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A thought experiment in which a person is scanned, destroyed, and rebuilt elsewhere, used to test whether survival requires physical continuity or only psychological continuity.
The teleportation problem is a thought experiment used to isolate what survival requires. A machine records the exact state of a person's body and brain, destroys the original, transmits the information, and assembles a molecule-for-molecule duplicate at the destination. The duplicate has every memory, every disposition, and the vivid sense of having stepped in one door and out another. The question is whether the person travelled or died and was replaced. It is the sharpest available test of theories of Personal identity and continuity, and it is the exact structure of destructive Mind uploading.
Derek Parfit gave the standard presentation.1 In the simple case the original is destroyed as the scan completes, so there is only ever one claimant. Most people who imagine themselves stepping into the booth report reluctance, but find it hard to say what is lost: the duplicate is physically identical, psychologically continuous, and would insist it is the same person.
Parfit then introduces the branch-line case. The scanner malfunctions: it copies without destroying. The original survives on Earth with fatal cardiac damage and a few days to live, while the replica walks around on Mars. Now the original can talk to the replica by radio. Very few people are willing to say the original should be indifferent to their imminent death because the replica exists — yet the replica's relation to the original is exactly the relation that, in the simple case, was supposed to constitute survival. Nothing about the replica changed. Only the presence of a competitor did.
Why the variant does the workIf identity depended only on the intrinsic relation between two people, the branch-line replica would be the original whether or not the original survives. That it apparently is not shows the criterion has an extrinsic component — whether anyone else has an equal claim — which is a strange feature for a fact about a person to have.
Responses divide roughly into three camps, and the division is stable across audiences.
Survivalists hold that the duplicate is the person. What makes someone the same person is the pattern of psychological connections, and the pattern is preserved. On this reading, ordinary survival through dreamless sleep or general anaesthesia already involves an interruption in experience, and nobody treats waking up as being replaced.
Physical-continuity theorists hold that the person died. Experience is a process running in a particular physical system; when the system is destroyed the process ends, and starting an identical process elsewhere begins a new life. On this view anaesthesia is disanalogous because the physical substrate continues to run, metabolizing and maintaining itself, throughout.
Deflationists, following Parfit, hold that the question has no further answer. There is a psychological relation that holds and a physical relation that does not, and once both are specified there is nothing left to discover. Asking whether the person "really" survived is like asking whether a rebuilt club is really the same club.
Experimental work confirms the split is genuine rather than an artefact of philosophical training. Studies of ordinary intuitions find that people give different verdicts depending on framing, on whether the scenario is described in first or third person, and on which mental features are said to be preserved.2 Judgements about whether someone remains the same person weight moral character heavily, more than autobiographical memory.3
Identity is not what matters. Parfit's own move is to grant that the duplicate is not straightforwardly identical to the original while denying that identity was ever what mattered. What matters is psychological connectedness and continuity, and these are preserved. The branch-line intuition, on this account, is a failure of imagination rather than a discovery.
Closest continuer. Robert Nozick's theory makes the duplicate the original when no better candidate exists, and not the original when the biological body survives.4 This delivers the intuitive verdict in both cases at the cost of making survival depend on facts about other people — so whether one survives can be changed by an event elsewhere, after the fact.
Four-dimensionalism. Treating persons as extended in time, with momentary stages as parts, lets one say that two person-worms overlapped before the scan. This resolves the logic of branching but does not tell anyone stepping into the booth what to expect.
Non-branching clauses. Some accounts define survival as psychological continuity provided it has not taken a branching form. Critics regard this as writing the intuition into the definition rather than explaining it.
The most influential practical response is to change the case. Instead of scanning and rebuilding, replace the brain's components one at a time, each substitute reproducing the function of what it displaces, while the person remains awake. There is never a moment of destruction, never two claimants, and never a gap in the running process — so all three camps can agree that the person survives, provided each replacement genuinely preserves function. Weaker versions of the same move underlie incremental framings of Human–AI merger and of the Posthuman condition: a person augmented in small steps never confronts a moment of transfer.
This is why gradual routes dominate serious discussion of substrate change, and it is the identity half of the argument for Substrate independence. David Chalmers, who defends the functionalist premise, is notably more confident about gradual uploading than about the destructive kind.5 The catch is engineering: the required devices would have to reproduce a single neuron's computation, including dendritic nonlinearity and neuromodulatory response, inside living tissue — far beyond anything achieved in Neuroprosthetics or contemplated under Medical nanorobots. The philosophically comfortable route is the technically hopeless one.
Physics offers no duplication machine. Quantum teleportation, proposed in 1993 and demonstrated in laboratories since, transfers an unknown quantum state from one system to another using entanglement and a classical channel.6 It necessarily destroys the state at the origin, because the no-cloning theorem forbids copying an unknown quantum state.7 It is closer to Parfit's simple case than to the branch line, and it says nothing about identity, since the relevant question is whether brains require quantum-state fidelity at all. Most neuroscientists think they do not: synaptic transmission is a thermally noisy chemical process, and proposals that quantum coherence is functionally important remain a minority position.
The problem is not idle. Destructive scanning for Whole brain emulation has the teleporter's exact structure, minus the reassembly of a body. Brain preservation and Cryonics raise a milder version: the process is not obviously destructive, but it does stop, and whether restarting counts as continuation depends on the same disagreement. The weaker products sold as Digital immortality do not raise the problem at all, since a model trained on someone's messages has no continuity claim of any kind — a distinction the marketing tends to blur.
Nothing in Connectomics, Neural correlates of consciousness research, or Machine consciousness work settles it, and no one has described a result that could. The physical facts in the teleporter case are stipulated and complete. What remains is a decision about which description of those facts to accept, and the practical question of who gets to make that decision when a real procedure is on offer.
bookParfit, D. Reasons and Persons. Oxford University Press, 1984. ↩
paperNichols, S. and Bruno, M. "Intuitions about personal identity: An empirical study." Philosophical Psychology, 2010.↩A vignette study of lay intuitions: it records what non-philosophers say about the cases, not which theory of identity is correct.
paperStrohminger, N. and Nichols, S. "The essential moral self." Cognition, 2014. ↩
bookNozick, R. Philosophical Explanations. Harvard University Press, 1981. ↩
bookChalmers, D. J. Reality+: Virtual Worlds and the Problems of Philosophy. W. W. Norton, 2022.↩A general-audience philosophy book; the uploading discussion is one strand of a broader argument about virtual worlds.
paperBennett, C. H. et al. "Teleporting an unknown quantum state via dual classical and Einstein-Podolsky-Rosen channels." Physical Review Letters, 1993.↩A theoretical proposal; experimental demonstrations of the protocol came several years afterwards.
paperWootters, W. K. and Zurek, W. H. "A single quantum cannot be cloned." Nature, 1982. ↩