Category: identity
Associated thinker(s): Derek Parfit, David Wiggins, Sydney Shoemaker
Original publication or approximate period: Twentieth-century personal-identity debates; Parfit, Reasons and Persons (1984)
Difficulty: Advanced
Themes: identity, branching, survival
Related experiments: teletransporter, brain-transplant
Kind: thought experiment
Research checked: 2026-10-30
Suppose a person’s brain can be divided so that each functioning half supports the relevant psychology. Each half is transplanted into a different recipient. Both resulting people remember the original life and continue its intentions, but they now occupy separate bodies and can disagree. The scenario assumes a level of preserved functioning that ordinary medical procedures do not establish.
Can one person survive as two people without being identical to both?
Strict identity is transitive: if the original were identical to each successor, the successors would be identical to each other. But they are distinct. Fission therefore pressures any theory that treats psychological continuity alone as sufficient for identity.
Let O be the original and L and R the later successors. If O = L and O = R, transitivity yields L = R. Yet L and R can have different experiences and locations. Rejecting both identity claims does not erase their memories or their ties to O. The important next step is to examine which practical concerns depend on those ties: promises, compensation, inheritance, anticipation, and affection may not all require the same answer. The logic constrains an identity theory while leaving substantive questions about treatment of the successors.
Nonbranching theories hold that branching prevents identity, even if either single branch would have preserved it. Multiple-occupancy theories describe coincident persons or shared temporal stages before division. Parfit argues that identity need not be what matters most in survival: the psychological relations may remain important without a single unique future self.
This is not a claim that a hemispherectomy produces two people, nor that actual split-brain patients neatly instantiate the scenario. The relevant possibilities are stipulated for conceptual analysis. Calling both successors equally similar avoids the identity problem only if similarity was the question being asked.
Asymmetric fission preserves one successor more closely than another and tests closest-continuer views. Fusion, the reverse-style scenario of combining psychological histories, raises different questions about which relations constitute one life.
Historical association: Twentieth-century personal-identity debates; Parfit, Reasons and Persons (1984). The sources above support the scenario and its intellectual context. Where an original work was not retrieved in full, its account is mediated by the named academic reference; no claim of full primary-source verification is made. The explanation and reflection questions here are original editorial prose.