Lecture 13: Fission cases and the shift from identity to what matters

What this lecture covers

This lecture completes the course's assessment of the three theories of personal identity by showing that the body theory faces the very same duplication problem as the personality theory, using a "fission" case in which a brain is split between two bodies. Kagan then shows that even the soul theory only avoids a no-branching rule under a specific, costly metaphysical assumption. Having found all three theories stuck with the same awkward patch, he pivots to a new question: rather than asking what it takes to survive, we should ask what actually matters to us in survival, using two scrubbed-memory thought experiments to argue that bare survival, on any of the theories, is not enough.

After watching, you should be able to explain the fission case and why it threatens both the body and personality theories, state what the no-branching rule requires, and explain Kagan's argument that survival without a continuous personality does not give us what we really want.

Key ideas

• The fission case: if a person's brain is split, half transplanted into one torso and half into another, both resulting people wake up believing they are the original, with equally strong claims, which is structurally identical to the personality-duplication problem.
• No branching rule for the body theory: to handle fission, the body theory must add the same patch as the personality theory: you are the same person only if there is no equally good competitor for your body or brain.
• The no branching rule feels external: whether you're the same person as your past self would depend on facts about what happened somewhere else entirely, which conflicts with the idea that identity should depend only on facts about you.
• The soul theory's possible advantage: if souls are simple and simples cannot split, then in a fission case the soul goes entirely to one side or the other, avoiding the need for a no-branching rule, though which side gets it may be unknowable from either inside or outside.
• A cost for the soul theory: if the soul theory is right and one side of a fission case is a full person (conscious, rational, creative) without the original soul, that undermines the earlier argument that we need to posit souls to explain personhood at all.
• Kagan's own leaning: he says he goes back and forth over his career but currently leans toward the body theory (with the no-branching rule), while granting the personality theory remains a live option.
• Survival versus what matters: Kagan proposes that the real question we care about may not be strict personal identity ('will that future person be me?') but whether survival preserves what we actually value.
• Scrubbed-soul and scrubbed-brain thought experiments: even if you survive, in the sense required by the soul theory or the body theory, with your soul or brain reincarnated but stripped of all memories and personality, Kagan argues this gives you nothing you actually wanted from survival.

Walkthrough

Introduction: the personality theory and no branching (0:00)

Kagan reviews last lecture's conclusion: duplicating a personality forces the personality theory to add a no-branching rule, since neither duplicate can be uniquely correct, both cannot be the same person in two places, and yet mere possession of the personality was supposed to be sufficient for identity. He restates why this rule feels implausible, since it makes identity depend on facts unrelated to you personally.

Fission is not allowed: the body theorist's no branching rule (5:29)

Kagan constructs the fission case: an accident destroys his torso, his brain survives, and surgeons transplant one hemisphere into Jones' torso ('Lefty') and the other into Smith's torso ('Righty'). Since both wake up with equally strong brain-based claims to being Shelly Kagan, the body theory faces the identical problem the personality theory faced, and needs its own no-branching rule to avoid saying both, or neither, are him without further explanation.

The metaphysics of soul-splitting (19:31)

Kagan asks what the soul theory would say about the same fission case, which depends on whether souls can split. If they can, the soul theory faces the identical problem and needs its own no-branching rule. If souls are simple and cannot split, the soul goes entirely to one side, avoiding the rule, though we might never be able to tell which side, even from the inside. Kagan notes this apparent advantage comes at a cost: if the other side wakes up as a fully conscious, rational person without a soul, that undermines the earlier argument that souls are needed to explain personhood at all.

What matters in survival? Refocusing the question (35:57)

Concluding that all three theories need a no-branching rule and that he personally leans toward the body theory without being certain, Kagan pivots to a new question: instead of asking what makes a future person literally me, we should ask what actually matters to us about survival. He illustrates this with the soul theory: if reincarnation preserved your soul but wiped all memory and personality clean, he argues that satisfying the soul theory's criterion for 'you' surviving would still not give you anything you actually cared about, comparing it to a trivial 'knucklebone theory' of identity.

Conclusion: bodily survival without personality is not enough (47:43)

Kagan runs the same argument against the body theory: if the body theory is correct and a future 'John Doe' has your brain but has suffered complete, irreversible amnesia with no trace of your beliefs or memories, that person would technically be you, yet this bare survival gives you nothing you wanted. He concludes that what people actually care about in survival may not be strict identity at all, but continuity of personality, setting up next lecture's question of whether 'what matters' really does come down to sameness of personality.

Before you watch

• Watch the previous lecture on Bernard Williams' torture cases and the duplication problem for the personality theory, since this lecture directly extends that problem to the body theory.
• Keep the brain-centered version of the body theory in mind, since the fission case depends on splitting the brain between two bodies.

Check your understanding

1. Describe the fission case and explain why it creates the same problem for the body theory that duplication created for the personality theory.
2. What is the no branching rule, and why does Kagan find it counter-intuitive for personal identity to depend on it?
3. Under what metaphysical assumption can the soul theory avoid needing a no branching rule, and what problem does that assumption create instead?
4. Explain Kagan's scrubbed-soul thought experiment and what it is meant to show about the soul theory.
5. Why does Kagan suggest that the real question we care about might be 'what matters in survival' rather than 'do I survive'?