Sunday, February 27, 2005

Vague Identities

In my previous post on Personal Identity, I suggested that the idea of a 'pure ego', independent of all physical and psychological facts, is just senseless. I presented Parfit's reductionist view as an alternative. According to Parfit, identity merely consists in physical and psychological continuity. It is not some 'further fact' to be discovered on top of these more mundane facts.

(I personally think psychological continuity is all that matters. You can test your own intuitions with this fun game at The Philosophers' Magazine.)

In this post and the next, I want to discuss some of the interesting thought-experiments Parfit presents in Reasons and Persons. The first shows that personal identity is a vague concept, so (remarkably!) questions like "am I about to die?" do not always have a determinate answer. The second shows that personal identity does not explain our unity of consciousness, because it is possible for one person to have two independent streams of consciousness simultaneously. The third shows that identity is not what matters. It doesn't matter if you die, so long as another person connected to you in the right way lives on.

Once we understand identity in terms of physical or psychological continuity, it should come as no surprise that we can have indeterminate results. After all, such continuities can vary widely, and it would be arbitrary to impose some particular threshold to specify what counts as 'identity' or not. This idea is illustrated by the following thought experiment.

Imagine a mad scientist could replace parts of your brain with someone else's. Now consider the spectrum of cases where he replaces none, 1%, 2%, ..., 99%, or 100% of your brain. It's clear in the leftmost cases, where the procedure has minimal effect, that the resulting person would still be you. It's equally clear in the rightmost cases that the resulting person is not you, for your entire brain has been destroyed and replaced by someone else's. But what about the middle cases in the spectrum? Where do you draw the line? Any decision would be arbitrary. Personal identity is supposed to be deeply significant, so it is implausible to claim that it could be affected by something so trivial as a couple of cells either way.

So to hold that personal identity is determinate, as non-reductionists must, is both arbitrary and trivializing. We should instead recognize that the concept is vague. There are some cases where there is no determinate answer as to whether a person's identity will persist. This is not because of ignorance - we can know all the facts about their physical and psychological continuity. Personal identity is not some 'further fact' on top of this. (See the Parfit quote in my earlier post.) It may simply be unclear whether 'identity' is the best description to give of these facts.


  1. I'm not convinced Parfit avoids the need for a 'further fact'. That is, he needs not just continuity, but continuity relevant to preserving personal identity. Any change the body undergoes, for instance, would exhibit continuity, simply because that's the way physical change works. (Psychological change is trickier, but we can presume it the same for our purposes; the point would be moot for most physicalists anyway, given the issue with physical change.) Parfit himself occasionally talks of "full continuity", and I am not convinced that that 'full' isn't in fact dragging in a 'further fact', i.e., something that must be preserved through the continuity.

    And I think this actually gives away the store to the ego theorist, who doesn't have to hold that the ego is a 'pure ego' unmodified by anything psychological and physical; she just has to posit a unifying subject to which the relevant psychological and physical acts are referred. In other words, the old idea that to actually have continuity through change, something must remain through it, a subject of the change, which is a precondition for saying "There is real continuity here". And, whatever its nature (or even if it changes over time itself) when we are talking about ordinary personal identity, we just call this subject of change the ego, or person, or what have you, because that's the type of change we're looking at. (I think, in fact, that this is precisely what has traditionally been intended by ego theories, and it is what Parfit misses about them.) What happens in the typical philosophy of mind Crazy Sci-Fi Cases is irrelevant unless we have a handle on whether there is a subject of change in those cases. And the reason there is sometimes no answer to whether we will persist through a change proposed by these cases is that the cases are not clear and precise enough about what is going on for us to get a handle on whether there is a subject of change.

    I think the reason, though, that personal identity is supposed to be deeply significant is precisely Locke's reason: whatever the metaphysical reality, we simply can't do without personal identity forensically: i.e., morally and legally. We need it to have a viable theory of responsibility. And this also requires that we have more of a handle on what is actually going on in the change than the Crazy Cases give us.

    So in other words, (1) identifying something as a relevant continuity seems to require a 'further fact' itself; (2) the Crazy Cases aren't clear enough to tell us anything metaphysically; (3) the Crazy Cases aren't clear enough to tell us anything forensically; (4) it is utterly unclear why we are using them at all (I tend to follow Kathleen Wilkes's line in Real Persons on this whole issue). But it's been ages since I've read Parfit, so he might have some reasonable response to this that I've forgotten.

  2. I'm not sure I follow your objections here. Psychological continuity is presumably both relevant and significant. (It is distinct from physical continuity, even for physicalists, for consider the teletransportation case discussed in my earlier post.) You seem to be suggesting (in your first paragraph) that any change will trivially exhibit some continuity. But it's clear in the far spectrum cases that the appropriate psychological (and even physical) continuity is lost - your entire brain has been replaced! The resulting person will behave entirely different from your present self. These are all mundane natural facts. None of this seems to require any extraordinary 'further facts'.

    In your second paragraph, you question whether change is possible without a persisting core. I'm not sure why we need any sort of 'personal' core (i.e. ego) however. The subject of change could be merely physical and/or psychological. There was a change in this brain, or to those psychological relations, or whatever. I'm not sure that we need to bring an underlying 'self' into any of those explanations. To suggest otherwise would seem to beg the question against the reductionist.

    I also don't see what is unclear in the Spectrum case I described. It seems clear to me? (Though no doubt Parfit does a better job of describing it than I have. And I did make some changes for the sake of brevity.) We could know all the physical and psychological facts about such a scenario, and it wouldn't make the problem go away.

    Anyway, you seem to be ignoring the central point, which is that a non-reductionist seems stuck imposing an arbitrary and trivializing 'threshold' at some point in the spectrum. If you believe in some sort of all-or-nothing ego (pure or not), it either persists or it doesn't. There's no room for indeterminacy, is there? But it's absurd to say that you can remove 40 billion neurons without destroying my identity, but 40 bil. + 1 will result in my destruction. So I'm not sure how one can hold to a belief in determinate irreducible egos in the face of the spectrum case. Instead we should hold that what matters is the appropriate sorts of continuity (which is not 'all or nothing', so avoids these problems). I'll present further reasons in support of that conclusion in my next post...

  3. An analogy with regard to the spectrum argument:

    Suppose someone predicts that, if you keep adding grains of sand to a pile, at some critical level, the pile will shift along its slopes. And suppose a doubter were to say, Ah, but if n grains of sand won't do it, it is absurd to say that n+1 grains will. The doubter, of course, is simply wrong. There is no constant number n for every pile of sand; but for any given pile of sand there is a critical point at which the prediction will come true. Nor is this 'arbitrary'; it's just unpredictable, which is an entirely different thing.

    Parfit, of course, hasn't done the scientific work to see what happens through a real spectrum case; no one has. So he's really just putting forward a speculation as if it showed something real. This is what is unclear about the spectrum case: we simply do not know enough to say what would actually happen. Likewise, it is mere speculation that, if we knew all the physical and psychological facts about such a scenario, it wouldn't go away. (I find it a little funny that you talk about total brain replacement as a mundane natural fact. It isn't, of course; it's a Crazy Case. But that wasn't my point about 'further fact'; my point there is that continuity does not suffice, because we need a further fact to determine the relevant or appropriate continuity.)

    On your other points, briefly (I have to head off to the airport soon):

    (1) The reason we regard continuity as broken is not that there hasn't been continuity (for all we know in this case there might be) but because we think at some point nothing relevant remains to serve as unitary subject of change.

    (2) Why do you think a unitary subject of change involving psychological continuity wouldn't be a personal core?

    Split brains is a much better type of argument, despite the ambiguities in the evidence, because we do know something about split brains. The spectrum case is merely plausible science fiction, and proves nothing, as far as I can see.

    Well, I'm off....


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