Personal identity and continuity

Personal identity and continuity is the problem of what makes a person existing at one time the same person as one existing at another, and what has to be preserved for that person to survive a change. It is an old question in philosophy that acquires practical force whenever a technology proposes to interrupt, copy, or reconstruct a mind — which is why it recurs across Mind uploading, Cryonics, gradual neural replacement, and advance directives in dementia. Nothing in neuroscience answers it, because it is not a question about what happens but about how to describe what happens.

§The classical positions

John Locke separated the person from both the soul and the animal. A person, he wrote, is a thinking intelligent being that can consider itself as itself across time, and personal identity extends only as far as consciousness — effectively, memory — extends backwards.¹ The proposal explains why a person is responsible for past acts they remember and severs identity from bodily persistence.

Two objections followed quickly. Joseph Butler charged the account with circularity: to remember doing something is already to remember oneself doing it, so memory presupposes identity rather than constituting it. Thomas Reid's brave officer case sharpened it: an old general remembers being a young officer, the young officer remembered being a flogged schoolboy, but the general remembers nothing of the schoolboy. Memory is not transitive; identity is. The standard repair replaces direct memory with overlapping chains of psychological connection — psychological continuity — which restores transitivity at the cost of the intuitive simplicity Locke had.

§Parfit's reductionism

Derek Parfit's Reasons and Persons reframed the field.² Parfit argues that persons are not separately existing entities over and above brains, bodies, and interrelated mental events; personal identity consists in facts describable without presupposing it. From this he draws a conclusion most readers find harder than the metaphysics: identity is not what matters in survival. What matters is Relation R — psychological connectedness and continuity — and R can hold to a degree, can hold with more than one future person, and can hold without identity.

The practical upshot is that some questions have no answer because they are questions about how to describe a case, not about a hidden fact. Parfit's combined spectrum imagines a series in which one person's brain and body are progressively replaced by another's; somewhere along it, asking whether the resulting person is the original is, on his view, an empty question.

Why this is the crux for uploading

If Parfit is right, an upload that inherits full psychological continuity carries everything worth caring about, and its being a copy is not a further loss. If he is wrong and identity requires something more — a continuously running physical process, say — then destructive scanning is death with a convincing survivor. The engineering cannot decide between these. See The teleportation problem.

§Fission and the branching problem

The case that breaks most theories is division. Suppose each hemisphere of a brain is transplanted into a different body, and both resulting people have the original's memories and character. They cannot both be the original, since they are two and identity is one-to-one. They cannot be neither, since each has as good a claim as an ordinary survivor. Choosing one arbitrarily is unmotivated.

The available responses divide neatly. Parfit accepts that identity fails while what matters is preserved twice over. David Lewis, taking persons to be four-dimensional worms of person-stages, holds that two people were present all along, sharing stages before the split.³ Robert Nozick's closest-continuer theory makes identity depend on whether a candidate is the best continuer, with the awkward result that whether a person survives can depend on what happens to someone else.⁴ Every option gives up something ordinarily assumed about persons.

Fission is not purely hypothetical. Commissurotomy patients, whose corpus callosum has been severed to control epilepsy, show hemispheric dissociations under controlled testing that Thomas Nagel took as evidence that the unity of consciousness may not be all-or-nothing.⁵ The clinical picture has since become more complicated, and researchers dispute how divided such patients really are.

§Rivals: animalism and narrative

Animalism, defended by Eric Olson, holds that each of us is a human animal, and that a person persists exactly as long as the organism does.⁶ Its main argument is the too-many-thinkers problem: if a person is distinct from the animal sitting in a chair, then two thinkers occupy the chair. Animalism denies that psychological continuity is even necessary for survival — a patient in a persistent vegetative state is still the same animal — and denies that any upload could be a person's continuation, since the animal remains behind or dies.

Narrative accounts, associated with Marya Schechtman, treat identity as constituted by an ongoing self-told story that organizes experience into a life.⁷ These accounts are better at capturing what people mean when they say someone is "not the same person" after a trauma, and worse at delivering the sharp yes-or-no answers that law and metaphysics ask for.

§What people actually believe

Experimental work suggests folk judgements track something narrower than psychological continuity in general. Nina Strohminger and Shaun Nichols found that participants judging cases of neural change treated moral traits — honesty, cruelty, compassion — as the most identity-defining part of a mind, more so than memories or personality broadly.⁸ Related studies find that intuitions about survival shift with how a case is framed, and that people give inconsistent answers to structurally identical scenarios.

This is relevant to technologies that alter mental function without ending life. Reports of personality change after Deep brain stimulation were widely cited as an identity problem, though several reviews have since argued that the empirical basis is thinner than the philosophical literature assumed and that much of it rests on small numbers of case reports. Similar questions attach to a Memory prosthesis, to pharmacological Moral enhancement, and more broadly to the debates surveyed in Bioethics of enhancement.

§Practical stakes

Advance directives and dementia. Ronald Dworkin argued that a person's earlier, competent wishes should govern their later demented self, on the grounds that a life has critical interests that outlive the capacity to express them.⁹ Rebecca Dresser replied that the later person's present experiential interests are real and may conflict, and that precedent autonomy can amount to one person disposing of another. The disagreement is exactly the psychological-versus-biological criterion applied at a bedside.

Preservation and revival. Whether Brain preservation preserves a person depends on which criterion holds and on what is physically retained; the two questions are usually run together. Under animalism, revival of a preserved brain does not obviously return the same animal. Under psychological criteria, it depends entirely on whether the information supporting continuity survived, which is an empirical matter addressed by Connectomics and not yet settled.

Very long lives. A person who lives for centuries under something like Longevity escape velocity may retain no direct psychological connection to their earliest self. On Parfit's account the strength of Relation R fades with distance, which weakens ordinary reasons for prudential concern about one's own remote future and complicates arguments about consent to continued existence taken up in Right to die and the duty to live.

Emulation and copying. If Substrate independence holds and Whole brain emulation works, copies become cheap, and every institution built around one-person-one-body — property, criminal responsibility, voting, consent — encounters cases it was not designed for. The weak-form products marketed as Digital immortality raise a smaller version of the same problem now, since a convincing imitation of a dead person has no continuity claim at all but is treated by users as though it did.

§Open problems

The deepest difficulty is that the competing theories agree on all the physical facts. No measurement distinguishes a world in which Relation R is what matters from one in which strict identity is, and no result from Neural correlates of consciousness research bears on it. That leaves the question to be settled, if at all, by which set of intuitions people are willing to give up — and by what legal systems decide when a case arrives that cannot be postponed.

§See also

• The teleportation problem
• Mind uploading
• Substrate independence
• Brain preservation
• Cryonics
• Digital immortality
• Whole brain emulation
• Machine consciousness
• Head transplantation

§References