Theo Todman's Web Page - Paper Abstracts

1. Animalism¹⁸
2. What is an animal?¹⁹
3. The thinking-animal argument²⁰
4. Are there animals?²¹
5. Can animals think?²²
6. Too many thinkers²³
7. Revisionary linguistics²⁴
8. Animalism and our identity over time²⁵
9. Further objections²⁶

1. What sort of things might we be? Let us begin our study of answers to this question with the view that we are animals₂₆: biological organisms₂₇, members of the primate species [Homo sapiens₂₈. This has a certain immediate attraction. We seem to be animals. When you eat or sleep or talk, a human animal eats, sleeps, or talks. When you look in a mirror, an animal looks back at you. Most ordinary people suppose that we are animals. At any rate if you ask them what we are, and make the question clear enough to indicate that “animals” is one of the possible answers, they typically say that it is obviously the right answer. Few people would deny that we are animals. No one is going to feel immediately drawn to any of the alternative views – that we are bundles of perceptions, or immaterial substances, or non-animals made of the same matter as animals, say. Compared with those proposals, the idea that we are animals looks like plain common sense₂₉.
2. But things are not so simple. As we saw earlier, the appearance that we are animals may owe merely to our relating in some intimate way to animals – to our having animal bodies₃₀, if you like – rather than to our actually being those animals. The weight of authority is overwhelmingly opposed₃₁ to our being animals. Almost every major figure₃₂ in the history of Western philosophy denied it, from Plato and Augustine to Descartes, Leibniz, Locke, Berkeley, Hume, and Kant. (Aristotle₃₃ and his followers are an important exception.) The view is no more popular in non-Western philosophy₃₄, and most philosophers writing about personal identity today either deny outright that we are animals or say things that are incompatible with it. We will come to the reasons for this unpopularity later.
3. The view that we are animals has become known as animalism₃₅. Because animalism is easily confused with similar-sounding claims, I will say something about how I understand it. Animalism₃₆ says that each of us is numerically identical₃₇ with an animal: there is a certain organism, and you and it are one and the same. This would not bear stating but for the fact that some philosophers who deny that we are identical with any animal nonetheless insist on saying that we are animals. What they mean is that we “are” animals in some loose sense: in the sense of having bodies₃₈ that are animals, or of being “constituted by” animals, or the like. We are animals in something like the sense₃₉ in which an actor playing Lear is a king. That is not animalism.
4. This terminological point calls for a brief comment. I wish I could persuade philosophers not to state views according to which we are non-animals by saying that we are animals. It forces us to express the view that we are really animals – that we are animals in the ordinary, straightforward sense in which we are people₄₀ – with the ugly phrase ‘we are numerically identical with animals’. This is linguistically perverse₄₁: the most obvious interpretation of the sentence ‘That is an animal’ is surely that the denoted object really is an animal, and not that it relates in some way to something else that really is an animal. It is also tendentious₄₂: it makes animalism sound complicated and difficult when it ought to be simple and intuitive. Likewise, stating the view that we are non-animals constituted by animals (for instance) by saying ‘we are animals’ makes it sound simple and intuitive when it ought to be complicated and difficult. I, for one, refuse to play this mug’s game₄₃. When I discuss the view that we really are animals, I will state it by saying ‘we are animals’. And I will state the view that we are non-animals constituted by animals by saying ‘we are non-animals constituted by animals’. I encourage others to do the same₄₄.
5. Anyway, animalism says that we are animals, not that people in general are; so it is compatible with the existence of people who are not animals (gods or angels, say), and of animals – even human animals that are not people. Animalism is not an account of what it is to be a person, and implies no answer to the personhood question₄₅ of §1.6⁴⁷.
6. The view that we are animals may call to mind the idea that we are identical with our bodies₄₇. What does animalism say about this? Is it₄₈ the same as the view that we are our bodies? Does it at least entail that we are? I find these questions hard to answer₄₉. Suppose that a person's body, or at least a human person's body,
   1. Must always be a sort of animal: none of us could possibly have a non-animal body.
      
   And suppose also that
   2. None of us could ever be an animal other than the animal that is his body.
   If these assumptions₅₀ are true, then our being animals amounts to our being identical with our bodies. But are they true?
7. I don’t know. Someone might doubt whether a person's body must always be an animal. It is often said that we could have partly or wholly inorganic bodies: “bionic” bodies with plastic or metal parts, say, or even entirely robotic bodies. But no biological organism could come to be₅₁ partly or wholly inorganic. If you cut off an animal's limb and replace it with an inorganic prosthesis, the animal only gets smaller₅₂ and has something inorganic attached to it₅₃. It doesn’t acquire prosthetic parts₅₄. If you were to replace all an organism’s parts with inorganic prostheses, it would no longer be there at all₅₅. You couldn’t point to an inorganic machine and say truly, “That machine developed in its mother’s womb.” So it seems to me, anyway. If this is right – if we could acquire inorganic bodies, but no organism could become inorganic – then replacing some or all of your parts with inorganic gadgets could give you a body that was not an organism₅₆: a body that was at most partly organic. In that case you could be identical with your body without being an animal – or else be an animal without being identical with your body. Being an animal would be something different from₅₇ being your body, even if ordinarily, when our bodies are wholly organic, the two conditions coincide.
8. What it is right to say here depends on whether having some of your parts replaced by inorganic bits could give you a partly inorganic body (one that was not an animal), or whether it would only cause your body to shrink and become attached to those inorganic bits (as the animal would). And that depends in turn on what thing someone's body is. It depends, in other words, on what it is for a thing to be someone's body. For any objects x and y, what is necessary and sufficient for x to be y's body? What does it mean to say that
   1. A certain thing is your body, or that
   2. Your body is an animal, or that
   3. Someone might have a robotic body?
   Unless we have some idea of how to answer these questions₅₈, we shall have no way of saying whether someone might be identical with his body without being an animal or vice versa.
9. I have never seen a good account of what makes something someone's body (see₅₉ van Inwagen 1980, Olson 1997: 144-150, 2006a). I don’t know how to complete the formula₆₀ ‘necessarily, x is y’s body if and only if…’. Because of this I have no idea what would happen to₆₁ someone’s body if some of a human animal’s parts were replaced with organic prostheses; and I therefore have no idea whether someone could be his body without being an animal or vice versa. So I cannot say how animalism relates to the thesis that we are our bodies. More generally, I find the word ‘body’ unhelpful and frequently misleading in metaphysical discussions. (§2.5⁶³ below gives an example of the sort of confusion it can cause.) For the sake of convenience I will sometimes use₆₃ the term 'x's body' to mean
   1. The human animal intimately connected with x:
   2. The animal we point to when we point to x,
   3. The animal that moves when x moves,
   4. The animal that x would be if x were an animal at all,
   5. …and so on.
   This is merely a stipulation, however, and does not pretend to reflect the way other philosophers use the word 'body’.
10. Here is another delicate matter. Suppose someone said, "We are animals, but not just animals₆₄. We are more than mere biological organisms." Is that compatible with animalism? Does animalism say that we are nothing more than animals? That we are mere animals?
11. The answer depends on whether being "not just" or "something more than" an animal is compatible with being an animal. And that in turn depends on the import of the qualifications 'not just' and 'more than'. If a journalist complains that the Cabinet is more than just the Prime Minister, she means that the Cabinet is not the Prime Minister: it has other members too. If we are more than just animals in something like that sense, then we are not animals at all₆₅; at best we may bear some intimate relation to those animals we call our bodies. That may be because we have parts that are not parts of any animal, such as immaterial souls. On the other hand, we say that Descartes was more than just a philosopher: he was also a mathematician, a Frenchman, a Roman Catholic, and much more. That is of course compatible with his being a philosopher. We could certainly be more than just animals in this sense₆₆, yet still be animals. We could be animals, but also mathematicians or Frenchmen or Roman Catholics. There is nothing "reductionistic₆₇", in the pejorative sense of the term, about animalism. An animal can have properties other than being an animal₆₈, and which do not follow from its being an animal. At any rate there is no evident reason why not. Despite its ugly name, animalism does not by itself imply that our behavior is determined by a fixed, "animal" nature, or that we have only crudely biological properties, or that we are no different in any important way from other animals. We could be unique among animals₆₉, and yet be animals.
12. Finally, animalism does not say that we are animals essentially₇₀; for all it says, our being animals might be only a contingent or perhaps even a temporary feature₇₁ of us, like our being philosophers. Whether we could be animals contingently depends on whether human animals are animals contingently: whether it is possible for something that is in fact a human animal to exist without being an animal. Animalism implies that we have the metaphysical nature of human animals; but what that nature amounts to₇₂ is a further question (see below). My own view, and that of most philosophers, animalists or not, is that animals are animals essentially₇₃; but few arguments for or against our being animals turn on this claim.

1. Saying that we are animals will tell us little about what we are unless we have some idea of what sort of thing an animal is. I mean by 'animal' what biologists mean by it: animals are biological organisms, along with plants, bacteria, protists, and fungi. Animals are what zoologists study. Someone might say that ‘animal’ in the ordinary sense of the word means nothing more than ‘animate being’ – a thing that can move and perceive – and that whether animals in this sense are biological organisms is an open question. If that is the case, then my use of the word ‘animal’ is not the ordinary one, and I ought to have used the term ‘organism’ or ‘animal in the biological sense’ instead.
2. Anyway, here is a brief sketch of what I take to be the metaphysical nature of animals. The view I will offer has [controversial elements}₇₄, but it is nonetheless widely held. (More detailed accounts more or less consistent with mine are found in₇₅. van Inwagen 1990b: §14, Hoffman and Rosenkrantz 1997: ch. 4 and Wilson 1999: ch. 3.)
3. As I see it, animals, including human animals, have more or less the same metaphysical nature as other biological organisms. This is not to deny that some animals may have properties of considerable metaphysical interest – rationality and consciousness, for instance – that no plant or fungus could ever have. But if we ask
   1. What organisms are made of,
   2. What parts they have,
   3. Whether they are concrete of abstract,
   4. whether and under what conditions they persist through time,
   … and the like, I believe that the answer will be more or less the same for human organisms as it is for plants and fungi. So we need an account of the metaphysical nature of organisms generally₇₆.
4. I take it that
   1. Organisms are concrete particulars.
   2. They are substances₇₇, and not events or states or aspects of something else.
   3. They persist through time; moreover
   4. They continue to be organisms when they persist.
   5. I will assume for the present that they do not have temporal parts, though we will revisit this assumption in Chapter 5⁷⁹.
   6. I also assume that organisms are made up entirely of matter: they have no immaterial or non-physical parts.
   Descartes thought that each normal human animal was somehow attached to an immaterial substance that is necessary for a thing to think rationally, but not necessary for it to be alive in the biological sense. If this were true, I take it that the animal would be the material thing₇₉, and not the object made up of the material thing and the immaterial one.
5. Organisms differ from other material things by having lives. By a life I mean more or less what Locke meant (1975: 330-31): a self-organizing biological event₈₀ that maintains₈₁ the vastly complex internal structure of an organism. The materials organisms are made up of are intrinsically unstable, and must therefore be constantly repaired and renewed₈₂, else the organism dies and its remains decay. An organism must constantly take in new particles, reconfigure and assimilate them into its living fabric, and expel those that are no longer useful to it. An organism's life enables it to persist and retain its characteristic structure despite constant material turnover₈₃.
6. There may be things besides organisms that are in some sense alive: certain parts of organisms, such as arms, and things made up of several organisms, such as packs of wolves. They are not organisms because they lack lives of their own₈₄. My arm's tissues are kept alive by the vital processes of the human animal it is a part of: there is no self-organizing biological event of the right sort to be a life going on throughout my arm and nowhere else.
7. Organisms have parts₈₅: vast numbers of them. A thing is alive in the biological sense by virtue of a vastly complex array of biochemical processes, and the particles caught up in these processes are parts of the organism. (If Aristotle₈₆ thought that organisms were mereologically simple, that is presumably because he thought that matter was homogeneous and not particulate.) Owing to metabolic turnover, organisms are made up of different parts at different times₈₇.
8. What are the parts of an organism? Where does an organism leave off, and its environment begin? Where an organism's boundaries lie has presumably to do with the spatial extent of its life₈₈. But just how its life determines its boundaries₈₉ is not obvious. It is tempting to say that an organism is made up at a given time of just those particles that are caught up in its life – its metabolic activities₉₀ – at that time. If you are an organism, you extend all the way to the surface of your skin and no further because that is the extent of your biological life₉₁. Your clothes, or a prosthetic limb₉₂, are not parts of you
   1. Because damage to them is not repaired in the way that damage to your living fabric is repaired,
   2. Because they are not nourished by your blood supply,
   3. Because their parts are not renewed and replaced in the way that parts of your kidneys are,
   … and so on. Neat though this view is, however, some find it too restrictive. They say that the particles in an animal's hair or in the dead heartwood of an ageing tree are parts of the organism, despite no longer being caught up in its life (Ayers 1991: 225₉₃.). We needn’t settle this matter for present purposes₉₄.
9. As for identity over time, I am inclined to believe that an organism persists if and only if its life continues₉₅. This has the surprising consequence that an organism ceases to exist when the event that maintains its internal structure₉₆ stops and cannot be restarted – that is, when the organism dies. Whatever is left behind – the organism's lifeless remains or its corpse or what have you – is something other than the organism. Strictly speaking, there is no such thing as a dead organism: no organism can be alive at one time and dead at another. I believe this because I have never seen a plausible alternative account of what it takes for an organism to persist (Olson 2004: 269-271₉₇.). It is not a wholly eccentric view: in addition to Aristotle (see Furth 1988: 156-157₉₈.) and Locke (1975: 330-331), it has several contemporary advocates₉₉. (van Inwagen 1990: 142-158, Hoffman and Rosenkrantz 1997: 159, Wilson 1999: 89-99). It is controversial, however, and nothing I say in this book turns on it. The persistence conditions of human animals will concern us again in §2.7¹⁰¹ and §7.7¹⁰².

1. Why suppose that we are animals? Well, there are about six billion human animals walking the earth – the same as the number of human people. For each of us there is a human animal, and for every human animal (pathological cases aside₁₀₃, perhaps) there is one of us. Those animals are very like ourselves: they sit in our chairs and wear our clothes; they do our work and read our newspapers and chat with our friends. They appear to be so like us, both physically and mentally, that it is hard to tell the difference. These apparent facts pose a formidable obstacle to anyone who would deny that we are animals: the thinking-animal problem.
2. There is a human animal intimately related to you, which some call your body. Consider that animal’s mental properties. It would seem to have mental properties. You have mental properties, and the animal has the same brain and nervous system as you have (and the same surroundings too, if that is relevant). It went to the same schools as you did, and had the same teachers. It shows the same behavioral evidence of mentality as you do. What more could be required₁₀₄ for a thing to have mental properties? In fact the animal seems to be mentally exactly like you: every thought or experience of yours appears to be a thought or experience on the part of the animal. How could you and the animal have different thoughts? But if the animal thinks your thoughts, then surely it is you. You could hardly be something other than the thing that thinks your thoughts₁₀₅.
3. Consider what it would mean if you were not the animal. The animal thinks. And of course you think. (We can’t suppose that the animal thinks and you don’t think. Nor can we suppose that you don’t exist, when your animal body thinks.) So if you were not that thinking animal, there would be two beings thinking your thoughts: there would be the thinking animal, and there would be you, a thinking non-animal. We should each share our thoughts with an animal numerically different from us. For every thought there would be two thinkers₁₀₆.
4. Or perhaps the animal located where you are doesn’t think, or doesn’t think in the way that you do. Something might prevent it from thinking. Someone might even suppose that it was a mistake to concede the existence of an animal sitting there in the first place: maybe there is strictly speaking no such thing as your body. In any case, there are just three alternatives to your being an animal:
   1. There is no human animal where you are;
   2. There is an animal there, but it doesn’t think in the way that you do; or
   3. There is an animal there, and it thinks exactly as you do, but you are not it.
      
   There is no fourth possibility₁₀₇. The repugnancy of these three alternatives seems to me a powerful reason to suppose that you are an animal. Let us consider them.

1. If you are not an animal, the reason may be that there is no animal that you or anyone else could be. How could there be no human animals? What reason could anyone have for believing this?
2. A number of general metaphysical principles are incompatible with the existence of animals. For instance, some versions of idealism entail that there are no material objects at all (so I should describe those views, anyway); and if there are no material objects, then there are no biological organisms. But let’s not discuss idealism₁₀₈. Another example is the principle that nothing can have different parts at different times. According to this principle₁₀₉, whenever something appears to exchange an old part for a new one, the truth of the matter is that the object composed of the old parts ceases to exist (or else begins to disperse₁₁₀) and is instantly replaced by a new object composed of the new parts. Yet organisms by their very nature constantly exchange old parts for new ones. If nothing could ever survive a change of any of its parts, then organisms are metaphysically impossible₁₁₁; what we think of as an organism is in in reality only a series of "masses of matter" that each take on organic form for a brief moment- -until a single particle is gained or lost – and then pass that form on to a numerically different mass.
3. The principle that nothing can change its parts is both theoretically elegant and strikingly implausible₁₁₂ (we will return to it in §7.3¹¹⁴-§7.4¹¹⁵). But few opponents of animalism deny the existence of animals. They have good reason not to: anything that would rule out the existence of animals would also rule out the existence of most of the things we might be if we were not animals. If there are no animals, then there are no beings constituted by animals, for instance, and no temporal or spatial parts of animals. And if nothing can change its parts, then persisting bundles of perceptions are no more possible than animals. If there are no animals, there will be few items remaining among the furniture of the earth₁₁₅ that we might be.

1. The second alternative to our being animals is that the animals we call our bodies exist but don’t think in the way that we do. (Let any sort of mental activity or state count for present purposes as thinking.) There are two possibilities here:
   1. that human animals don’t think at all, and
   2. that they think but not as we do.
      
2. Consider first the idea that they don’t think at all. You think, but the animal sitting there doesn't. The reason for this can only be that the animal cannot think: it would certainly be thinking now if it were able to. And if that animal cannot think now, no human animal can ever think, for no human animal is better suited for thinking that it is. Presumably no biological organism of any sort could think. The claim, then, is that animals, including human animals, are no more sentient or intelligent than stones; in fact they are necessarily incapable of thought. It may still be that most human animals relate in some intimate way to thinking beings – to us – and stones do not; and it might be appropriate for certain purposes to describe this fact loosely by saying that human animals are more intelligent than stones₁₁₆. But strictly speaking human animals would have no mental properties whatever.
3. That would be surprising. Human animals seem to think. Could this really be only a misleading appearance? If human animals and other organisms cannot think, there must surely be some impressive explanation of why they can’t – that is, some account of what prevents them from using their brains to think.
4. One possible explanation is that nothing can think: there is no such thing as thinking, any more than there is such a thing as phlogiston (a chemical substance once thought to be a constituent of solid matter and released in combustion). This view is known as eliminative materialism₁₁₇. But no opponents of animalism that I know of accept it. If it were true, it could not be the case that our identity through time consists in psychological continuity, or that we have our mental properties essentially; and that would leave little reason to suppose that we are anything other than animals (see §2.8¹¹⁹ and §2.9¹²⁰).
5. Suppose eliminative materialism is false. In that case, the reason why human animals cannot think must presumably be that they have some property that prevents them from thinking – a property that we, who clearly can think, lack. (Or maybe they lack a property of ours that is necessary for thought.) The most obvious candidate for such a property is being material. If any material thing could ever think, surely it would be some sort of animal; so if animals cannot think, we should expect the reason to be that only an immaterial thing could think₁₂₀. You and I must therefore be immaterial. Of course, simply denying that any material thing could think does nothing to explain why it couldn't; but those who hold this view have said many things that would, if they were true, explain why no material thing could think. So you might expect anyone who denies that you and I are animals to deny that we are material things of any sort. But this is not so: many opponents of animalism claim to be materialists₁₂₁. They cannot explain human animals' inability to think by appealing to the fact that animals are material.
6. They might say that human animals cannot think because they are mere bodies, and mere bodies cannot think. It could only be some sort of joke, the idea goes, to say that Newton’s body believed₁₂₂ in absolute space, while Leibniz’s body disagreed. Since we think, it would follow that we are not our bodies, and therefore not animals. But that wouldn't mean that we are immaterial: we might be material things other than our bodies.
7. Now even if this is a reason to believe that animals cannot think, it does nothing to explain why₁₂₃ they can’t. That a human animal is someone’s body and that it is somehow absurd to say that someone’s body thinks tells us nothing about why a human animal, call it what you will, should be unable to think. It makes that claim no less surprising or easier to believe. (Compare₁₂₄: if Professor Hawking tells us that light cannot escape from a black hole, that is a reason to believe it, but no explanation of why it is so.)
8. In any case, it is hardly an impressive argument against animal thought. I grant that there is something odd about the statement that Newton’s body believed in absolute space. But a statement can be odd without being false. Though it sounds preposterous to say₁₂₅ that there is a liter of blood in my office, it is nevertheless true: I am in my office, and there is a liter of blood in me. The statement is odd because it suggests that blood is stored in my office in something like the way it is stored in blood banks, which really would be preposterous. The statement that Newton’s body believed in absolute space might be odd for a similar reason. For instance, the reason it sounds wrong might be₁₂₆ that it suggests the false claim that believing in absolute space is in some sense a “bodily” property.
9. In any case, the oddness of saying that Newton’s body believed in absolute space should not lead us to infer that the phrase ‘Newton’s body' denotes something of Newton’s – a certain human organism – that was unable to think. Compare the word 'body' with a closely related one: mind. It is just as odd to say₁₂₇ that Newton’s mind was tall and thin, or indeed that it had any other size or shape, as it is to say that Newton’s body believed in absolute space. But no one would conclude from this that Newton had some mental thing with no size or shape. That would be a poor argument for substance dualism. We cannot always substitute the phrase 'Newton’s mind' for the name 'Newton' without something going wrong; but it is doubtful whether any important metaphysical conclusion follows from this. We ought to be equally wary of drawing metaphysical conclusions from the fact that we cannot always substitute the phrase 'Newton's body' for the name 'Newton' without something going wrong.
10. Anyone who wants to explain why some material objects can think but animals cannot has his work cut out for him. I know of just two possible explanations worth considering. The first says that animals cannot think because they are too big₁₂₈. The true thinkers are brains, or perhaps parts of brains. A whole animal can be said to think only in the derivative sense of having a thinking brain as a part, much as a car is powerful in the sense of having a powerful engine as a part. Animals are stupid things inhabited by clever brains. I will take up this idea in Chapter 4¹³⁰.
11. The second, which is far more interesting, is due to Shoemaker₁₃₀. (1984: 92-97, 1999, 2004). He says that animals cannot think because they have the wrong identity conditions₁₃₁. Mental properties, he says, have characteristic causal roles. For you to be hungry, for instance, is for you to be in a state that, among other things, is typically caused by your having low blood sugar, and which tends to cause you to act in ways you believe would result in your eating something nourishing. Now your hunger is a state that tends to combine with your beliefs – not mine – to cause you, and no one else, to behave in certain ways. That is part of the nature of hunger. More generally, for you to have any mental property is at least in part for you to be in a state disposed to combine with certain of your other states to cause you, and no one else, to do certain things.
12. But that, Shoemaker claims, is to say that any being whose later states or actions are caused in the appropriate way by your current mental states must be you₁₃₂. Now suppose your cerebrum₁₃₃ is put into my head tomorrow. Then your current mental states will have their characteristic effects₁₃₄ in the being who ends up with that organ, and not in the empty-headed thing left behind. The subject of those states – you – must therefore go along with its transplanted cerebrum₁₃₅. It follows that₁₃₆ psychological continuity of a sort must suffice for you to persist through time. More generally, the nature of mental properties entails that psychological continuity suffices₁₃₇ for anything that has them to persist. Since no sort of psychological continuity suffices₁₃₈ for any organism to persist – no human animal would go along with its transplanted cerebrum₁₃₉ – it follows that no organism could have mental properties₁₄₀. The nature of mental properties makes it metaphysically impossible for animals to think. However, material things with the right identity conditions would be able to think. Shoemaker believes that human organisms typically "constitute₁₄₁" such things.
13. It is important to see just how surprising this view is₁₄₂. Suppose you and I are physically just like human animals. (Shoemaker more or less accepts this.) Then the view implies that beings with the same physical properties and surroundings can differ radically in their mental properties. In fact this happens regularly: every human person coincides with an animal physically indistinguishable from her – a perfect physical duplicate – that has no mental properties whatever. There are physically identical beings, in identical surroundings, that differ as much in their mental properties as we differ from trees. Mental properties fail to supervene on physical properties in even the weak sense that any two beings with the same physical properties will have the same mental properties. A thing's having the right physical properties and surroundings does not even reliably cause it to have any mental properties.
14. I find Shoemaker’s argument against animal thought unpersuasive. It doesn’t seem absolutely necessary that the characteristic effects of a being’s mental states must always occur in that very being. In fact it seems that it would not be so in fission cases₁₄₃. Suppose your cerebrum is removed from your head and each half is implanted into a different empty head. Then your mental states have their characteristic effects in two different people. If the nature of mental states entails that the donor must be identical with the recipient in the “single” transplant case, it ought to entail that the donor must be identical with both recipients in the double transplant. But that, as Shoemaker himself agrees, is impossible.
15. There is more to say about Shoemaker’s argument, but this is not the place for it (see Olson 2002c₁₄₄.). What if human animals do think, but not in the way that we do? There are two possibilities here.
    1. One is that they have different mental properties from us: for instance, they are conscious but never self-conscious₁₄₅.
    2. The other is that human animals have the same mental properties as we have, but they have them in a different way: for instance, they think only in the derivative sense of relating in a certain way to us, who think in a straightforward and non-derivative sense₁₄₆.
    By itself, however, neither of these suggestions does anything to solve the thinking-animal problem₁₄₇. It would be just as surprising if human animals were incapable of having the sorts of thoughts that we have, or if they could not think in the sense that we do, as it would be if they could not think at all. It would demand the same sort of explanation, and the prospects for finding one would be similar. It is hard to see what opponents of animalism would gain by proposing such a view.

1. Suppose human animals think in just the way that we do: every thought of yours is a thought on the part of a certain animal. How could that thinking animal be anything other than you? Only if you are one of at least two beings that think your thoughts. (Or maybe you and the animal think numerically different but otherwise identical thoughts. Then you are one of at least two beings thinking exactly similar thoughts.) If you think, and your animal body thinks, and it is not you, then there are two thinkers there, sitting and reading this book. Call this the cohabitation view. It is unattractive in at least three different ways₁₄₈.
2. Most obviously, it means that there are far more thinking beings₁₄₉ than we thought: the overcrowding problem. Defenders of the cohabitation view – and it has its defenders – typically respond by proposing linguistic hypotheses. They propose that the things we ordinarily say and believe about how many people there are do not mean or imply what they appear to mean or imply. They interpret, or reinterpret, our ordinary, non-philosophical statements and beliefs in a way that would make them consistent with the cohabitation view. When I write on the copyright form that I am the sole author of this book, for instance, I might seem to be saying that every author of this book is numerically identical with me, which according to the cohabitation view is false. But it may not be obvious that that is what I mean. Perhaps I mean only that every author of this book bears some close relation to me: that they all share their thoughts with me, say, that they exactly coincide with me. In that case the ordinary statement that I am the sole author of this book comes out true, even if strictly speaking the book has at least two authors. My wife is not in any ordinary sense a bigamist₁₅₀, even if she is married both to me and to this animal. At any rate it would be badly misleading to describe our relationship as a ménage à quatre.
3. The general idea is that whenever two thinking beings relate to one another in the way we relate to our animal bodies, we "count them as one" for ordinary purposes (Lewis 1993₁₅₁). Ordinary people – people not engaged in metaphysics – have no opinion about how many numerically different thinkers there are. Why should they? What matters in real life is not the number of numerically different thinkers, but the number of non-overlapping thinkers. Human people and thinking human animals don’t compete for space. The world is overcrowded only in a thin, metaphysical sort of way and not in any robust quotidian sense.
4. If this is right, the cohabitation view is consistent with everything we ordinarily say and believe about how many of us there are. But that does not entirely deprive the overcrowding problem of its force. Philosophers of language who know their business can take any philosophical claim, no matter how absurd₁₅₂, and come up with a linguistic hypothesis according to which that claim is compatible with everything we say and think when we’re not doing philosophy. If I say that I had breakfast before I had lunch today, there is no doubt something I could be taken to mean that would make my statement compatible with the unreality of time₁₅₃. But that would not make it any easier to believe that time is unreal – not much, anyway. For the same reason, the mere existence of the hypothesis that we “count” philosophers by a relation other than numerical identity does little to make it easier to believe that there are two numerically different philosophers₁₅₄ sitting there and reading this now. That is because that linguistic hypothesis seems to most of us to be false₁₅₅.
5. In any case, the troubles for the cohabitation view go beyond mere overcrowding. The view makes it hard to see how we could ever know that we were not animals₁₅₆. If there really are two beings, a person and an animal, now thinking your thoughts, you ought to wonder which one you are₁₅₇. You may think you're the person – the one that isn't an animal. But since the animal thinks exactly as you do, it ought to think that it is a person. It will have the very same grounds for thinking that it is a person and not an animal as you have for believing that you are. Yet it is mistaken. If you were the animal and not the person, you would still think you were the person. So for all you know, you are the one making the mistake. Even if you are a person and not an animal, it is hard to see how you could ever have any reason to believe that you are₁₅₈. Call this the epistemic problem.
6. The cohabitation view is unattractive in a third way as well. If your animal thinks just as you do, it ought to count as a person₁₅₉. It satisfies every ordinary definition of 'person': it is, for instance, "a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places," as Locke put it. But no one supposes that your animal body is a person numerically different from you – that we each share our thoughts with another person. If nothing else, that would contradict the popular claim that people – all people – have properties incompatible with those of animals (see §2.7¹⁶¹ below). It would also mean that some human people are animals, even if others are not. And if human animals are not only psychologically indistinguishable from ourselves but are also people in their own right, it is even more difficult to see how anyone could have any reason to believe that she was not one of the animal people.
7. If ordinary human animals are not people, on the other hand, despite having the same mental properties as people, all familiar accounts of what it is to be a person are too permissive. There could be non-people whose inner life was entirely indistinguishable from ours; indeed, there would be at least one such non-person for every genuine person. That would deprive personhood of any psychological or moral significance. For that matter, it would make it a real epistemic possibility that we are not people. I can verify easily enough that I am rational, self-conscious, and so on; but how could I assure myself that I have that extra feature required for personhood that rational human animals lack? Call this problem – that our animal bodies would be people different from ourselves – the personhood problem₁₆₁.

1. Some say that the epistemic problem₁₆₂ has a linguistic solution (Noonan 1998₁₆₃, forthcoming). They make two surprising claims.
   1. First, they say, not just any rational, self-conscious being, or more generally any being with our mental capacities, is a person. To count as a person, a thing must have not only the appropriate mental qualities, but something else as well: it must persist by virtue of psychological continuity₁₆₄, or have those mental qualities essentially₁₆₅, or the like. Call this extra feature F. That beings must have F in order to fall within the extension of the word ‘person’ is supposed to be a contingent fact about how we use₁₆₆ that word. Human animals lack F₁₆₇, and therefore do not qualify as people, despite being psychologically indistinguishable from ourselves. That is the first claim.
   2. The second is that the word 'I' and other personal pronouns, at least in their most typical uses, refer only to people₁₆₈: that's why we call them personal pronouns. A being that says 'I' in normal circumstances refers thereby to the person who says it. This too is supposed to be a contingent fact about how we use language₁₆₉.
      
2. These two claims, together with the cohabitation view, yield the startling conclusion₁₇₀ that first-person utterances (and presumably first-person thoughts as well) do not always refer to the beings that utter or think them. In particular, when your animal body says 'I', it doesn't refer to itself, as it isn't a person. But presumably you have F; so you are a person, and when you say or think 'I', you do refer to yourself. Since your animal body says and thinks just what you say and think, its first-person utterances and thoughts therefore refer to you – the person who produces them – rather than to itself. If it says, "I am hungry," it means not that it itself is hungry, but that you are. More to the point, if the animal says or thinks, "I am a person and not an animal," it does not say falsely that it is a person and not an animal, but truly that you are. So neither you nor the animal is mistaken₁₇₁ about which thing it is.
3. Call this linguistic hypothesis – that personal pronouns refer only to people and that people by definition have F – personal-pronoun revisionism₁₇₂. How would it solve the epistemic problem? Suppose there are two beings thinking your thoughts: an animal, and also a nonanimal with psychological persistence conditions – a psychological continuer for short. Better, suppose that you know this. Suppose further that having psychological persistence conditions is the extra person-making feature F. Now imagine wondering which of the beings thinking your thoughts you are, the animal or the psychological continuer. How could you work out the answer to this question?
4. Well, as a competent speaker of English you would know at least implicitly
   1. That each occurrence of the word 'I' refers only to the person who utters it. You would also know, or be able to work out,
   2. That something counts as a person only if it is a psychological continuer, which according to pronoun revisionism is true by definition. And of course you know
   3. That you are whatever you refer to when you say 'I'. These are supposed to be linguistic and conceptual facts that we can know a priori. Given that a psychological continuer thinks your thoughts, it follows from these claims
   4. That you are a person and a psychological continuer.
   If you know that animals are not psychological continuers, you can infer from this that you are not an animal – even if you share your thoughts with an animal psychologically indistinguishable from you. You can therefore know that you are a psychological continuer and not an animal. You can know which of the beings thinking your thoughts you are. That would solve the epistemic problem₁₇₃.
5. There is much to be said about this proposal (I discuss it at greater length in₁₇₄ Olson 2002b; see also Zimmerman 2003: 502-503). I will make just one comment. We are supposing that the human animals that walk and talk and sleep in our beds have the full range of human attitudes and emotions, and are psychologically indistinguishable from ourselves₁₇₅. (We discussed the view that human animals are psychologically different from ourselves in the previous section.) Now consider your understanding of the word ‘person’. In particular, think of the sense of the word that informs your use of the personal pronouns. What features must a being have in order for you to call it a person in that ordinary sense? What must it have in order to be a someone rather than a something, a he or a she rather than an it? If something were psychologically indistinguishable from yourself, or from one of your close friends, would you refuse to call it a person or a someone until you were told whether it persists by virtue of psychological continuity? That seems to be no part of what we ordinarily mean by ‘person’. If human animals really are psychologically just like ourselves, they will count as people in any ordinary sense of the word. It couldn’t turn out that₁₇₆ half of the rational, self-conscious, human-sized beings that we know and love and interact with in daily life are not people. Human animals may fail to satisfy some specialized philosophical sense of ‘person’, owing to their having the wrong persistence conditions or on some other trivial grounds. But they are surely people in the sense of the word that informs our ordinary use of the personal pronouns.
6. Maybe it isn’t always clear to us what we mean by our words. Some ordinary words may mean something very different from what they seem to mean. Perhaps we cannot dismiss personal-pronoun revisionism as absurd. But it is hardly part of an attractive alternative₁₇₇ to animalism.

1. Those who say that we are not animals will probably want to argue either that₁₇₈ human animals cannot think in the way that we can, or that we can somehow know that we are not the human animals that share our thoughts. Neither prospect looks promising. That, to my mind, is the principal case for our being animals₁₇₉. What is the case against₁₈₀ it?
2. Historically, the main reason for denying that we are animals is hostility to materialism₁₈₁. The conviction that no material thing, no matter how complex, could ever think in the way that we do is clearly incompatible with our being animals. But few philosophers₁₈₂ set much store by it nowadays. The main contemporary objection to animalism has to do with our identity over time, the most popular account of which is that we persist by virtue of some sort of psychological continuity₁₈₃. That rules out our being animals, for no sort of psychological continuity is either necessary or sufficient₁₈₄ for a human organism to persist.
3. To see that it isn’t necessary,
   1. Consider the fact that each human animal starts out as an embryo₁₈₅ incapable of any sort of mental activity. There is no psychological continuity of any sort between an adult human animal and the embryo it once was: the adult animal’s mental properties cannot derive in any way from those of the embryo, for the embryo had none. The embryo is₁₈₆ the adult human organism, yet there is no psychological continuity between the embryo as it started out and the full-grown animal as it is today. A human animal can therefore persist without any psychological continuity whatever. Or
   2. Consider what would happen if you were to lapse into a persistent and irreversible vegetative state₁₈₇. The result of this would be a human organism that was clearly alive, in the biological sense in which an oyster₁₈₈ is alive: it would breathe spontaneously, digest its food, fight infection, heal wounds, and so on. It would presumably be the very human organism₁₈₉ that was once able-bodied: no one supposes that a human animal that lapses into a persistent vegetative state thereby ceases to exist and is replaced by a new animal. But the animal would no longer be capable of any mental activity. Again, a human animal can persist despite complete psychological discontinuity.
   If any sort of psychological continuity is necessary for you to persist, then your animal body existed before you did, and it could outlive you. But nothing existed before it itself existed, and nothing can outlive itself. It follows that you are not that animal₁₉₀.
4. Now the claim that psychological continuity is necessary for us to persist may sound unattractive₁₉₁. Those who have actually suffered the misfortune of having a loved one lapse into a persistent vegetative state do not often believe that that person has literally ceased to exist₁₉₂, and that the living thing lying on the hospital bed is a numerically different being. (They may say that their loved one’s life no longer has any value, or that he ought to be allowed to die; but that is another matter₁₉₃.) Nor does this attitude appear to rest on the mistaken belief₁₉₄ that there is some sort of psychological continuity in these cases. And when we see an ultrasound picture of a 12-week-old foetus, most of us are inclined believe that we are seeing something which, if all goes well, will come to be₁₉₅ a full-fledged human person, even though it now has no mental properties. (This is something that most parties to debates over the morality of abortion₁₉₆ agree on.) We don’t ordinarily suppose that the foetus cannot itself become a person, but can only give rise to a person numerically different from itself.
5. In fact animalism appears to be compatible with everything we believe about our persistence in real-life situations. In every actual case, the number of people we think there are is the same as₁₉₇ the number of rational human animals. Every actual case in which we take someone to survive or perish is a case where a human animal survives or perishes. Or at least this is so if we leave aside religious beliefs – our being animals may be incompatible with our being resurrected or reincarnated (though some leading philosophers of religion disagree₁₉₈: see van Inwagen 1978, Zimmerman 1999, Merricks 2001a).
6. But animalism conflicts with things we are inclined to say about science-fiction stories. This appears to show a deep and widespread conviction that some sort of psychological continuity is sufficient for us to persist₁₉₉.
7. Imagine that your cerebrum is put into another head. The being who gets that organ, and he alone, will be psychologically continuous with you on any account of what psychological continuity is: he will have, for the most part anyway, your memories, beliefs, and other mental contents and capacities; he will have your “first-person perspective₂₀₀”; he will take himself to be you₂₀₁; all these mental properties will have been continuously physically realized throughout the process; and there are no troublesome rival claimants. If any psychological facts suffice for you to persist, that being would be you: you would go along with your transplanted cerebrum. And many people are convinced that you would indeed go along with₂₀₂ your transplanted cerebrum.
8. What about your animal body? Would it go along with its cerebrum? Would the surgeons pare that animal down to a lump of yellowish-pink tissue, move it across the room, then supply it with a new head, trunk, and other parts? Surely not₂₀₃. A detached cerebrum is no more an organism₂₀₄ than a detached arm is an organism: if the animal went along with the cerebrum, it would have to cease being₂₀₅ an animal for a time and then become an animal once more when the transplant is complete. More importantly, think of the empty-headed thing₂₀₆ left behind when your cerebrum is removed. It is an animal. If the surgeons are careful to leave the lower brain intact, it may even remain alive₂₀₇. It seems to be₂₀₈ the very animal that your cerebrum was a part of before the operation. The empty-headed being into which₂₀₉ your cerebrum is to be implanted is also a living human organism. And putting your cerebrum into its head surely doesn’t destroy₂₁₀ that organism and replace it with a new one.
9. So there appear to be two human animals in the transplant story. One of them loses its cerebrum and gets an empty head. That organ is then fitted into the empty cranium of the other animal, which is thereby made whole again. The surgeons move an organ from one animal to another, just as₂₁₁ they might do with a liver. No animal moves from one head to another. Even though there is full psychological continuity between the cerebrum donor and the recipient, they are not the same animal. Thus, no sort of psychological continuity suffices₂₁₂ for a human animal to persist through time. One human animal could be psychologically continuous in the fullest possible sense with another human animal₂₁₃.
10. The conviction that you would go along with your transplanted cerebrum is therefore incompatible with your being an animal. Your animal body would stay behind if your cerebrum were transplanted. If you would go along with your transplanted cerebrum, then you and that animal could go your separate ways. And of course a thing and itself can never go their separate ways. It follows that you are not that animal, or indeed any other animal. Not only are you not essentially an animal. You are not an animal at all, even contingently: nothing that is even contingently an animal₂₁₄ would move to a different head in a cerebrum transplant.
11. So the principal case against animalism₂₁₅ is this: If we were animals, we should have the persistence conditions of animals, conditions which have nothing to do with psychological facts. Psychology would be completely irrelevant to our identity over time. Cerebrum transplants would be no different, metaphysically, from liver transplants: you could donate your cerebrum to someone else, just as you could donate your liver. But that is absurd. Psychology clearly is relevant to personal identity. You would go along with your transplanted cerebrum; you wouldn’t stay behind with an empty head. Therefore we are not animals.
12. Taken in isolation, the transplant argument may look strong. Why deny that we should go along with our transplanted cerebrums? Isn’t it obvious that that is what would happen? But we have seen how this “transplant conviction” could be wrong: it would be wrong if we were animals. Would it really be so surprising if it were wrong? To my mind, it would be surprising if it were right₂₁₆. That would mean either that human animals cannot think, or that you are one of two beings thinking your thoughts, and one of those beings would not go along with its transplanted cerebrum. That would be surprising.
13. In any case, there are other reasons to doubt the transplant conviction. For one thing, the sort of psychological continuity that would hold between you and the recipient of your cerebrum could hold between you and two future beings. If your cerebrum were divided₂₁₇ and each half implanted into a different head, at least one of the resulting beings would be mistaken in thinking that she was you, for the simple reason that one thing (you) cannot be numerically identical with two things₂₁₈. Someone can be fully psychologically continuous with you and yet not be you: psychological continuity is not sufficient₂₁₉ for us to persist. That undermines the judgment that the one mentally continuous with you in the original transplant story would be you. If the claim that anyone psychologically continuous with you must be you fails to hold in fission cases, it might fail₂₂₀ to hold in cerebrum transplants too.
14. For another, the transplant conviction gets much of its support from a questionable assumption about our practical attitudes – "what matters in identity₂₂₁", as the jargon has it. Imagine that your cerebrum is about to be transplanted into my head. The empty-headed being left behind will then get a new cerebrum. The hospital has only enough morphine for one of the two resulting people; the other will suffer unbearable pain₂₂₂. If we asked you before the operation who should get the morphine, how would you choose? (Imagine that your motives are entirely selfish.) Most people say that you would have a strong reason to give the morphine to the one who ends up with your cerebrum. You would have less reason, if any, to give it to the other person. This may lead us to infer that you would be₂₂₃ the one who ends up with your cerebrum.
15. But this inference is questionable. Many philosophers doubt whether your selfish interest in the welfare of the person who gets your cerebrum must derive from the fact that he or she is you. In the double-transplant case, they say, you would have a selfish reason to care about the welfare of both offshoots. Better, you would have the same reason₂₂₄ to care about the fission offshoots as you would have to care about the one who got your whole cerebrum. Yet neither of the fission offshoots would be you. In that case the concern you would have for the person who got your cerebrum in the single transplant case would not support the claim that he or she would be you, thus depriving the transplant conviction of what appears to be its principal support₂₂₅.
16. If the transplant conviction is false, why did anyone ever accept it? Well, someone’s being psychologically continuous with you is strong evidence₂₂₆ for her being you. Conclusive evidence, in fact: no one is ever psychologically continuous with anyone other than herself in real life. That makes it easy to suppose that the one who gets your cerebrum₂₂₇ in the transplant case would be you, even if, because we are animals, it isn’t so.
17. Here is another reason why someone might find the transplant argument a conclusive refutation of animalism. Suppose there are, in addition to human animals, thinking non-organisms₂₂₈ that would go along with their transplanted cerebrums, or more generally beings that persist by virtue of some sort of psychological continuity. And suppose that such a being thinks your thoughts. Then there would be two beings that are otherwise equally good candidates₂₂₉ for being you, except that one has the sort of persistence conditions we believe you to have and the other (the animal) doesn’t. Would it not be perverse, in that case, to suppose that you are the second being? That would make animalism look plainly wrong. I believe that many advocates of the transplant argument do assume that certain non-animals think our thoughts₂₃₀. Few of them give any reason to accept that metaphysical claim, however, and some such reason is surely needed. We will consider some reasons for it in Chapters 3 and 5₂₃₁. But even if assuming that human animals coincide with thinking non-animals would make the transplant argument an irresistible attack on animalism, it would not make it a strong argument for any positive view about what we are. That is because of the thinking-animal problem: the difficulty of knowing that₂₃₂ we are anything other than the animals thinking our thoughts.

1. We have seen that animalism conflicts with traditional thinking about our identity over time. Here are some further objections₂₃₃.
2. First, animalism seems to imply that you and I are only temporarily and contingently people₂₃₄. At least this is so on most proposed accounts of what it is to be a person. Every human animal was once an embryo with no mental properties. If being a person implies having certain mental properties – rationality and self-consciousness or the like – then each human animal was once a non-person. Even if a thing need only have the capacity to acquire the relevant mental properties in order to count as a person, so that unthinking embryos might be people, human animals in a persistent vegetative state will not count as people, and any human animal could end up in such a state.
3. I don’t want to argue about what it is to be a person. (I don’t find it an interesting question₂₃₅.) The important fact is that our being animals would make our having mental features of any sort a temporary and contingent condition of us – even if it is our normal or proper condition. It would mean that any of us could exist at a time without having any mental properties whatever at that time, or even the capacity to acquire them. What is more, any of us could have existed without₂₃₆ having any mental properties at any time: any of us could have died six weeks after conception. Your being able to think or experience would be no more essential to you than your being a philosopher. It would not, so to speak, be part of your being₂₃₇.
4. Second, animalism appears to entail that there are no persistence conditions for people as such₂₃₈: no persistence conditions that necessarily apply to all people and only to people. The persistence conditions of human animals presumably derive from their being animals, or organisms. That makes their persistence conditions no different from those of oysters, which are not people by anyone’s lights. If so, then our being animals implies that we have the same persistence conditions as some non-people. Animalism is also consistent with there being people whose persistence conditions are different from ours₂₃₉: inorganic people such as gods or angels, for instance. If there could be such beings, it would not be necessary that all people have the same persistence conditions. People’s persistence conditions, and for that matter their metaphysical nature in general, would derive not from their being people₂₄₀, but from their being animals, or immaterial substances, or whatever metaphysical sort of person they are. Person would not be a kind₂₄₁ that determines the identity conditions of its members.
5. Some philosophers see in these implications a grave objection to animalism (Baker 2001: 218-20₂₄₂). They find it absurd to suppose I might be a person only temporarily and contingently. We might as well say₂₄₃ that the moon is only temporarily and contingently a material object. This thought appears to be incompatible with our being animals.
6. The claim that there are no persistence conditions for people as such is said to conflict with the very idea of personal identity (Baker 2001: 124₂₄₄). To think about personal identity, the objection goes, is to inquire into the identity conditions of people as such – identity conditions that things have by virtue of being people. That, they say, is why we call it personal identity₂₄₅. If there are no such conditions, as animalism seems to imply, then there is no such thing as personal identity – an implication that is also taken to be absurd.
7. I suppose these objections have some force. That you and I are people essentially is an attractive claim. But it doesn’t seem obvious. If we take seriously the idea that a person could be an organism, and we accept that organisms have mental properties only contingently, and we take being a person at a time to entail having mental properties at that time, then we can understand well enough how someone might be a person only contingently. And if nothing else, the thinking-animal problem₂₄₆ shows that our being organisms is a claim that we must take seriously.
8. That we must have our persistence conditions by virtue of being people, so that there must be persistence conditions for people as such, is another interesting conjecture. Here is how I see it. You and I have many important properties. We are people. We are also (let us suppose) material, composed of parts, biologically alive, sentient, and awake. For that matter, we might also be philosophers, Hindus, women, or Ukrainians. What principle dictates that our being people must determine our identity conditions₂₄₇, rather than any of these other properties? None that I know of. It may be plausible on the face of it; but its incompatibility with our being animals₂₄₈ looks like an excellent reason to doubt it.
9. One further objection to animalism is that it implies the wrong account of what determines how many of us there are at any one time (Lowe 1996: 31₂₄₉) – a topic sometimes called "synchronic identity₂₅₀". If we are animals, then the number of human people at any time will always be equal to₂₅₁ the number of human animals that have whatever it takes to be a person at that time. And what determines the number of animals is presumably a matter of brute biology. Perhaps it is determined by the number of biological lives in the sense sketched in §2.2²⁵³. But many philosophers, beginning with Locke, have assumed that the number of people or thinking beings at any given time is determined not by brute biology but by psychological facts: facts about mental unity and disunity.
10. My mental states are unified in the sense of being disposed to interact with one another, and not with any others, in an especially direct way. For instance, my desire to get a train to London will tend to combine with my belief that this train goes to London to cause me to board it. My desires don't interact with your beliefs in this way to produce action. That, the idea goes, is what makes it the case that my desires and my beliefs are the states of a single person₂₅₃, whereas my desires and your beliefs are not. More generally, mental states belong to the same person or thinking being just when they relate to one another in this way (Shoemaker 1984: 94- 97₂₅₄). So the number of people, or thinking beings generally, is necessarily equal to the number of unified systems of mental states. Call this the psychological individuation principle.
11. This principle looks incompatible with animalism. It seems possible for an animal to have disunified mental states – supposing that an animal can think at all, anyway. It may even be possible for an animal to have a mental life that is no more unified than yours is with mine: perhaps a single human animal could be the home of two unified mental systems. This might happen in an extreme case of multiple personality₂₅₅ – not in any actual case, but in a case that we could imagine by extrapolating from actual cases. The psychological individuation principle implies that such an animal would be the home of two people.
12. This doesn't yet show that the psychological individuation principle conflicts with animalism. Animalism doesn't say that all people are animals. Why couldn't we normal human beings be animals, while people with extreme split personality are something else? But that would be an uncomfortable view₂₅₆. What sort of things would the people in those unusual cases be? They must be something. Perhaps they would be bundles of mental states, or parts of brains. But if an animal with split personality could house two or more such non-animal people, we should expect your animal (which I take to be normal and mentally unified) to house one non-animal person. And if there is a non-animal person within you, it will be hard to maintain that you are the animal. How could you ever know which person you are₂₅₇? Animalism at least strongly suggests that for every animal there can be at most one human person, no matter how disunified that animal's mental states might be; and that appears to be incompatible with the psychological individuation principle.
13. As I see it, the psychological individuation principle is yet another debatable conjecture (Olson 2003b₂₅₈). In §6.3²⁶⁰ I will argue that it is incompatible with our being material things of any sort, and is best combined with the view that we are bundles₂₆₀ of mental states.
14. I believe that the most serious worries for animalism are very different from those we have considered here. We will come to them in Chapter 9₂₆₁. In the meantime let us turn to the other views of what we are.

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