Metaphysics – Personal Identity Theories

Introduction to the Personal-Identity Debate

• Metaphysics module shifts focus from free will & time to the question of personal identity.
• Framing question: “Are you the same person you were 10 years ago?”
  • Only two direct answers: Yes or No.
  • In everyday speech many say No (citing physical & psychological change).
  • Philosophically, the question is ambiguous because it hides two kinds of “sameness.”

Two Kinds of Sameness

• Qualitative Identity
  • Being alike in properties/qualities (height, hair length, preferences, memories, etc.).
  • You are not qualitatively identical to your 10-year-old self.
• Numerical Identity
  • Strict, one-to-one, mathematical sameness: $x=y$.
  • Despite changes, you are numerically identical with your 10-year-old self (same single continuing entity).
  • Courts, moral responsibility, everyday tracking of persons assume numerical identity (e.g., professor murder thought-experiment).

Core Guiding Question

• “Why are you numerically identical with 10-year-old-you and not with 10-year-old-me?”
• Goal: formulate a theory of personal identity – a criterion that makes $A$ at one time the very same person as $B$ at another time.

Theory 1: Physical Continuity (Body) Theory

• Slogan: “I have (virtually) the same body I had at 10.”
• Formal criterion: $A\text{ is the same person as }B \iff B\text{ is physically continuous with }A.$
  • “Physically continuous” = same living organism gradually changing, no sudden replacement.
• Intuitive support:
  • Our body is usually how we re-identify people.
  • Explains why I’m me and not my sibling: my body traces back to baby-me, not to sibling’s body.

Counterexample: Locke’s “Prince & Cobbler” / Freaky Friday

• Historical origin: John Locke’s 1689 Essay Concerning Human Understanding.
• Pop-culture retelling: Freaky Friday (2003) mother–daughter body swap.
  • Overnight, Lindsay Lohan (daughter) wakes in Jamie Lee Curtis’s (mother’s) body and vice-versa.
• Analysis:
  1. Let $A$ = daughter’s body before swap.
  2. Let $B$ = mother’s body after swap (now housing daughter’s psychology).
  3. Movie intuition: $A$ and $B$ represent the same person (the daughter) even though they are not the same body.
• Violation pattern:
  • LHS of criterion true, RHS false ⇒ theory fails.
• Moral: Body ≠ essence of personhood; a person could migrate to a new body.

Theory 2: Psychological Continuity Theory

• Inspired by Locke’s reply: identity travels with mind, not flesh.
• Criterion: $A\text{ is the same person as }B \iff B\text{ is psychologically continuous with }A.$
  • “Psychologically continuous” ≈ same memory network, consciousness, character traits, intentions, etc.
• Strengths:
  • Handles body-swap intuitions.
  • Fits responsibility practices that track memory/agency.

Initial Objection: Amnesia Case

• Severe retrograde amnesia wipes all past memories.
• Two interpretive options:
  1. Still same person (numerical sameness survives): then criterion fails because continuity broken.
  2. Not same person (identity ends when memories end): criterion survives but yields unintuitive verdicts about hospital patients.
• Classroom polls split ~50 %. Not universally decisive.

Devastating Objection: The Transitivity (Fission) Problem

1. Medical background
   • Hemispherectomy: real surgery removing one cerebral hemisphere; patients retain memories & personality with only half-brain.
2. Thought-Experiment: Divided-Brain Transplant
   • Step 1 : Remove left hemisphere; transplant to Body L.
   • Step 2 : Remove right hemisphere; transplant to Body R.
   • Both hemispheres individually support full psychological continuity.
3. Consequences under Psychological Theory:
   • Pre-operation You numerically identical to L (shares mind).
   • Pre-operation You numerically identical to R (shares mind).
   • By transitivity of identity (if $x=y$ & $y=z$ then $x=z$) ⇒ L identical to R.
   • Absurd: two distinct post-op bodies can’t be one person.

• Conclusion: Criterion cannot handle fission scenarios; mind-based identity leads to contradiction.

Comparative Summary of Problems

• Physical Continuity Theory
  • Fails in any conceptually possible body-swap (Prince & Cobbler, Freaky Friday, Star Trek “transporters,” etc.).
• Psychological Continuity Theory
  • Struggles with total amnesia (depending on intuition).
  • Suffers major defeat via fission/transitivity objection.

Open Questions & Further Directions

• If neither body nor psychology alone suffices, options include:
  • Hybrid theories: require some combination (e.g., “same brain + overlapping memories”).
  • Closest-continuer / best-candidate accounts: pick one branch after fission.
  • Four-dimensionalism / Stage theory: persons = series of temporal parts.
  • Soul / Non-physical-substance views (Descartes): identity grounded in immaterial soul unaffected by swaps or fission.
  • Narrative identity: unified life story constructed over time.
• Practical & ethical stakes:
  • Legal responsibility in brain-injury cases.
  • Organ transplantation, cloning, teleportation technologies.
  • End-of-life decisions & advance directives (does “future-me” count as me?).

Concept Check & Study Prompts

• Distinguish clearly between qualitative and numerical identity; provide everyday examples of each.
• Re-write both theories’ identity equations and be prepared to apply them to new scenarios.
• Explain Locke’s argument using your own mini body-swap narrative.
• Walk through each step of the divided-brain fission paradox; pinpoint exactly where transitivity triggers contradiction.
• Formulate your own criterion of personal identity—defend it against both body-swap and fission cases.

Connections to Earlier Material & Real-World Relevance

• Echoes earlier metaphysics of material objects (Ship of Theseus): what makes one ship through time?
• Ties to ethics: punishment, promises, prudence rely on stable persons.
• Cognitive science & neuroethics: hemispherectomy data shapes philosophical theorising.
• Pop culture (Freaky Friday, Face/Off, Black Mirror) offers intuitive laboratory for identity puzzles.

Key Formulae & Logical Tools

• Identity is transitive: $(\forall x\,y\,z)[(x=y \land y=z) \rightarrow x=z].$
• Physical Continuity Criterion: $A=B \;\text{iff}\; \text{ContinuousBody}(A,B).$
• Psychological Continuity Criterion: $A=B \;\text{iff}\; \text{ContinuousMind}(A,B).$
• Counterexample schema: Find case with $A=B$ but ¬Criterion, or Criterion but $A\neq B$.

Take-Home Message

• Neither “I am my body” nor “I am my mind” provides an uncontested solution.
• Personal identity remains an open problem—demanding a theory that survives both swap and split challenges.
• Next steps: explore hybrid or entirely new accounts capable of reconciling our intuitions and avoiding contradiction.