Personal identity

DOI: 10.4324/9780415249126-V024-2
Version: v2,  Published online: 2011
Retrieved February 23, 2017, from

3. The fission objection to psychological theories

While hypothetical cases are important to the defence of psychological theories they can also be used to reveal their defects. One case that has been especially challenging is the ‘fission’ case, in which a person splits into two continuants, each psychologically continuous with the person at the moment of splitting. Consider Derek Parfit’s (1984) version of this case. We are to imagine someone with complete redundancy between the two hemispheres of their brain, so that all of their memories and psychological attributes are stored twice over, once in each half of their brain. We are to imagine also that this man is one of a set of triplets. Speaking from the point of view of this person, Parfit offers the following case:

My body is fatally injured, as are the brains of my two brothers. My brain is divided, and each half is successfully transplanted into the body of one of my brothers. Each of the resulting people believes that he is me, seems to remember living my life, has my character, and is in every other way psychologically continuous with me. (Parfit 1984: 254–5)

Parfit then asks us to consider what happens to the original person in this case.

Call the triplet who receives the left half of the brain ‘Lefty’, the triplet who receives the right half of the brain ‘Righty’, and the triplet whose brain is transplanted ‘Donor’. There seem only four possibilities: (1) Donor is identical to both Lefty and Righty; (2) Donor is identical to Righty but not Lefty; (3) Donor is identical to Lefty but not Righty; (4) Donor is identical to neither Lefty nor Righty. The first possibility seems ruled out by the transitivity of identity. It is difficult to make a plausible case that Lefty and Righty are identical to one another; they lead separate lives and might not even know of each other’s existence. If Lefty and Righty are not identical to each other, however, transitivity prohibits them from both being identical to Donor.

Options (2) and (3) are implausible because Lefty and Righty each have exactly the same relation to the original person, and it is hard to justify the claim that one but not the other might be identical to him. Option (4) is problematic because each of Righty and Lefty bears the relation to Donor that the psychological continuity theory says constitutes identity. If only one hemisphere were transplanted, resulting in only one psychological continuer, the psychological continuity theory would say unequivocally that that continuer was identical to Donor. Since Righty and Lefty each have everything necessary to be identical to Donor, how could neither be?

Citing this article:
The fission objection to psychological theories. Personal identity, 2011, doi:10.4324/9780415249126-V024-2. Routledge Encyclopedia of Philosophy, Taylor and Francis,
Copyright © 1998-2017 Routledge.

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