demiurge fashions the cosmos on the model of an intelligible Living Creature, and the lesser gods who assist him fashion mortal bodies whose souls are drawn from the same mixture as the world-soul, though diluted and less perfectly ordered. Human souls are created first, and the lower animals arise from human souls that have failed in their first incarnation to achieve rational governance over passion and appetite. Those who lived unwisely by following sensual pleasure are reborn as women; those who were "simple-minded" and reckoned only by sight pass into the nature of birds; still lower forms of failure produce land animals; and those wholly given over to irrational impulse descend into water creatures and shell-fish. The mythological vehicle does not exhaust Plato's meaning: beneath it lies the principle that the grade of soul possessed by a creature determines its rank in nature, and that the animal kingdom exhibits a hierarchy whose highest expression is the contemplative life of reason.

This hierarchical framework rests on Plato's psychology of three soul-parts. In the Republic and Phaedrus he distinguishes reason, spirited emotion, and appetite. Animals share in the lower parts but either lack reason altogether or possess it only in a subordinate and disordered form. The Phaedo presses the matter further: the soul at its best is akin to the invisible, unchanging, and divine; in attaching itself to bodily pleasures, it becomes beast-like, dragged back into the visible world "never arriving at the pure and simple, the everlasting and immortal." The philosopher who purifies the soul from bodily entanglement stands at the opposite end of the scale from the creature that is nothing but body and sensation. Whether the soul that animates Plato's animals is continuous with or merely analogous to the sensitive soul Aristotle would later analyze is a question that bears on the broader problem of continuity or hierarchy in the world of living things, as discussed under the idea of Life and Death.

Plato's treatment of animals is also inseparable from his account of Forms. In the Timaeus, the Demiurge fashions the visible world by reference to an intelligible model — the Living Creature (to zōon) — and the particular animal species are images of this eternal pattern. Species, for Plato, are thus not nominal groupings but reflections of real intelligible kinds, a conviction that places him in sharp contrast to later debates about whether natural classifications are discovered or invented. The reality and fixity of species, which Aristotle would address on biological rather than metaphysical grounds and which Darwin would later dissolve genealogically, originates in this Platonic conviction that the sensible order is measured by the intelligible.

Plato established that the question of animal nature is inseparable from the question of soul, and that soul admits of grades corresponding to the degree of rational control over desire and passion. Aristotle converted this mythological and metaphysical account into a program of biological research, retaining the tripartite psychology but grounding it in the functional activities of living bodies rather than in a metaphysics of degradation and reincarnation.

Aristotle is the first thinker to study animals systematically, and the scope of his investigation extends across the whole of the animal kingdom. The History of Animals catalogs hundreds of species by habitat, diet, mode of reproduction, and social behavior. The Parts of Animals examines the organs and structures of different classes of animal and asks what purpose each one serves. His method is observation and comparison rather than speculation, and his data were so ample and his classifications so expert that the major divisions of his scheme remained intact in the taxonomy constructed by Linnaeus two thousand years later. The question of how animals should be classified, whether by similarity of structure or by manner of life, is treated more fully under the idea of Definition.

The foundation of Aristotle's biology is the doctrine of soul set forth in De Anima. Soul, for Aristotle, is the form of a natural body that has life potentially. It is the organizing principle that makes matter alive. He distinguishes three grades: the nutritive or vegetative soul, which plants possess and by which they grow, nourish themselves, and reproduce; the sensitive soul, which animals add to these vegetative powers and by which they perceive, feel pleasure and pain, desire, and move from place to place; and the rational soul, which belongs to man alone. Each higher grade includes the powers of those below it. The distinction between plant and animal life is drawn sharply by reference to faculties absent in plants: sensation, appetite, and local motion. Whether this distinction represents a difference in kind or merely in degree is a question that bears on the broader problem of continuity or hierarchy in the world of living things, as discussed under the idea of Life and Death.

Aristotle further maintains that nature does nothing in vain, that every part of an animal exists for a purpose, and that biological form is as lawful as geometry. The animal is not a random assemblage of matter but an organized whole whose parts cooperate toward the good of the organism. This teleological conviction, that structure is to be understood by reference to function, governs his anatomical investigations and his account of animal generation and embryonic development alike.

Aristotle's zoology provided the framework within which natural history was conducted for two millennia. Galen followed his teleological principles into detailed anatomical research. Aquinas adopted his tripartite psychology as the basis for distinguishing human from animal nature. Darwin, while replacing Aristotle's fixed species with populations that change over time, retained his insistence that form and function in the animal body are inseparable and that both require explanation.

Lucretius inherits from Epicurus the conviction that everything, including life, is composed of atoms moving through void. On this view, animals are not ensouled in Aristotle's sense but are configurations of matter whose apparent purposefulness arises from the properties of their constituent particles. The soul is itself corporeal, a texture of very fine atoms distributed through the body. Different animals exhibit different temperaments because the proportions of wind-atoms, fire-atoms, and air-atoms in their souls differ; lions are fierce and deer timid by virtue of their material composition alone.

The fifth book of On the Nature of Things offers a naturalistic account of the origin and extinction of species. The earth once produced many creatures, some monstrous, some viable. Those fitted for survival by strength, speed, cunning, or usefulness to humans persisted; those that could neither protect themselves nor serve another kind died out. No divine craftsman shapes each species for a purpose. Nature, on this account, experiments blindly, and what works endures. This is not natural selection in the sense that Darwin later gave that term, but the resemblance between the two accounts is close enough that Darwin himself took notice of the passage. The broader question of whether species are fixed or subject to change is treated under the idea of Change.

Lucretius further denies that the organs of animals were made for their functions. The eye was not made for seeing nor the leg for walking; the organ came first, the use after. To see purpose in animal bodies is, on this view, to read the story backward. This denial of final causes in biology stands in direct opposition to Aristotle's teleology and, later, to the tradition of natural theology which argues from the fitness of organic parts to a cosmic designer. The question of whether formal and final causes belong to the study of nature, or should be separated from material and efficient causes, is one that Francis Bacon would take up and that continues to divide opinion, as discussed under the idea of Cause.

Lucretius was nearly forgotten for a thousand years, but when his poem resurfaced in 1417, his naturalistic account of animal origins re-entered European thought. The issue of spontaneous generation as opposed to procreation runs through Aristotle, Lucretius, Aquinas, Harvey, and Darwin, and Lucretius remains an important voice in that continuing discussion.

Galen follows Aristotle in the conviction that nature does nothing in vain, and he carries this principle into a sustained program of anatomical research. In On the Natural Faculties he limits his investigation to the functions common to all living things, such as growth and nutrition, which he calls "natural" effects. These he distinguishes from feeling and voluntary motion, which are "peculiar to animals" and which he terms "psychic" effects. His restriction to the vegetative functions in animals rather than in plants may reflect the fact that, for the naturalists of antiquity, the biological functions of vegetable matter did not yield their secrets readily enough to observation.

Comparative anatomy is central to Galen's method. By dissecting the same organ across different species, he identifies common structural principles underlying diverse forms. The heart of a pig and the heart of an ape perform the same work and share the same essential architecture, though they differ in size and detail. His On the Usefulness of the Parts is a sustained demonstration that every structure in the animal body is fitted to its function: the hand has its particular arrangement of bones and tendons because it is the instrument of instruments; the eye its specific humors and membranes because it must receive light without distortion. The question of how bodily structures are adapted to their functions, and whether this adaptation implies a designing intelligence, is treated more fully under the idea of Nature.

Galen reads the animal body as a text written by a supremely rational craftsman, whom he calls Nature or the Demiurge. Every anatomical feature is, on this view, evidence of foresight. The physician who understands the purpose of each part can intervene when disease disrupts the design. Medicine, for Galen, presupposes teleology; if the body were not purposefully organized, the physician would have no rational basis for treatment. His insistence on finding a functional purpose for every organic structure stands as a rejoinder to those who, like Lucretius, would explain animal bodies without reference to final causes.

Galen's teleological anatomy provided the authoritative framework for understanding animal bodies throughout the medieval period and into the Renaissance. Harvey, who made the discovery of the circulation of the blood, worked within the tradition of Aristotle and Galen, and his insistence on the functional interdependence of heart, arteries, veins, and lungs is a direct continuation of Galen's method. Descartes would retain the view that the body is a legible mechanical artifact but would remove the provident designer from the account entirely.

Aquinas follows Aristotle in distinguishing three kinds of soul, but he further differentiates four grades of life. "There are some living things," he writes, "in which there exists only vegetative power, as the plants. There are others in which with the vegetative there exists also the sensitive, but not the locomotive power; such are immovable animals, as shellfish. There are others which besides this have locomotive power, as perfect animals, which require many things for their life, and consequently movement to seek the necessaries of life from a distance. And there are some living things which with these have intellectual power, namely, men." The distinction between perfect and imperfect animals, based on the presence or absence of locomotion, adds a refinement to Aristotle's scheme that bears on the question of continuity or hierarchy in the world of living things, as discussed under the idea of Life and Death.

On Aquinas's view, animals perceive the world through external and internal senses; they form images, estimate what is harmful or beneficial, and remember. A sheep sees a wolf and flees, not because it reasons about danger, but because a natural estimation, built into its sensitive nature, moves it to avoid what threatens. This estimative power is, in Aquinas's terms, sophisticated but not rational. Animals act by natural instinct, and though their behavior is "voluntary" in the sense that it involves awareness of the objects toward which it is directed, it is the opposite of free choice. It is determined by the inborn pattern of the instinct and does not leave the animal free to act or not to act, or to act one way rather than another. The nature of animal instincts and their relation to habit is considered under the ideas of Emotion and Habit.

If the animal soul were rational, the architecture of Christian ethics, which presupposes that the agent could have done otherwise, would require reconsideration. Aquinas draws the line between human and animal nature not merely in degree but in kind: the human soul is intellectual and therefore subsistent, capable of existing apart from the body after death; the animal soul perishes with the body it informed. Yet Aquinas does not limit the comparison to psychology alone. He observes that man must resort to reason and the use of his hands where other animals are guided by instinct. "The products of reason take the place of hair, hoofs, claws, teeth, and horns," the fixed means of defense natural to other animals. The question of whether this difference is one of kind or degree is treated further under the idea of Man.

Aquinas's framework governed the Latin West's understanding of animals for three centuries. His separation of instinct from reason and of sensitive soul from intellectual soul set the terms that Montaigne would challenge from the side of skepticism and Descartes would press further from the side of mechanism.

Montaigne doubts that man can lay claim to any special attributes or excellences that set him above the beasts. In the "Apology for Raymond Sebond," the longest of his essays, he argues that "it is not upon any true ground of reason, but by a foolish pride and vain opinion, that we prefer ourselves before other animals, and separate ourselves from their conditions and society." Against every claim that reason elevates humans, Montaigne assembles counter-examples drawn from the behavior of animals: bees communicate direction and distance; the fox tests the ice before crossing; animals grieve, cooperate, and deceive. If these behaviors do not constitute reason, Montaigne asks, then by what evidence do we know that our own inner processes differ?

Montaigne does not claim that animals are rational in Aristotle's sense, but rather that the line between instinct and reason is less clear than the Scholastic tradition supposed. "The defect that hinders communication betwixt them and us, why may it not be on our part as well as theirs?" Montaigne thinks we have no grounds for believing that "beasts, by natural and compulsory tendency, do the same things that we do by our choice and industry." Rather, he continues, "we ought, from like effects, to conclude like faculties." We infer animal cognition from behavior; we infer our own cognition from behavior as well. The presumption of a radical difference rests, on this view, on pride rather than evidence. The broader question of whether human and animal intelligence differ in kind or only in degree is treated under the idea of Mind.

Montaigne does not conclude that humans and animals are identical. He concludes that we do not know where we stand. If animals share with us some measure of intelligence, feeling, and social life, then the sharp boundary that Aquinas drew between sensitive and intellectual soul begins to appear uncertain. "Why does the spider make her web tighter in one place and slacker in another?" Montaigne asks. "Why now one sort of knot and then another, if she has not deliberation, thought, and conclusion?" Such observations, drawn from the ordinary behavior of animals, tend to dissolve the confidence with which earlier thinkers separated instinct from reason.

Montaigne's skepticism about the human-animal boundary posed a challenge that subsequent thinkers could not ignore. Descartes constructed the beast-machine hypothesis in part as a reply to arguments of the kind Montaigne had advanced. Hume drew a different conclusion: if animals reason by experience, then perhaps reason itself is a natural faculty, less exalted than the philosophical tradition had supposed.

Harvey's discovery of the circulation of the blood, announced in De Motu Cordis (1628), is often treated as a simple addition to the stock of anatomical knowledge, but its significance for the debate about animal nature runs deeper. Harvey arrived at the circulation by a method indebted to Aristotle and Galen — he insisted on finding a functional purpose for each structure of the animal body — yet his conclusions departed radically from both predecessors. Where Galen had held that blood moved outward from the liver in a slow tide and was consumed by the tissues, Harvey demonstrated by calculation and experiment that the volume of blood expelled by each beat of the heart was far too large for this account to hold; the blood must return from the periphery and be pumped out again in a continuous circuit. The discovery united anatomy and physiology in a single functional account and showed that the animal body, understood teleologically, could yield results that Aristotle and Galen had not reached by observation alone.

Harvey's On the Generation of Animals (1651) extends the same method to the questions of reproduction and embryonic development that Aristotle's Generation of Animals had first systematically addressed. Harvey reads as a sustained conversation with Aristotle, accepting the framework of formal and final causes while revising specific conclusions about the respective roles of male and female in generation. Against those who held that the embryo was preformed in the semen, Harvey argued by observation that the embryo develops epigenetically — each part arising in sequence from an undifferentiated beginning, so that all animals come from an egg (ex ovo omnia). His posture toward the tradition was characteristic of the Renaissance: "respect for our predecessors and for antiquity at large inclines us to defend their conclusions to the extent that love of truth will allow." Yet he was equally insistent that no one should suppose all truth was engrossed by the ancients, and that the safer way to knowledge lay in questioning "things themselves rather than by turning over books." The problem of spontaneous generation versus procreation, which runs through Aristotle, Lucretius, and Aquinas, receives its early modern treatment in Harvey and its definitive dissolution in Darwin.

Harvey's position in the tradition is pivotal in another respect. Descartes, reading Harvey's work on the heart, accepted the circulation as established but gave it a wholly mechanistic explanation: the heart heats the blood by internal fermentation, with no need for any force analogous to a deliberate muscular contraction. Harvey rejected this: the heart acts by a vital motion that is purposive and cannot be fully reduced to the movement of particles. The debate between Harvey and Descartes marks one of the sharpest seventeenth-century confrontations between teleological and mechanical accounts of animal life. In insisting that final causes remain indispensable in biology even after the new physics, Harvey anticipates objections to the beast-machine hypothesis that would be pressed from within natural history rather than from theology or metaphysics.

Harvey made the organization of the animal body a matter of detailed empirical investigation undertaken in the spirit of, but not limited to, the Aristotelian tradition. Darwin acknowledged the great books of biology — Harvey's among them — as the background against which the theory of descent had to be measured; and the functional interdependence of organ systems that Harvey established became the framework within which later physiology asked its questions about adaptation and natural selection.

Descartes maintains that animals are machines or automata, possessing neither soul, thought, nor feeling. When soul is identified with intellect as "a thing which thinks, that is to say a mind or an understanding, or a reason," and when soul is further conceived as a spiritual and immortal substance, then, as Descartes argues, the conclusion follows that animals do not have souls. Everything in the physical world apart from the human mind, animals included, belongs to res extensa and operates by the laws of mechanics alone. "If there had been such machines, possessing the organs and outward form of a monkey or some other animal without reason," Descartes claims that "we should not have had any means of ascertaining that they were not of the same nature as those animals." Hobbes reaches a similar position by a different route, asking "what is the heart, but a spring; and the nerves, but so many strings; and the joints, but so many wheels?"

The argument for the beast-machine rests on two observations. First, no animal uses language in the relevant sense: parrots mimic sounds but never compose novel sentences expressing thought. Second, no animal displays the universal adaptability that reason provides. Animals perform specific tasks with great skill, sometimes exceeding human ability, but they cannot transfer that competence to new domains. These two marks, the absence of flexible language and the absence of general intelligence, are for Descartes sufficient evidence that animal behavior is wholly mechanical. The conception of the animal as a machine or automaton is, in his hands, not merely a biological thesis but a consequence of the fundamental division between thinking substance and extended substance on which his entire philosophy rests.

The implications of the beast-machine extend beyond physics to morals. If animals could be shown to think, the division between mind and matter would be called into question. But if animals are indeed without thought or feeling, then the use of animals for human purposes, including vivisection, raises no ethical difficulty. The question of man's moral obligations toward animals, and whether charity if not justice should govern our treatment of them, is one on which Plutarch, Spinoza, and Marcus Aurelius hold views quite different from those implied by the Cartesian position, as discussed under the idea of Good and Evil.

Descartes forced the question of animal cognition into its sharpest form. After him, the question was no longer whether animals have souls in Aristotle's sense but whether they have minds at all. Locke and Hume, in their different ways, defined their positions on animal intelligence partly in reaction to the beast-machine thesis.

Locke refuses the Cartesian position that animals are mere machines. Anyone who has watched a dog recognize its master, a bird learn a tune, or a parrot associate words with objects can see that animals perceive, remember, and reason in a limited way. "I think beasts do have some ideas," Locke writes, "and are not bare machines as some would have them." To deny them all mental life is, on his view, to contradict the obvious evidence of observation. The question, then, is not whether animals think at all, but how their thinking differs from ours.

The answer lies in abstraction. Animals perceive particulars and retain particular ideas. A dog distinguishes its master from a stranger and connects the smell of game with the chase. But it cannot form the general idea "animal" or "triangle" or "justice." It cannot reason from universal principles to particular conclusions. It operates always among the concrete and the singular. Human understanding, by contrast, rises from particulars to generals, frames abstract propositions, and draws inferences across domains. "The having of general ideas," Locke concludes, "is that which puts a perfect distinction betwixt man and brutes." This single power of abstraction separates the human mind from even the cleverest animal. The question of what abstraction is and how it relates to other cognitive operations is treated more fully under the idea of Knowledge.

Locke's position occupies a middle ground between Descartes' total denial of animal mentality and Montaigne's skeptical leveling of the human-animal distinction. It preserves continuity at the lower levels of cognition, perception, memory, and simple association, while drawing a firm line at abstraction. In this way it concedes much to the animal while reserving the capacity for general thought to the human alone. The distinction also bears on the problem of definition and classification, since the capacity to form general ideas is, on Locke's account, what makes it possible to define species and sort the variety of nature into kinds, as discussed under the idea of Definition.

Locke's middle position shifted the debate from metaphysics to psychology: the question was no longer whether animals possess souls or are machines, but what kinds of mental operations different creatures perform. Hume would accept Locke's concessions to the animal but would question whether abstraction marks so firm a boundary as Locke supposed.

Hume titles his chapter "Of the Reason of Animals" and states his position plainly: "No truth appears to me more evident, than that beasts are endowed with thought and reason as well as men." Animals learn from experience. A dog scalded by hot water avoids it afterward. A bird that has found food in a certain place returns there. These are instances of what Hume calls causal reasoning: the animal connects past experience with future expectation, and its behavior adjusts accordingly. This is the same process, custom or habit, by which humans form causal beliefs. If we call it reason in our own case, Hume argues, consistency requires us to call it reason in theirs.

The argument draws its force from Hume's prior analysis of human reasoning. In the preceding sections of the Treatise, he has shown that human causal reasoning is not a product of abstract logic. We do not deduce that fire will burn from first principles; we expect it because we have experienced it. Our reasoning about causes is habitual, not rational in the Scholastic sense. If that is so, then the supposed gulf between human reason and animal instinct narrows. Both operate on the same principle: experienced conjunction produces expectation. Locke's claim that abstraction marks a firm boundary between human and animal minds begins, on Hume's analysis, to appear less certain. The question of whether causal reasoning is the product of reason or of habit is treated more fully under the idea of Experience.

If the difference between human and animal cognition is one of degree rather than kind, then the philosophical tradition may have drawn its most important line in the wrong place. On Hume's view, reason is not a divine endowment separating man from the beasts but a natural capacity shared across species, varying in complexity but not in principle. This position aligns Hume, at least in tendency, with Montaigne's earlier skepticism about human exceptionalism, though Hume arrives at it by a different route. Where Montaigne argued from the impressive performances of animals, Hume argues from the limitations of human cognition. The question of what distinguishes man from animal, and whether the distinction is one of kind or degree, is considered further under the idea of Man.

Hume's assertion that the difference between human and animal cognition is one of degree prepared the ground for Darwin's later argument that all differences between species are differences of degree accumulated over time. The continuity Hume asserted on philosophical grounds, Darwin would seek to demonstrate on biological ones.

Rousseau's Discourse on Inequality (1754) reopens the question of what distinguishes man from animal on terms quite different from those that had governed the debate in the preceding century. Descartes had sought the mark in the absence of flexible language and universal reason; Locke had placed it in the capacity for abstract ideas. Rousseau observes that animals too display prudence, communicate in their fashion, and show signs of sentience. If this is granted, then reason as commonly understood — the capacity to infer, to pursue ends, to solve problems — appears to be shared in some degree. The crucial distinction lies elsewhere. Nature commands every animal, and the brute obeys. Man receives the same impulse, but at the same time recognizes in himself the freedom to yield or to resist. This consciousness of freedom is one mark that separates the human from the animal; but it is the second mark — perfectibility — that Rousseau takes to be the deeper and more distinctive feature of human nature.

Perfectibility (perfectibilité) is a term Rousseau coins for the open-ended capacity to change over time, to develop new faculties, and to be altered by circumstance and experience in a way that progressively transforms the creature itself rather than merely its behavior. Animals have instincts that are fixed at birth and remain constant across generations; a beaver today builds as its ancestors built. Human beings have virtually no fixed instincts; what they are is largely the product of history, circumstance, and habituation. This plasticity is at once the source of all human progress and the condition of all human misery. The animal is adapted to its environment by nature; man must adapt himself and his environment by art. Perfectibility means that man can ascend to reason and virtue, but equally that he can sink below the animal, becoming idle, vicious, or enslaved in ways that no brute ever could. Rousseau draws the contrast explicitly: "The animal cannot deviate from the rule prescribed to it . . . man, even when it would be advantageous for him to do so, deviates to his own injury."

Rousseau's account of perfectibility bears on the question of animal intelligence that Montaigne had pressed skeptically and that Darwin would later approach through natural history. For Rousseau, the difference between human and animal cognition is not that humans reason and animals do not, but that human cognitive capacities are open and malleable in a way that animal capacities are not. The animal's intelligence, such as it is, is bounded by its nature; the human being's nature is, in a peculiar sense, to have no fixed nature. This structural openness distinguishes Rousseau's position from both Descartes (who denied intelligence to animals entirely) and Darwin (who located the difference in degree of associative capacity along a continuous scale). The relation between perfectibility and the natural condition of man is treated more fully under the ideas of Man and Progress; its bearing on the formation of human society is considered under State and Custom and Convention.

Rousseau's perfectibility became one of the most productive ideas in nineteenth-century anthropology and philosophy of history. Darwin read the Discourse with attention, though he explained the gap between human and animal nature through natural selection rather than an original human endowment. The problem of whether the human capacity for cultural self-transformation constitutes a difference in kind from animal adaptability remains, in much the form Rousseau gave it, a contested question at the intersection of biology, philosophy, and social thought.

Darwin's contribution to the discussion of animal nature is twofold: he proposes a new mechanism by which species originate and change, and he offers a new principle for their classification. The Origin of Species establishes natural selection acting on heritable variation as the cause of the diversity of animal forms. On this view, animals are not designed for their environments by a provident craftsman but are shaped by the differential survival and reproduction of variant individuals across generations. The question of whether the origin of animals is to be explained by creation or by evolution is one of the oldest in the tradition; it runs through Aristotle, Lucretius, Aquinas, and Harvey, and receives its fullest treatment under the idea of Change.

With respect to classification, Darwin departs from the Aristotelian tradition in principle rather than in the correction of observational errors. Where Aristotle and all taxonomists before Darwin classify animals by reference to their similarities and differences, Darwin makes inferred genealogy or descent the primary criterion. "All true classification," he maintains, is "genealogical," and "community of descent is the hidden bond which naturalists have been unconsciously seeking." The Natural System, on Darwin's view, must be "strictly genealogical in order to be natural." This replacement of a static principle of classification with a dynamic one has consequences for the broader question of whether the classes that taxonomists construct represent distinct natural forms or are somewhat arbitrary and artificial, a problem treated more fully under the idea of Definition.

In The Descent of Man, Darwin extends his argument to the human-animal boundary. Human reason, morality, language, and social life differ from their animal counterparts in degree, not in kind. Dogs display loyalty, apes use tools, birds communicate with calls that function like language in rudimentary form. Darwin, like William James after him, makes the difference between human and animal intelligence a matter of greater or less power to associate ideas, with the consequence that human instincts are much more modified by learning and experience than the instincts of other animals. The philosophical question of whether this difference in degree amounts to a difference in kind, which had occupied the tradition from Aristotle through Hume, receives from Darwin a biological restatement but not, perhaps, a final resolution.

Darwin changed the terms of the debate about animals permanently. After him, the question of whether species are fixed natural kinds became inseparable from the question of their genealogical descent. The problems of classification, the nature of species, and the continuity or hierarchy of the world of living things, which had been discussed under different assumptions from Aristotle onward, are all reconsidered in light of the theory of evolution.

William James devotes one of the longest chapters of The Principles of Psychology (1890) to instinct, and his treatment challenges the assumptions shared by most earlier participants in the debate about animal nature. The tradition had distinguished sharply between instinct, which governs animals, and reason, which governs men — with Aquinas holding the line categorically and Descartes sharpening it into the beast-machine hypothesis. Darwin had blurred the line by insisting on continuity between human and animal mental faculties. James presses the continuity argument further, but his more distinctive contribution is a claim about the nature of instinct itself. An instinct, he argues, is not a fixed, blind mechanism that unfolds the same way in every encounter. "Every instinctive act, in an animal with memory, must cease to be blind after being once repeated, and must be accompanied with foresight of its end just so far as that end may have fallen under the animal's cognizance." Once an animal has performed an instinctive act and experienced its results, that act can no longer be purely instinctive: it is now accompanied by memory, expectation, and something like anticipation. The instinct has been, in James's phrase, modified by experience.

James develops this argument by attending to the variety of instincts that higher animals possess. Man, he notes, "has a far greater variety of impulses than any lower animal," and every one of these impulses "taken in itself, is as blind as the lowest instinct can be." Human behavior is not less instinctive than animal behavior; it is more instinctive, but modified throughout by memory and inference. Where Aquinas had seen instinct and reason as the alternative means provided respectively for animal and human life, James sees them as intertwined throughout the vertebrate world, with the balance varying by degree of nervous system development. A dog's response to a situation it has encountered before is shaped by what it previously experienced; even apparently reflexive behavior in higher animals contains elements of learned expectation. The question of whether this constitutes genuine intelligence — whether a creature that modifies its behavior through experience can be said to think — James connects to the broader debate about mind and its relation to function, treated more fully under the idea of Mind.

On the question of the intellectual contrast between brutes and men, James is willing to locate a genuine difference, though it is one of degree. The capacity that most distinguishes human from animal minds is the power to associate ideas by similarity, which underlies abstraction. "The most elementary single difference between the human mind and that of brutes is the deficiency on the brute's part to associate ideas by similarity," so that "characters, the abstraction of which depends on this sort of association, must in the brute always remain drowned." Darwin had already observed a similar difference; James sharpens it by connecting it to his functional account of how minds work. Like Darwin, he assembles anecdotes to show that animals exercise their wits and learn from experience, but these demonstrations of animal sagacity serve to clarify the specific cognitive difference rather than to deny it. The result is a position that takes seriously both the intelligence of animals and the distinctively human capacity for abstract thought, without treating the line between them as absolute.

James's account of instinct had the effect of dissolving one of the firmest distinctions in the tradition: the clean separation between the kind of cognition animals perform and the kind humans exercise. For James, the difference is one of degree and type of associative capacity, not of faculty. His treatment of animal instinct as continuous with human impulse, while refusing to collapse the distinction between them entirely, set terms that subsequent psychology and ethology would find difficult to improve upon.