Philosophy of Mind Part 7: Personal Identity

The Question

Two questions lurk in "personal identity":

• Synchronic identity: What unifies the various mental and physical events happening right now into one person?
• Diachronic identity: What makes a person at one time the same as a person at another time?

The diachronic question generates most of the puzzles — and most of the practical stakes (responsibility for past actions, prudence about future suffering, the meaning of survival).

Locke's Memory Criterion

John Locke, in the 1694 chapter "Of Identity and Diversity" added to the second edition of his Essay, separated the question of man (the same biological organism) from the question of person (the same self). Personal identity, he proposed, consists in continuity of consciousness — specifically, in the chain of memories linking present to past.

You are the same person as the boy who climbed the tree because you can remember climbing it. The prince whose memories were swapped into a cobbler's body would, on Locke's view, be the prince still — wherever the memories go, the person follows.

Modern responses use overlapping chains — A is identical to D not because A directly remembers D's experiences, but because A-B-C-D form a continuous chain of memory connections.

Body & Brain Criteria

Alternative theories ground identity in physical continuity. Crudely, the body criterion: same body, same person. Refined: same brain, since other parts can be replaced. The brain criterion has the advantage of intuitive support — most of us would say a heart transplant doesn't change personhood, but a brain transplant would.

Consider Sydney Shoemaker's thought experiment: brain transplant. Brown's brain is placed in Robinson's body, producing a creature ("Brownson") with Brown's memories, character, and skills, but Robinson's body. Who is this? Almost everyone says Brown. So even the body criterion concedes the brain — and ultimately mental continuity — is what matters.

Parfit's Reductionism

Derek Parfit's Reasons and Persons (1984) is the most important book on personal identity in the 20th century. Through a barrage of thought experiments, Parfit argued for two radical conclusions: (a) personal identity is not what matters; and (b) personal identity is reducible to facts about psychological continuity, which is just a matter of degree.

Teletransportation

The teletransporter scans you on Earth, destroys you, and reassembles a perfect duplicate on Mars from local atoms. The duplicate has all your memories, character, plans. Is it you? Most people initially say yes — until told a glitch occurred and the original on Earth was not destroyed; both wake up. Now there are two of you. They cannot both be you (identity is transitive). So neither is — even though one is qualitatively perfect.

Fission

Imagine your brain is split and each hemisphere placed in a body, both surviving with continuous memories and character. You cannot be both (transitivity again), but there is no principled reason to favor one over the other. So you are neither — yet ordinary survival is just "what would have happened" if one hemisphere had been destroyed. Fission is "as good as ordinary survival in every way that matters" — without identity.

Narrative Identity

Paul Ricoeur (in Oneself as Another) and Alasdair MacIntyre (in After Virtue) shifted the question. The self is not a metaphysical substance to be located; it is a story we tell. Coherence is achieved, not discovered. Each life is a narrative with characters, themes, conflicts, transformations. The author, narrator, and protagonist are one person — and being a person is precisely engaging in this self-authoring activity.

The narrative view fits with the modern psychological literature on identity formation (McAdams), explains why life-disrupting events (illness, war, trauma) can feel like they "kill the old self," and accommodates change in a way Lockean memory cannot.

The Buddhist Parallel

The Buddhist doctrine of anatta (Pali) or anatman (Sanskrit) — "no-self" — anticipates Parfit's reductionism by 2,400 years. The Buddha taught that the apparent self is composed of five aggregates (skandhas): material form, sensation, perception, mental formations, and consciousness. None of these is a self; the self is not in any of them, nor is it the sum of them. It is, like a chariot, a useful designation for an arrangement of parts, with no further entity behind the appearance.

The convergence is striking. Parfit himself acknowledged it in his preface to Reasons and Persons, calling Buddhism the "no-self" tradition that had reached his conclusions millennia earlier. Western and Eastern paths to the same disquieting truth: the substantial self we feel ourselves to be is, on close examination, not what we thought.