Under Derek Parfit's theory of identity, we should direct our concern to future selves not because they are identical to us, but because they bear some special relation to our current self. He used Psychlogical Continuity and Connectedness. As an "intuition pump" he proposes split brain thought experiments, where you are anesthetized and have your brain split into two functioning parts and then you re-awaken. Who will you awaken as? He proposes that you awaken as both, as a split identity, as both are psychologically connected to you. He extends this logic to say that as long as there is a mind out there that bears such a connection to you, you exist as a multiple-identity.

My question is about interpretation: It is all well and good to say you exist as a split-personality in some abstract sense, but subjectively, you exist as multiple loci of experience. How should one explain, subjectively, the experience of undergoing such a brain split?

My initial thinking is that if you have 1 mind now, and 2 after the split, then subjectively I would explain to the patient that there is a 50% change he will wake up with a right lobe and 50% with a left lobe. Upon waking, both copies of yourself will see that that is true, as they will have had a prior experience of having both lobes, and each will find they have one. The "you" here is one of the two halves, but you don't know which one. However, I think a classical-probability description seems to fit what you would actually experience.

If that is the case, consider a slightly revised experiment: You are in ill health and old, so you want to transfer yourself to a healthier body. To do this, your brain is stopped, its configuration copied into the healthier body and both are reanimated. By previous logic, you have 50% chance of waking as either person. The one that wakes as the healthy person declares success and moves on, the other remains and is disappointed that they wasted their money. Lets say that each run of this costs $100k. How much money should you bank to ensure you will end up in a healthy body with 99% probability?

Based on the above, you want (1/2)^N = .01, so N=-ln(.01)/ln(2) = 6.6 = 7 trials to ensure at least 99% probability of success. So, you should bank $700k for your ill-selves to spend on repeat procedures so that the "current" you has a good chance of being in a healthy body. Of course, there will be one you left with memory of 7 unsuccesful tries. With infinite money, the probability that you will end up in a healthy body approaches 1 almost surely. I.e., the probability that the memories of a randomly chosen copy will contain a success is 100% as n-> infinity.

I thought this was an interesting thought experiment so I wanted to share it with the group. Any thoughts or suggestions on alternative viewpoints or calculations?

  • Does Parfit do anything with the (1) no-cloning theorem (2) dissociative identity disorder (formally called 'multiple personality disorder')?
    – labreuer
    Oct 25, 2013 at 19:51
  • Not that I know of. From your links, it looks like (1) is about subatomic particles in QM (2) is a psychological problem. His writings are more about how we should direct our future concern.
    – user4634
    Oct 25, 2013 at 19:55
  • @Eupraxis1981: (1) talks about quantum state, which isn't just about subatomic particles. If consciousness depends on quantum state, it's not clear it can be copied! (2) interests me because people with DID seem similar to your "brain cut in half" thought experiment.
    – labreuer
    Oct 26, 2013 at 6:49
  • @labreuer: OK, I understand. I honestly don't know what Parfit's views are on either. Haven't read any myself.
    – user4634
    Oct 27, 2013 at 16:11

1 Answer 1


You've an interesting line of thought in that we could have "identity by degree", but it's a little misleading with respect to Parfit. In part III of his book Reasons and Persons he offers a series of thought experiments, mostly aimed at untangling Identity from Survival. He goes so far as to write that in the case of duplication, you survive as two people, but the further question of identity - "which you is you" - is an empty question, and meaningless. I take by your mention of "multiple-identity" you're familiar with all that, but it bears repeating.

Part of the problem we have is equating subjectivity with identity, but that's wrong, and disastrously so. One subjective perspective isn't the same from one moment to the next in the same person, leaving us with either

  • a complex account of psychological connectedness just to claim that a person at age six is the same person at age sixty (and which may have many real counterexamples such as Alzheimer's sufferers), or
  • dualism, with a distinctive Cartesian flavor.

Parfit's concern is to point out that multiple subjective viewpoints can belong to the "identical person" for split-brain patients, or for duplicated people of any degree, and his response to your experiment would likely be that even the person waking up with less than half of a mind (whether we're talking about behavioral patterns, intentions, or memories) would, barring some existential crisis, find it rational to continue as both people. So there's no need to perform the experiment multiple times in the first place.

  • Thanks Ryder for your thoughtful response :) I agree, and I hope I wasn't implying otherwise, that under Parfit's theory of identity all the mind-copies in my experiment would be part of the same "identical person". I also agree that no specific mental content is needed for psychological connectedness. My discussion was concerning subjectivity of the experiment, not who was the "real" you. If a person now (T=0) wanted to have a subjective experience in a new body at some point T>0 via this method, then they would need to perform this multiple times, as they could subjecvtively wake as either
    – user4634
    Oct 27, 2013 at 16:12
  • To clarify my thought experiment: Each of the two subjectivities has equal claim as the successor of the original. However, each does not experience the other's viewpoint. Therefore, what I am getting at is not "tracking the path of a soul" but examining how you would end up being the subjectivity that has the desired outcome, and not the other, when both are continuations of you and you have no soul that can pick. Both are you, but you want to be one of the two "subjective components" of the split identity (preferred half). I'm not trying to metaphysically establish "identity by degree"
    – user4634
    Oct 27, 2013 at 16:15
  • @Eupraxis1981, it's still a bit muddy when you say "but you want to be one of the two 'subjective components' of the split identity" -- but then, if you're more interested in something like a game-theoretic evaluation of value to place on "which you" you want to be, there's a view from Nozick that's been called the closest continuer theory [ iep.utm.edu/nozick/#H4 ] which your experiment seems (to me, at least) to resemble a bit, though you're defining the probabilities more rigorously, which is always helpful.
    – Ryder
    Oct 29, 2013 at 9:06
  • Thanks for the link Ryder. I was thinking about that "muddyness" after I wrote my comment to you. I guess what I am trying, perhaps not too well, to articulate is that, even in our current life, we have a subjective sense of continuity, and per Parfit, psychlogical connectness is just as good as ordinary survival, hence we can see ourselves as surviving in each of the two copies. If a person is about to undergo this procedure, I think it is subjectively most accurate to describe what will happen "next" when they awake by giving a probabilistic description.
    – user4634
    Oct 29, 2013 at 12:57
  • 1
    (cont'd, final) I have confirmed this understanding with Parfit himself though personal communication. He confirmed that there is no difference, in terms of survival, between, for example, traveling to mars via spaceship (lets say while sleeping/unconscious) and having your brain and body digitally copied, destroyed, and reassembled on mars. So, in both cases, you would be justified in saying "your" next experience will be to awake on mars. Now, if you are copied after you lose consciouness, with one staying on earth, then in a subjective sense, you have halved your chances of getting to mars.
    – user4634
    Oct 29, 2013 at 13:10

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