Lecture 11: The body theory and the personality theory of identity

What this lecture covers

Continuing the search for the metaphysical 'glue' of personal identity, this lecture moves past the soul theory to examine two physicalist-friendly alternatives: the body theory, which says sameness of person tracks sameness of body, and the personality theory, which says it tracks sameness of personality. Kagan works through puzzles about bodily resurrection and reassembly, argues for a brain-centered version of the body theory, and then shows how the personality theory can be refined using an analogy to a rope made of overlapping fibers.

After watching, you should be able to state both theories, explain why Kagan favors following the brain over the torso, and explain how the personality theory handles gradual change over a lifetime.

Key ideas

• Body theory of personal identity: you are the same person over time just in case you have the same body; unlike sameness of soul, sameness of body can in principle be tracked and verified.
• Bodily resurrection puzzle: if the body is reassembled after death (as in some religious views of Judgment Day), whether that is really the same body echoes a disagreement between the watch case (reassembled watch seems to be the same watch) and van Inwagen's block tower case (a duplicated tower is not the original).
• Not all body parts are equally essential: losing a hand, an arm, or even a lung does not intuitively end a person's identity, but destroying the brain does, suggesting the brain matters more than the rest of the body ('torso').
• The brain version of the body theory: personal identity follows the brain, not the torso; a brain transplant is really a body transplant for the brain's owner.
• Partial brain and split-brain thought experiments: if enough of the brain (perhaps even half, hypothetically) preserves the personality, identity follows that portion, raising open questions about cases where both halves are transplanted into different bodies.
• Personality theory of personal identity: you are the same person just in case you have the same personality (the ongoing pattern of beliefs, memories, desires, goals, and fears), regardless of whether the body or brain persists.
• Compatibility with physicalism and dualism: both the body theory and the personality theory can be held whether or not one believes in souls; a dualist could still deny that sameness of soul is what matters for identity.
• The rope analogy: personality persists through gradual change the way a rope stays one rope through overlapping fibers, none of which spans its full length; identity requires overlap and continuity, not identical content at every moment.

Walkthrough

Review of the soul theory's weakness (0:00)

Kagan briefly recaps why the soul theory of personal identity is hard to accept: it implies a person could effectively die and be replaced every few minutes if the underlying soul changed, even while personality stayed constant, which he and Locke found implausible. He clarifies that rejecting the soul theory is a claim about what matters for identity, not necessarily a claim that souls don't exist.

The body theory of personal identity (3:07)

Kagan introduces the body theory: same body, same person, which can be tracked, unlike the mysterious sameness of a soul. He explores whether bodily resurrection could preserve identity, comparing a reassembled watch (which he's inclined to call the same watch) with Peter van Inwagen's example of a rebuilt wooden block tower (which he agrees is a mere duplicate, not the original). He leaves open which analogy better fits a resurrected body, noting the theory allows for gradual changes, like losing weight or shedding atoms, without becoming a different body.

Equating the brain with identity: implications of the body theory (25:47)

Using the Star Wars example of Luke Skywalker losing a hand, Kagan argues that most body parts are not essential to identity, but the brain is, since it houses the personality. He proposes the refined 'brain view': personal identity follows the brain rather than the rest of the body, so a so-called brain transplant is better described as the recipient's torso being transplanted onto the original brain. He raises a hypothetical about splitting a brain between two bodies as an open puzzle to revisit later.

Physicalists and the personality theory (32:35)

Kagan introduces a third option: the personality theory, on which the key to being the same person is having the same evolving set of beliefs, memories, desires, and goals, independent of whether the same body or brain persists. He notes this view is compatible with both physicalism and dualism. To handle the fact that personalities change constantly, he offers the rope-and-fibers analogy: identity requires a pattern of overlapping continuity over time, not an unchanging core.

Comparing the views: assessment by torture (46:39)

Having set the soul theory aside, Kagan frames the real choice as between the body theory and the personality theory, which normally agree but can be pulled apart in science-fiction scenarios. He previews a method for testing intuitions: imagining a case where a person's body and personality end up in different places, then asking which one you would want spared from torture, as a guide to which one you really believe is you.

Before you watch

• Watch the previous lecture on the soul theory of personal identity and Locke's objection, since this lecture builds directly on rejecting that view.
• Keep the earlier train and car examples of identity across time in mind, since the body and personality theories both build on that framework.

Check your understanding

1. Why does Kagan think a reassembled watch is the same watch, but van Inwagen's reassembled block tower is not the same tower?
2. Explain why Kagan favors the brain over the torso as the essential part of the body for personal identity.
3. How does the personality theory of personal identity differ from the body theory, and how can both be compatible with physicalism?
4. Use the rope-and-fibers analogy to explain how a personality can persist even though someone's beliefs and memories are always changing.
5. Why does Kagan propose using a thought experiment about torture to test whether the body theory or personality theory better captures what matters to us?