The Nature of Identity

Identity Questions

• There are many things we might mean by ‘personal identity’ (Olson 2022: §1).

  1. Outside of philosophy, perhaps the main questions about identity – as in the phrase ‘identity politics’ – seem to concern those categories that we persons fall into that we take to be most central or characteristic of how we see ourselves as individual people – e.g., the categories teacher, or parent, or woman, or queer.

  2. But philosophers – and here we are identifying as philosophers! – are often more concerned with somewhat different questions about persons.

     1. The persistence question: ‘What does it take for a person to persist from one time to another—to continue existing rather than cease to exist?’
     2. The what are we? question: ‘What sort of things, metaphysically speaking, are you and I and other human people?’
• We’ll focus on the persistence question; the hope is that the answers to ‘what it takes’ will tell us something about what persons are.

Identity

• But even focusing on the persistence question, a prior question presses: How can there be a problem of identity?:

  The topic of identity seems to many of us to be philosophically unproblematic. Identity, we will say, is the relation that each thing has to itself and to nothing else. (Hawthorne 2003: 99)

• This makes it clear that we are talking of numerical identity (‘\(x\) is one and the same thing as \(y\)’, or \(x=y\)) – not qualitative sameness, as in ‘identical’ twins.

• Perfect qualitative sameness does allow us a gloss on identity: \(x\) is identical to \(y\) iff for every property or feature \(P\), either \(x\) and \(y\) ‘both’ have \(P\), or ‘both’ lack it.

  • This is more informative phrased negatively: \(x\) and \(y\) are non-identical iff there is some feature \(x\) has and \(y\) lacks – some difference between them.

• But this is no analysis, since identity is surely more conceptually basic than quantification over properties.

What is the Problem of Identity, Anyway?

• But there are both epistemic and metaphysical problems to do with identity.

• The epistemic problems have to do with the relationship between identity and indiscriminability (Williamson 2013).

  • Clearly indiscriminability is necessary for identity (if you can tell \(x\) and \(y\) apart, then they aren’t the same thing). But it’s not sufficient (sometimes things differ even though you can’t tell them apart). Very often, however, our judgments of identity must rest on evidence deriving from facts about (in)discriminability – hence the epistemic problem.

• The metaphysical problems have to do with so-called criteria of identity:

  If we are to use the symbol \(a\) to signify an object, we must have a criterion for deciding in all cases whether \(b\) is the same as \(a\), even if it is not always in our power to apply this criterion. (Frege 1884: §62, my emphasis)

Criteria of Identity

• Here’s a proposed criterion of identity for sets.

  Extensionality

  \(∀𝑥∀𝑦(𝑥=𝑦↔∀𝑧(𝑧∈𝑥↔𝑧∈𝑦))\).

  Any two sets are identical iff they have the same members (any member of one is a member of the other).

  • This is certainly not generally applicable; persons aren’t identical just when they have the same (i.e., no) members.

• Another example: Frege’s criterion for sameness of directions (1884: §64).

  Same Direction

  For any lines \(x\) and \(y\), the direction of \(x\) is identical to the direction of \(y\) iff \(x\) and \(y\) are parallel.

• A criterion of identity for some sortal term \(F\) is a set of necessary and sufficient conditions for two instances of the sort to be the same thing.

  • Note that our two examples have quite different form: extensionality gives criteria for same set as which only relies on properties of sets; the Fregean definition gives criteria for same direction as which refers to properties of things which have directions, not properties of directions themselves. Williamson (2013: §9.1) calls these ‘one-level’ and ‘two-level’ criteria of identity.

Confusion about Criteria

• The idea of a criterion of identity is a bit mangled. Frege talks of criteria for ‘deciding’, but also talks of being unable to apply it – he gestures at an epistemic reading while seemingly trying to give a metaphysical reading of ‘criterion’.

• Moreover, while the epistemic question – what procedure should we use to decide when we are encountering the same object again? – seems to make sense, the metaphysical question – what facts make it the case that we are encountering the same object again? – doesn’t even seem to admit of a non-trivial answer: for surely it is the facts about identity which answer it.

  I now think the term ‘criterion of identity’ and its associated ideology do far more harm than good in philosophy; we should abandon both. The only theory of identity itself we need is a completely general, purely logical one. (Williamson 2013: 170)

Questions of Identity: Picking Out Targets

• Another metaphysical question about identity, which picks up on a feature of Frege’s proposal about directions, is non-trivial: under what conditions is it the case that a thing picked out one way is the same \(F\) as a thing picked out another way?
• This is best understood as a two-level question: what relation among occasions of picking out can be used to track the identity of the entities picked out?
• What is it to pick out something? Examples:
  • Pointing: the thing I’m pointing at now
  • Description: the person with the red hat over there.
    
• These can be modelled using a description function \(\delta\) such that the output \(\delta(x)\) is an object, and the input \(x\) is an occasion.
  • E.g., consider ‘the thing I’m pointing at now’, which takes an occasion of pointing \(x\) as input, and yields a thing as output.
• The use of a function \(\delta\) is no accident, because functions (in logic) correspond to descriptions in natural language.
  • Every functional expression \(\delta(x)\) can be translated into a natural language expression of the form the … of \(x\).

Identity and Occasions

• Then we can ask: is there any relation between inputs \(x\) and \(y\) that holds iff their values under \(\delta\) are identical?

• I.e.: is there an \(R\) such that \[\forall x \forall y (\delta(x)=\delta(y) \leftrightarrow Rxy)?\]

• We’re particularly interested in the case of identity over time, so often the inputs \(x\) and \(y\) will be indexed by times and places, and we’ll be seeking a relation among those occasions that tracks identity between their occupants.

• So we might pick out a distant mountain range by pointing, and ask ‘is the range we’re looking at now the same mountain range as the one we saw yesterday?’ The description is ‘the mountain range we’re looking at now’, where now is a temporal indexical.

• The occasions here could be the places we’re looking at; if so, here’s a proposal:

  Mountain

  The mountain range occupying the place \(x\) is the same as that occupying \(y\) iff \(x\) and \(y\) are ‘high-level connected’ – joined by a continuous spatial path that lies wholly above the general level of the surrounding land.

• Note that Mountain is a synchronic condition (relating inputs at the same time, but different places).

Personal Identity

Persons and Identity

• With some resources for asking identity questions we can approach the problem of personal persistence: what does it take to identify persons over time?

• Pointing, and other events of ‘person indication’, pick out person stages: persons at particular times and places.

  The phrase ‘personal identity’ can henceforth be used for the relation which the spatiotemporal location \(x\) has to the spatiotemporal location \(y\) just in case the person at \(x\) is the person at \(y\). Speaking strictly in this technical sense, personal identity is neither a relation between persons nor a species of identity. (Williamson 2013: 119)

• I point to a person on Monday, dubbing them \(\alpha\) (i.e., \(\alpha\) is stipulated to be synonymous with the description person Antony pointed to on Monday); I point to a person on Friday, dubbing them \(\beta\). And we then ask: what features of the person stages at those places make it true that they are stages of/occupied by the same person?

• This, unlike Mountain, is a diachronic relation, relating inputs at different times.

Candidate Answers

• Following our earlier example, our question can be phrased like this:

  Personal Identity

  When \(x\) and \(y\) are person stages, what (if any) relation must hold between them in order for the persons associated with those stages to be identical?

• Let \(\iota(x)\) mean ‘the person associated with person stage \(x\)’. Some candidate answers:

  Soul

  The person \(\iota(x)\) is identical to the person \(\iota(y)\) iff the soul possessed by \(x\) is the same one possessed by \(y\).

  Psychological

  The person \(\iota(x)\) is identical to the person \(\iota(y)\) iff the psychological state exemplified by \(x\) is connected and continuous with the psychological state exemplified by \(y\).

  Bodily

  The person \(\iota(x)\) is identical to the person \(\iota(y)\) iff the physical body occupying \(x\) is the same one present at \(y\).

Persons, Bodies, Animals

• Not only are there many candidate answers, there are many candidate questions of identity arising from each event of pointing.
• This is because we are not only persons – we are also animals … and physical bodies (‘masses of matter’) … and living things … etc.
  • Maybe I’m not a mere hunk of matter, but certainly there is one very intimately associated with what I’m pointing at now!
• For each of these sortal or kind terms (person, mass of matter, body), there are questions of identity too. So merely pointing and asking Is that the same thing I saw yesterday? is not yet to ask a precise question with a determinate answer.
• We may want to offer an account of personal identity that piggybacks on one of these: e.g., to say that a person is just a human animal, and they have the same persistence conditions.
• But we may offer very different answers for different sortals: it may be that something is now an \(F\) and a \(G\), but that the \(F\) it is will go on to differ from the \(G\) it is.
  • So if a child is a collection of particles and a person, they might appear to coincide now, but come apart in future as the child grows and changes.

Locke on Masses of Matter

• Locke says that for the mass of matter that constitutes us now,

  the Mass, consisting of the same Atoms, must be the same Mass, or the same Body, let the parts be never so differently jumbled: But if one of these Atoms be taken away, or one new one added, it is no longer the same Mass, or the same Body. (Locke 1689: §II.27.3)

  • So Locke thinks something like Extensionality holds of masses of matter: \(x\) is the same mass of matter as \(y\) iff they have exactly the same constituent particles.

• Even though I am here now a mass of matter, as well as a person, since the mass of matter can survive (according to Locke) arbitrary jumbling, then this mass (points to self) can survive changes that this person (points to self) cannot survive.

Locke on Personal Identity

we must consider what Person stands for; which, I think, is a thinking intelligent Being, that has reason and reflection, and can consider it self as it self, the same thinking thing in different times and places.…

For since consciousness always accompanies thinking, ’tis that, that makes every one to be, what he calls self; and thereby distinguishes himself from all other thinking things, in this alone consists personal Identity, i.e., the sameness of a rational Being: And as far as this consciousness can be extended backwards to any past Action or Thought, so far reaches the Identity of that Person; it is the same self now it was then; and ’tis by the same self with this present one that now reflects on it, that that Action was done. (Locke 1689: §II.27.9)

The Memory Theory of Identity

• Locke offers a view on what it is to be a person stage: ‘a thinking intelligent being that has reason and reflection’).

• He also offers an account of what it takes for person stages at different times to be stages of the same person: ‘sameness of consciousness’, where that in turn is indicated by memory, the ability to extend consciousness backward to some past mental state.

  Lockeanism

  A thinking intelligent being at one time \(x\) is the same person as a thinking intelligent being at another time \(y\) iff the consciousness of one of them can be extended, via memory, to that of the other.

  • Here ‘remembers’ must be factive: ‘false’ memories (the mental state one is in when it seems to you that you remember \(p\) even though \(p\) isn’t true) don’t suffice for personal identity.

Memory Lapses

Suppose a brave officer to have been flogged when a boy at school, for robbing an orchard, to have taken a standard from the enemy in his first campaign, and to have been made a general in advanced life: Suppose also, which must be admitted to be possible, that when he took the standard, he was conscious of his having been flogged at school, and that when made a general he was conscious of his taking the standard, but had absolutely lost the consciousness of his flogging.

These things being supposed, it follows, from Mr LOCKE’s doctrine, that he who was flogged at school is the same person who took the standard, and that he who took the standard is the same person who was made a general. When it follows, if there be any truth in logic, that the general is the same person with him who was flogged at school. But the general’s consciousness does not reach so far back as his flogging, therefore, according to Mr LOCKE’s doctrine, he is not the person who was flogged. Therefore the general is, and at the same time is not the same person as him who was flogged at school. (Reid 1785: 276)

• A Lockean response: this example shows the general’s consciousness ‘can be extended’ to the boy’s, even though he doesn’t directly remember it.

Psychological Continuity and Connectedness

• Locke’s is one historically important example of a view according to which personal identity is a relation of psychological connectedness between person stages. Locke says, if person stages \(a\) and \(b\) participate in the same extended consciousness – which is partly individuated by memory – then \(a\) and \(b\) are stages of the same person.
• Others offer different precise accounts of which psychological features matter (memory? consciousness? character traits? personality?)
• But all differ from those who offer bodily or other criteria of personal identity – perhaps the most prominent of which is animalism, the view that persons are human animals (Olson 1997).
  • They differ in cases where the psychological connections come apart from the bodily connections; maybe in supposed cases of ‘body swapping’ (Williams 1970).
• And these differences matter: consider the legal and ethical issues around brain dead stages, who lack psychological continuity with earlier stages, but who do stand in the same living organism relation to earlier stages.

Persons and Psychological Continuity

• Let \(R\) denote the relation of ‘mental continuity and connectedness’ (Lewis 1976: 55) that might hold between person stages.

• The details don’t matter – choose your favourite combination of memory, personality, etc., as long as it has the following features:

  1. \(R\) permits at most gradual psychological change; it doesn’t hold between stages that are radically discontinuous with each other; and
  2. That the sequence of psychological states each related by \(R\) are appropriately governed by (causal) psychological laws.

• A psychological theory of persons uses this \(R\) relation to define what a person is:

  I claim that something is a continuant person if and only if it is a maximal \(R\)-interrelated aggregate of person-stages. That is: if and only if it is an aggregate of person-stages, each of which is \(R\)-related to all the rest (and to itself), and it is a proper part of no other such aggregate. (Lewis 1976: 60)

The \(R\)-relation and Questions of Personal Identity

• Turn the psychological theory of persons into a theory of identity:

  \(\Psi\)-identity

  The person \(\iota(x)\) is identical to the person \(\iota(y)\) iff the person stages \(x\) and \(y\) are part of the same maximal \(R\)-related aggregate of person stages.

• That gives us some traction on some traditional questions:

  If you wonder whether you will survive the coming battle or what-not, you are wondering whether any of the stages that will exist afterward is \(R\)-related to you-now, the stage that is doing the wondering. … If you wonder whether this is your long-lost son, you mostly wonder whether the stage before you now is \(R\)-related to certain past stages. If you also wonder whether he is a reincarnation of Nero, you wonder whether this stage is \(R\)-related to other stages farther in the past. If you wonder whether it is in your self-interest to save for your old age, you wonder whether the stages of that tiresome old gaffer you will become are \(R\)-related to you-now to a significantly greater degree than are all the other person-stages at this time or other times. If you wonder as you step into the duplicator whether you will leave by the left door, the right door, both, or neither, you are again wondering which future stages, if any, are \(R\)-related to you-now. (Lewis 1976: 58)

Fission Problems

Parfit’s Problem

• Parfit (1971) is sympathetic to many aspects of the broadly psychological approach to personal identity originated by Locke and advocated by Lewis.
• But he is struck by a problem: the psychological relations can hold between person stages in ways that identity relations cannot hold between persons.
• That is: it might be that stages \(x\) and \(y\) are related by psychological connectedness and continuity – by any plausible \(R\) relation – but the person occupying \(x\) can’t literally be (numerically identical to) the person occupying \(y\).
  • This is in effect a converse problem to Reid’s challenge of the general, who is identical to the boy despite failing to be psychologically connected to him (in one sense of connected).

A Difficulty for Personal Identity: Fission

• His initial case is simple: a person ‘who, like an amoeba, divides’ (Parfit 1971: 4):

  We suppose that my brain is transplanted into someone else’s (brainless) body, and that the resulting person has my character and apparent memories of my life. Most of us would agree, after thought, that the resulting person is me. I shall here assume such agreement.

  Wiggins then imagined [another] operation. My brain is divided, and each half is housed in a new body. Both resulting people have my character and apparent memories of my life.

  What happens to me? There seem only three possibilities: (1) I do not survive; (2) I survive as one of the two people; (3) I survive as both. (Parfit 1971: 4–5)

• The case is both conceivable and scientifically possible (as in the case of brain bisection discussed by Nagel (1979)).

The Prima Facie Problem

• The basic idea: there are now two people, each of whom is psychologically continuous with the preceding person – but two cannot be one!
• If the person pre-fission is at \(x\), and the two descendants are at \(y_{1}\) and \(y_{2}\), then \(Rxy_{1}\) and \(Rxy_{2}\). It would then follow, if psychological connectedness sufficed for identity, as \(\Psi\)-identity maintains, that \(\iota(x)=\iota(y_{1})\) and \(\iota(x)=\iota(y_{2})\).
• By standard principles about identity, \(\iota(y_{1})=\iota(y_{2})\).
• But note that it is not true that \(Ry_{1}y_{2}\) – these two person stages are not appropriately connected. So \(\Psi\)-identity entails that \(\iota(y_{1})≠\iota(y_{2})\).
• Contradiction. \(\Psi\)-identity is false. So \(R\) is not a proxy for identity: whatever personal identity is, psychological continuity doesn’t track it.

Survival and Psychology

• We could simply follow the argument where it leads, and look for some other proxy for identity – same animal as, perhaps.

  • Yet there may sometimes be no successful relation of same animal as: the case of conjoined twins who are different persons and share an animal body shows that there is no relation amongst body stages which holds iff they are stages of the same person.

• And further reflection on the case emphasises the difficulties of giving up a psychological account of same person as, since our mental features are what really matter to us:

  When I consider various cases in between commonplace survival and commonplace death, I find that what I mostly want in wanting survival is that my mental life should flow on. My present experiences, thoughts, beliefs, desires, and traits of character should have appropriate future successors. My total present mental state should be but one momentary stage in a continuous succession of mental states. (Lewis 1976: 55)

Splitting Survival and Identity

• It is clear that the pre-fission person psychologically survives – having an appropriate mental successor after event \(E\) is, if Lewis and Parfit are right, just what it is to survive \(E\).
• Identity is a one-one relation: if \(x=y\) and \(x=z\) then also \(y=z\). But survival is not, for two reasons:
  1. If one post-fission person died on the operating table, I would survive. But ‘how could a double success be a failure?’ (Parfit 1971: 5). So I must survive in both.
  2. It is arbitrary to think that I can be one post-fission person at the expense of the other, since the psychological connectedness between me and them is symmetrical.
• So Parfit concludes: I survive as both; but since they aren’t identical, survival isn’t identity.
  • Survival is one-many: I can have many survivors (think of a duplicator which spits out two or more copies).

Survival Without Identity

• If I do survive as both, there remains the old question: who am I identical to? Obviously not both.

  [B]ecause these questions [about practical/ethical effects of survival] are important, [fission] does present a problem. But we cannot solve this problem by answering the question about identity. We can solve this problem only by taking these important questions and prizing them apart from the question about identity. After we have done this, the question about identity (though we might for the sake of neatness decide it) has no further interest. (Parfit 1971: 9)

• Parfit thinks, in fact, there is no true answer to the question, ‘who am I identical to in this case?’ And our puzzlement may disappear if we don’t think there has to be an answer – that we can be content just with facts about who survives.

  • Of course, that is unsatisfactory if (i) identity claims express propositions, and (ii) bivalence holds.
  • But maybe what Parfit really means is: we survive, and so it doesn’t matter what we are identical to.

A Parfittian Argument

1. Identity is a one-one relation and does not admit of degree (Parfit 1971: 10–11).

2. There exist some cases (cases of fission) in which what matters in survival is one-many (Parfit 1971, p. §I).

3. There exist some cases (cases of gradually diminishing psychological connectedness) in which what matters is partly a matter of degree (Parfit 1971, p. §IV)

4. Therefore what matters in survival isn’t identity; rather psychological continuity is what matters.

• That is: while the metaphysics of personal identity has been the subject of considerable discussion, really what is ethically and practically important is psychological continuity, whether there is one survivor or many. Though identity is sufficient for survival, it isn’t necessary.

When Identity Can be Important – Even for Parfit

Judgments of personal identity have great importance. What gives them their importance is the fact that they imply psychological continuity. This is why, whenever there is such continuity, we ought, if we can, to imply it by making a judgment of identity.

If psychological continuity took a branching form, no coherent set of judgments of identity could correspond to, and thus be used to imply, the branching form of this relation. But what we ought to do, in such a case, is take the importance which would attach to a judgment of identity and attach this importance directly to each limb of the branching relation. So this case helps to show that judgments of personal identity do derive their importance from the fact that they imply psychological continuity. (Parfit 1971: 12)

The Unpalatability of Parfit’s Conclusion

The problem begins with a well-deserved complaint that all this about mental connectedness and continuity is too clever by half. I have forgotten to say what should have been said first of all. What matters in survival is survival. If I wonder whether I will survive, what I mostly care about is quite simple. When it’s all over, will I myself – the very same person now thinking these thoughts and writing these words – still exist? Will any one of those who do exist afterward be me? In other words, what matters in survival is identity – identity between the I who exists now and the surviving I who will, I hope, still exist then. (Lewis 1976: 56)

• What can be more natural than to hope the best for myself in the future, and be fearful of cases in which there is no-one succeeding me who is me?
  • I wouldn’t be blasé about my impending death, for example, if the right symmetrical response to fission cases is that neither of the fission products is me – for then I die, to be replaced by two duplicates.

Survival and Identity: the Four-Dimensional Solution

Lewis’ Resolution of the Problem

• Lewis says: we need not choose, as Parfit demands we do, between survival and identity.

• For Parfit’s argument rests on a confusion about identity, one which we may express concisely like this: the mistake arises from thinking of person stages (spacetime locations occupied by persons) as if they are themselves persons.

• The relation of psychological continuity – which is involved in the two-level principle governing identity for persons (\(\Psi\)-identity) – is a relation between person stages. The relation of personal identity is a relation between persons.

• The fact that the formal character of the former relation isn’t that of the latter only shows

  that the two relations are different. And we should have known that from the start, since they have different relata. (Lewis 1976: 58)

The \(I\)- and \(R\)-relations

• What about the problem cases? In fission, where one person survives as two, we now can say: there is a person stage \(x\) at \(t_{0}\) which is \(R\)-related to two person stages \(y_{1}, y_{2}\) at \(t_{1}\).
• But, says Lewis, there is another relation of interest: the \(I\)-relation, the relation that person stages have to each other if they are stages of the same person.
  • I.e.: \(Ixy\) can be defined like this: \(Ixy\) iff someone occupying person stage \(x\) also occupies person stage \(y\).

  [I]f common sense is right that what matters in survival is identity among continuant persons, then you have what matters in survival if and only if your present stage is \(I\)-related to future stages. (Lewis 1976: 59)

• Parfit’s puzzle, in these terms, is this: aren’t fission cases those where two stages would have to be \(R\)-related and not \(I\)-related?

Responding to Fission cases

• Lewis replies: ‘I claim that the \(I\)-relation is the \(R\)-relation’ (Lewis 1976: 59). How can this be?

  • Since the \(R\)-relation holds between \(x\) and \(y_{1}\), so must the \(I\)-relation, so there must be a person of whom \(x\) and \(y_{1}\) are stages.
  • Likewise: there must be a person of whom \(x\) and \(y_{2}\) are stages.
  • But since \(y_{1}\) and \(y_{2}\) are not \(R\)-related, they are not \(I\)-related. So while they are both occupied by persons, they are not occupied by the same person.
  • So there are two people: both occupy \(x\); one occupies in addition \(y_{1}\); the other occupies \(y_{2}\).

• In Parfit’s fission case, there were two people all along!

  • Note I was careful in defining the \(I\)-relation to leave open the possibility that a given person stage could be associated with two people.

Partial Overlap Amongst Roads and Persons

Diagnosing the Fault in Parfit’s Argument

The \(I\)-relation will fail to be transitive if and only if there is partial overlap among continuant persons. More precisely: if and only if two continuant persons \(C_{1}\) and \(C_{2}\), have at least one common stage, but each one also has stages that are not included in the other. If \(S\) is a stage of both, \(S_{1}\) is a stage of \(C_{1}\) but not \(C_{2}\), and \(S_{2}\) is a stage of \(C_{2}\) but not \(C_{1}\), then transitivity of the \(I\)-relation fails. Although \(S_{1}\) is \(I\)-related to \(S\), which in turn is \(I\)-related to \(S_{2}\), yet \(S_{1}\) is not \(I\)-related to \(S_{2}\). In order to argue [as Parfit does] that the \(I\)-relation, unlike the \(R\)-relation, must be transitive, it is not enough to appeal to the uncontroversial transitivity of identity. The further premise is needed that partial overlap of continuant persons is impossible. (Lewis 1976: 62)

• Note that the intransitivity of the \(I\) relation isn’t the intransitivity of identity.
• This shows our earlier notation ‘\(\iota(x)\)’ is ill-formed: as a function, it presumes that there is just one person linked to the person stage \(x\); Lewis rejects this.
• Accordingly, our question about identity criteria from earlier ought to be rephrased as this: Is there any relation between person stages \(x\) and \(y\) such that there is a continuant person of which those stages are both parts?
  • To which Lewis answers: yes, the relation of mental connectedness and continuity.

A Puzzle About Coincidence

• In this location \(x\) before you is a person and a human animal.
• Suppose that at some later time I have a horrible accident and am left brain dead at \(y\). Then while \(x\) and \(y\) do not stand in relations of psychological continuity, they may stand in the relation of sharing a continuous intervening life. So the animal occupant of \(x\) is the same as the animal occupant of \(y\).
• So there is a single animal at both \(x\) and \(y\). But while there is a person at \(x\), there is not a person at \(y\). Two options:
  1. There is just one thing at \(x\) and \(y\); at \(x\) it has the properties being a person and being an animal; at \(y\) it has only the latter property.
  • Note counterintuitive consequences: you will survive long after you cease to be a person, since you are a mass of atoms.
  2. There is a person and an animal at \(x\), distinct from each other since they have different lifespans; so \(x\) is multiply occupied by two distinct but coincident things (Wiggins 1968). This is Lewis’ answer.

Person Stages

• Lewis’ elegant and ingenious reconciling solution to Parfit’s worries rests on one fundamental assumption: that person stages are distinct from persons. A moment’s thought shows that person stages at different times must be distinct from one another too (else we would end up collapsing fission cases into complete overlap cases).
• That is, person stages are merely instantaneous PARTS of persons. A person is like a long segmented worm, each segment of which is a person stage (hence this is sometimes called the worm view). A person is both spatially and temporally extended; we extend through space by having different parts at different places, and according to Lewis, we extend through time in the same manner (hence this is sometimes called four-dimensionalism).
• Why should we accept this? We introduced person stages to make formulating identity principles for persons easier; we didn’t postulate them as independently existing things!

References