Monday, February 28, 2005

A Self Divided

Continuing on from my previous post on personal identity...

Consider split brain patients, who have their two hemispheres surgically disconnected. The evidence suggests that this gives rise to two independent spheres of awareness, each with access to only one half of the visual field, and controlling one half of the body. Parfit offers a simplified account (p.245):
One of these people is shown a wide screen, whose left half is red and right half is blue. On each half in a darker shade are the words, 'How many colours do you see?' With both hands the person writes, 'Only one'. The words are now changed to read: 'Which is the only colour that you can see?' With one of his hands the person writes 'Red', with the other he writes 'Blue'.

Now imagine technology advances to a point where we could temporarily split our brains (e.g. by anesthetizing the corpus callosum):
When I disconnect my hemispheres, my stream of consciousness divides. But this division is not something that I experience. Each of my two streams of consciousness seems to have been straightforwardly continuous with my one stream of consciousness up to the moment of division. The only changes in each stream are the disappearance of half my visual field and the loss of sensation in, and control over, one of my arms.

[Ten minutes later...] I am about to reunite my mind. What should I, in each stream, expect? Simply that I shall suddenly seem to remember just having worked at two calculations, in working at each of which I was not aware of working at the other. This, I suggest, we can imagine. And, if my mind had been divided, my apparent memories would be correct. [Parfit, p.247]

How are we to explain the temporary disunity of consciousness? It is implausible to suggest that any new people came into existence for just those ten minutes. Rather, we should say that the one person had a (temporarily) divided mind, with two separate spheres of awareness, neither of which was aware of the other. It follows that a person is not just a 'subject of experiences', and cannot explain the 'unity of consciousness'.

Now suppose that your body and your identical twin's brain have been fatally injured. If your brain could be transplanted into his body, then you would survive. In fact, one hemisphere is enough - people have survived strokes or injuries that destroyed just one of their hemispheres. So imagine that one half of your brain is transplanted successfully, and the other half is destroyed. We must say in such a case that you survive in your twin's body.

But what if the other half is not destroyed? Suppose you have identical triplets. We split your brain in half and transplant each into different bodies. This seems to be an even better result for you than the previous scenario. It seems twice as good. However, it is one where your personal identity is lost.

We clearly have two surviving people, not just one. You cannot be both of them. They are entirely independent now - each with all the characteristics of an individual person. It is not like the 'divided mind' case when both streams soon recombined. Here the separation is permanent, so we cannot plausibly claim that you are both. However, it is similarly implausible to suggest that you are either one but not the other. There is no significant difference between them, so what could make you one rather than the other?

The best description is to say that you are neither surviving person. This may seem odd. You would have survived if just one transplant had been successful. But this result was even better - not one success, but two! Parfit explains (p.262):
You will lose your identity. But there are at least two ways of doing this. Dying is one, dividing is another. To regard these as the same is to confuse two with zero. Double survival is not the same as ordinary survival. But this does not make it death. It is further away from death than ordinary survival.

This entire scenario seems unexplainable for non-reductionists. Treating identity as a 'further fact' makes it impossible to apply it sensibly to the case of division. But for reductionists it is much easier. We can recognise that we know all the facts (e.g. there will be two resulting people, both psychologically continuous with me), and we can choose how best to describe it in terms of 'identity' talk. The best way to describe it is to say that we are identical to neither resulting person. But there is no rational significance to what we call things. Since the application of 'identity' talk becomes fairly arbitrary in such cases, it shows that identity is not what matters. Rather, it is the underlying facts involving psychological continuity (etc.) that matter. If we divide, then we lose our identity. But this doesn't matter, because people survive who stand in the right psychological relation to us. It cannot be called 'identity' because the relation does not hold uniquely. But the thought experiment shows that it is this relation, and not unique identity, that really matters.


  1. Patrick, that's a fair question. As a reductionist, I don't really think personal identity is any 'further fact' in these cases. The fundamental facts are exhausted by the physical and psychological facts, and the identity question is simply about how best to describe these fundamentals. I simply think the most intuitive answers in each case are those presented in the main post. But this is just a matter of labelling, and doesn't really signify any deep (metaphysical) difference between the two cases.

    Even so, we might ask what justifies labelling the two cases differently. There are two differences we might appeal to: temporary vs. permanent division, and one shared body vs. two independent bodies.

    I think the latter is largely a pragmatic issue. We want to hold the first person responsible for his actions during the temporary split. The 'division' results in two independent individuals, however, so it would clearly be inappropriate to treat them as the same person (though I do think both would inherit responsibility for, e.g., the crimes of their earlier self, because psychological continuity is what matters). Parfit asks us to imagine the two individuals playing tennis with each other, and points out that it would be ridiculous to describe this as one person playing against himself! Alternatively, they might go their separate ways, and bump into each other years later and not even recognise the other! So the two-body distinction pulls some intuitive weight, even though I do not think it is deeply significant. It seems possible for two persons to share one body, or one person to control two bodies (perhaps by some sort of remote control).

    The temporary/permanent distinction might be more deeply significant. The first person's hemispheres can recombine quite easily - almost "naturally" - to form a coherent whole. They were only separated for a few minutes, with no major changes befalling them during that time. The resultant person would be clearly the same as he was before the split. Moreover, he would identify with both halves of the split. He would be psychologically continuous with both of them, so it makes sense to say that it is all just one person.

    By contrast, imagine if the second people were to recombine after years of living independently. They could have had strikingly different experiences, and developed into quite different people (psychologically speaking). It would be more like merging two strangers. The result would be one person, but he would be a strange new person - a chimera formed from two quite different parts. I imagine he would feel psychologically confused and incoherent as he tried to make sense of his two disparate sub-selves. Eventually, reconciliation might occur. But the result would be someone quite different from any of the earlier people (including the original pre-split person). I think this difference could justify us describing them all as distinct people.

    "if I was pressed to say where it is I would say that part is in each of them."

    In a sense, I agree with you. There is a sense in which the original person 'lives on' as part of each after division. That is because each resultant person is psychologically continuous with the original person, and this relation is what matters. However, we cannot call such a case 'identity' because the logic of identity requires that it hold uniquely, whereas it is clear that, post-division, we have two people. The temporary split is different because it makes sense for us to understand each hemisphere as merely a part of the larger whole. In the case of division, we do not have such an obvious 'whole' to subsume both 'parts' under. Instead, each hemisphere seems to become a whole person in its own right.

    But I should emphasise again that, for a reductionist, this linguisitic difference should not be taken too seriously. We could make the two cases more similar to each other, until we reached a point where we couldn't decide how to describe it. (E.g. if the divided people recombined after just a week or two of living in separate bodies.) This reinforces my previous post, where I argued that personal identity is a vague (sometimes indeterminate) concept. The only real facts are those about physical & psychological continuity, etc. All else is wordplay :)

  2. One of the tricky things about split-brain cases is that the differences have to be teased out. Split-brain persons live lives that are virtually identical to themselves; it usually takes carefully designed experimental conditions to bring out any difference between split-brain cases and non-split-brain cases. In other words, the split-brain person is indistinguishable from the non-split-brain person except in contexts that make one hemisphere conflict with the other. Splitting a person's brain has very few consequences - much fewer than we would expect if the split-brain person were not a unified subject. Precisely one of the mysteries of split-brain cases (my understanding is that it still is so) is how best to account for both the high level of integration - it takes a hefty amount of work to uncover any disunity - and for the fact that you can discover such disunity. In other words, split-brain patients act neither as if they were perfectly unified subjects, nor as if they were simply two subjects of consciousness.

    Further, it seems too strong to say that the 'streams of consciousness' are independent; for one thing, it's virtually impossible to prevent the hemispheres from communicating with each other in split-brain cases, since the hemispheres appear, among other things (including some neural connections that remain), to read each other's effects (face-twitches, etc.) and interpret them as if they were their own. (Some of the most interesting split-brain experiments, for instance, are those in which the left brain interprets right-brain responses as its own, sometimes giving odd rationalizations of them, since in those experiments the right brain was shown completely different things than the left brain was). And, again, temporary brain-splitting would probably have no effect whatsoever on the way you act or perceive the world; what experiments on brain-bisection show is that there can be hemisphere conflicts, not that the subjects are two people.

    Another thing to keep in mind is that the subject-of-consciousness view is not committed to saying that a subject of consciousness implies unity of consciousness, but only that unity of consciousness implies a subject of consciousness; and that is not affected by split-brain cases, nor by any similar sorts of cases involving splitting or shattering of consciousness. In other words, for the subject view to explain unity of consciousness, it is not necessary that it hold that perfect unity of consciousness is necessary for there to be a subject of consciousness (in fact, one wouldn't expect it to be if the latter explains the former).

    I would actually say that, for reasons noted above, commissurotomy shows that the continuity thesis is probably wrong; split-brain patients simply don't in any intelligible sense divide into two persons, and it's only under certain sorts of conditions that any division is noticeable at all.

  3. Reminds me of a religious theory - when you die you all become part of the "god".

  4. Patrick, what if there ALWAYS WERE two half-pieces of bread, and cutting the loaf in half was simply separating them?
    Although the loaf is a bad example, as when split, each half is not the same as the whole.
    Kind of like cutting a person in half and saying "look, two people!"
    No it isn't - it's a bloody pair of legs sitting next to a gaping torso.

    Anyway, I have wandered. Here is an excerpt from the essay I wrote last night on the subject:

    "...In effect one would have split in half like an amoeba, and become two seemingly identical beings. Which one would be you? Parfit suggests that there are three possibilities: ‘(1) I do not survive; (2) I survive as one of the two people; (3) I survive as both.’²

    I agree with Parfit’s rejection of (3) – one cannot be two people at once. This is mathematical inconsistency, and lays waste to the very definition of ‘identity’.

    I also agree with Parfit’s initial assessment of the problem with (1): Supposing I survived with half my brain destroyed, I would still be alive; but if the other half of my brain, rather than being destroyed, was successfully implanted in another body, according to the theory I would die. In Parfit’s words: ‘How could a double success be a failure?’²
    He goes on to state his eventual agreement with (1), revising his initial ‘psychological continuity’ theory to accommodate fission, and bringing it consistent with (1) rather than (3). His amendment suggests that strong psychological continuity only ensures the connection between X and Y providing there is no third person-stage Z, who is also psychologically continuous with X. This implies the death of one’s identity during fission. Parfit justifies this idea by proposing the theory that whilst one’s personal identity does not survive during fission, this is not important to us. Instead, ‘what we really value and care about are the psychological connections, like quasi-memory and sameness of personality and intentions and values, that the “Psychological Continuity” theories appeal to.’³

    I disagree with Parfit’s revision of his psychological continuity theory, and with his dismissal of (2). I believe it can be compatible with (2) without drifting into inconsistency. Parfit asks, ‘what can make me one of them rather than the other?’²
    I believe there is an answer to this.

    Suppose every thing in the universe contained an infinite number of separate numerical identities. These numerically separate identities – within a person for instance – would be qualitatively identical, and impossible to discern. As long as no one ever fissions or fuses, this would never be apparent, as all the infinite numerical identities within a person would all be exactly the same. The only time these identities would differentiate themselves would be upon the instant of fission, when potentially separate identities within a person would be split asunder, and would be proportionally distributed amongst the newly separated bodies. For instance, let us say that you are about to be divided into two separate beings – A and B. You cannot tell whether your personal identity and consciousness will end within A or B. It is a 50 percent chance either way. After undergoing fission, you end up being A. The numerical identity of B is separate from you, and always was – you were simply always qualitatively identical until a moment ago. There is no way of telling how many identities are contained within a being, and it does not matter – they are simply an illustration of how personal identities exist, and there are potentially infinite identities because there can be potentially infinite fissions – and fusions – of a body."

    Apparently this theory has been well-supported, but I cannot find any references to back it up, so I basically just wrote it up myself. Hmmm.

  5. I suggest that when you divide you do become one of the people in as far as you ever perceive continuality you will perceive continuality into the new body.

    I think this mixes two concepts (A) that of duplication and (B) that of halving - for example are you still you of some of your brain cells die?
    And "which you are you if you duplicate?"
    In relation to B you could imagine a person who uses pathways largely in his left side of his brain - in that case he might carry more memories and more of himself in a sense on the left side. If he only used the right side of the brain at an instant in a sense that part would be lost to the let side forever - but in as far as we use both sides al the time and double enter the information then that is the assumption I am using here.
    In relation to A we just need to understand how human continuality works - i.e. that you could actually become two people.

    Anyway - to say you die by dividing is nonsense because you have brain cells dying all the time and memories being lost and so forth - in sense you are dividing all the time if that is death then there is no life beyond an instant.

  6. Why should we think that continuity of consciousness is constitutive of our identity in the first place? If I am a particular type of biological organism, viz., a human animal, it would seem that I would persist just so long as that human animal that I am persists, and the persistence conditions for biological organisms or animals don't seem to require either psychological continuity or continuity of consciousness.

    Also, if one thinks that identity over time is primitive, does that make one a non-reductionist?


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