Parfit on Personal Identity

The argument in plain language

Most of us think there is a determinate fact about whether some future person will be me. The future person either is me, in which case I have ordinary self-interested reasons to care about their welfare, or is not me, in which case my self-interested reasons drop away. Parfit attacks this picture from two directions.

He grants, with John Locke, that what makes someone a continuation of you is psychological continuity — a chain of overlapping memories, intentions, beliefs, character traits and projects. He calls a strong daily overlap "psychological connectedness," and a chain of connections over time "psychological continuity." So far this is recognisably Lockean.

What Parfit then shows, by means of one thought experiment after another, is that psychological continuity comes in degrees, can branch, and need not have an answer to the question "is this future person identical with you?" Once we see that, he argues, we should also see that the question of identity is not, after all, what we should care about.

The teletransporter thought experiment

Imagine a teletransporter on Earth and another on Mars. You step into the booth on Earth. A scanner records the exact state of every cell, including the connections in your brain. The scanner then destroys your body. The information is transmitted to Mars, where a constructor assembles an atom-perfect copy out of new matter. The copy steps out, blinks, walks home, and writes the same email you would have written. Did you just travel to Mars — or did you die in the Earth booth and a stranger was made on the other planet?

Parfit asks us to vary the case. Branch-line case. The Earth scanner malfunctions: it scans you successfully but does not destroy your body. The Mars copy is constructed as before. Now there are two of you — one on Earth, one on Mars — both with all your memories and intentions. The Earth body is told it has 24 hours to live before its scanned-tissue degrades. Which one is you?

The first answer most students give is: the Earth body, because that one is continuous with the original organism. But the Earth body and the Mars body are exactly equally psychologically continuous with the pre-scan you. If the Earth body had been destroyed at the moment of scanning, we would have said the Mars body is you. If it had not been destroyed, but the Mars body had failed to materialise, we would have said the Earth body is you. Now, with both surviving, we cannot say both are you, because identity is one-to-one. The judgement that the Earth body wins because of bodily continuity feels arbitrary in a way that, for Parfit, reveals something about the underlying concept.

Theories of personal identity, compared

	Soul (Cartesian)	Body / animalism	Brain	Memory (Locke)	Psychological continuity (Parfit)	Narrative (MacIntyre, Schechtman)	No-self (Buddhist anatta)
Carrier of identity	Immaterial soul	Living organism	Brain	Memories	Chain of mental states	Self-told life story	None — illusion
Survives teletransport?	Depends on soul-transfer	No	No (organism left)	Yes	Yes (and: identity isn't what matters)	If the story can continue	The question is wrong
Survives fission?	Depends on soul split	No (one-organism)	Maybe (split brain)	Problem case	Comes in degrees; identity fails	Two stories, two persons	The question is wrong
Era / source	Descartes 1641; older theology	Eric Olson 1997	Variants since 1970s	Locke 1694	Parfit 1971, 1984	Late 20th-century	Early Buddhism, c. 5th c. BCE
Determinacy	Always determinate	Usually determinate	Mostly determinate	Mostly	Often indeterminate	Indeterminate at edges	Indeterminate by design
Practical upshot	Religious survival	Identity = animal	Brain transplant possible	Loose ties via amnesia	Ego boundaries soften	Authorial responsibility	Egolessness, compassion

Parfit's position is not the most radical on the table — that honour goes to the Buddhist no-self view, which Parfit himself flagged as a kindred predecessor. But his is the most analytically detailed; he tries to show, case by case, that the only thing the ordinary concept of identity can do, the more relaxed concept of psychological continuity does just as well.

Worked example: the fission case

The teletransporter shows that psychological continuity can hold without bodily continuity. The fission case is meant to show why identity itself, even psychological identity, must be the wrong category.

You suffer an accident that leaves your two cerebral hemispheres functional but separated. Each is transplanted into a new body, identical to your old one. Each has half your brain's connections; remarkably, each preserves the full set of your memories, personality and intentions, because of a fortunate redundancy. Two people walk out of the hospital. Call them Lefty and Righty.

Question: which is you? Three answers, three failures.

1. "You are Lefty." But Righty has equally good claim. Whatever made Lefty count — psychological continuity, brain matter, memories — Righty has too.
2. "You are Righty." Same problem, by symmetry.
3. "You are both." Logically impossible. Identity is transitive: if you are Lefty and you are Righty, then Lefty is Righty. But Lefty and Righty are clearly distinct people: they go home to different addresses, age differently, may eventually disagree.
4. "You are neither." Equally strange. Each of them remembers being you, plans the project you started, loves the people you loved. To say neither is you would make ordinary survival look like a metaphysical lottery: if your brain had failed to split, you would have lived; because it split, you died, even though everything that ordinarily mattered to your survival is preserved twice.

Parfit's diagnosis is that the question "are you Lefty or Righty?" has no answer because it is the wrong question. What ordinarily matters about survival — that there be someone who remembers being you, carries on your projects, completes your unfinished business — is preserved in the fission case twice over. If you would have called single-survival "ordinary survival," double-survival is a kind of better survival. And yet by the rules of identity, you are now no one. The reasonable response, Parfit thinks, is to drop identity from the role we had given it.

Hence the slogan: identity is not what matters. What matters is psychological continuity, which can hold to one future person, to two, or in degrees. Survival is not all-or-nothing.

Major objections

The Williams brain-swap objection

Bernard Williams, in "The Self and the Future" (1970), pressed an objection that Parfit took seriously throughout his career. Imagine a scientist tells you that tomorrow you will be tortured. You feel ordinary fear. Now imagine the scientist tells you instead that tomorrow your memories will be replaced with someone else's, and then you will be tortured. Williams argues that you would still feel fear: there is something that is going to be tortured, and that something is you. The thought experiment seems to vindicate a more body-based, less psychological account of identity, against Parfit's preferred reading. Parfit replied that intuitions in such cases are unreliable; Williams replied that intuitions are all we have. The dialectic continues.

Animalism and the too-many-thinkers problem

Eric Olson's The Human Animal (1997) revived bodily theories under the heading of "animalism." If you are essentially the human organism, then teletransport kills you (because the organism on Earth is destroyed). Animalists also press the "too-many-thinkers" problem against psychological views: if a person and their human animal both think your thoughts, there are two thinkers in the same place, which seems absurd. Parfitians have responses, but the debate is alive.

The practical conclusions seem too strong

Parfit drew anti-egoist conclusions from the metaphysics. If your future self is not you in any deep sense — if survival is just continuity, and continuity can fade or branch — then the boundaries between persons matter less than common-sense morality assumes. Parfit took this to support more impartial, broadly utilitarian moral conclusions: less weight on one's own future, more weight on others. Critics argue that the metaphysics does not support so much. Even on a reductionist view, my future self has claims on me that strangers do not, because of distinctive psychological links.

Can identity really come in degrees?

Identity is normally thought to be all-or-nothing. Either A is identical to B or it isn't. Parfit's view that identity comes in degrees strikes some critics as a category mistake — what comes in degrees, they say, is something else (continuity, similarity), not identity. Parfit's reply is that he agrees identity is binary; that is exactly why we should not let identity carry the weight we usually give it, since the underlying reality (continuity) is graded.

Variants and afterlives

• Lockean memory theory. Locke's Essay (1694) chapter 27 launched the psychological tradition. Parfit's view is its most refined modern descendant.
• Sydney Shoemaker. Shoemaker's quasi-memory account fixed Locke's circularity problem — what makes a memory your memory, when "yours" already presupposes identity? — and prepared the ground Parfit built on.
• Animalism. Olson, Snowdon, and others argue that we are essentially organisms; teletransport kills you, fission produces two new organisms.
• Narrative identity. Charles Taylor, Alasdair MacIntyre, Marya Schechtman: persons are constituted by the stories they tell about themselves. Compatible with reductionism but emphasises a cultural and ethical dimension.
• Buddhist anatta. The doctrine that there is no enduring self underlying experience. Parfit himself remarked that his view brings Western philosophy closer to a tradition of thought it has long undervalued.
• Four-dimensionalism / perdurance. Persons are spacetime worms whose temporal parts are connected; fission becomes simply a worm that branches. Theodore Sider, David Lewis, and others develop this picture in conjunction with continuity-based views.

Why Reasons and Persons mattered

Parfit's 1984 book changed the landscape of moral philosophy more than almost any work of its generation. It was a single 540-page volume divided into four parts — on self-defeating theories, rationality and time, personal identity, and future generations — and it pursued an unfashionably wide-ranging argument for the conclusion that common-sense ethics, common-sense rationality and common-sense personal identity are all systematically mistaken in the same direction.

The personal-identity arguments had been published earlier — most famously in his 1971 Philosophical Review paper — but the book brought them together with their ethical consequences. The combination was distinctive. It also set up Parfit's later work, especially the two volumes of On What Matters (2011, 2017), in which he tried to show that the major moral traditions — Kantian, contractualist, consequentialist — converge once their best versions are developed.

The teletransporter has since become one of the most-cited philosophical thought experiments outside the trolley problem. It shows up in discussions of mind-uploading, brain-emulation, cryonics, and the prospect of digital persons — the cases where Parfit's reductionism stops being a thought experiment and starts being a problem applied ethicists need to settle.

Common confusions

• "Identity is not what matters" is not "you don't matter." Parfit thinks something matters about your future — psychological continuity. He just thinks identity is the wrong way to capture it.
• Reductionism is not eliminativism. Parfit is not saying persons do not exist. He is saying that persons are constituted by lower-level facts about brains, bodies and experiences, with no further metaphysical "self" over and above them.
• The teletransporter is not a prediction about technology. It is a thought experiment to test what we mean by survival. Whether such machines are ever built is irrelevant.
• Fission is not science fiction in principle. Real cases of split-brain surgery (corpus callosotomy) are not full fission, but they hint at the kind of psychological branching Parfit's argument requires only as a possibility.
• Parfit's view is not Buddhism. The convergence is real and Parfit acknowledged it, but the metaphysical framework, vocabulary and ethical conclusions are distinct. He was a Western analytic philosopher reasoning his way to nearby ground.