Version: v2, Published online: 2011
Retrieved February 24, 2017, from https://www.rep.routledge.com/articles/thematic/personal-identity/v-2
4. Responses to fission
Four-dimensionalist responses. Psychological theorists have used two major strategies to respond to the fission case. One is to take a four-dimensionalist approach to identity (e.g. Perry 1972; Lewis 1976). Four-dimensionalists see continuing objects as spread out over time in much the same way they are spread out over space. Just as a three-dimensional object is not wholly present at any one point in space, but only over the totality of the space it covers, so a four-dimensional object is not wholly present at any point in its history, but only over the whole of the time it occupies. What is present at a point in space is a spatial part of the spatially extended object, and what is present at a point in time is a temporal part (‘time-slice’) of the temporally extended object. On the four-dimensionalist view of persons a person at time t 1 is never, strictly speaking, identical to a person at time t 2.At most a person-time-slice at t 1 and a person-time-slice at t 2 can be time-slices of the same extended person. The four-dimensionalist can thus analyse the fission case as a case in which two distinct temporally extended persons overlap for a time, sharing a number of time-slices.
If the Figure 1 represents the fissioning of Donor into Righty and Lefty, the four-dimensionalist will view the entire span from A to B as one person, and the entire span from A to C as a distinct person. AB and AC share a segment, but they are different people, each numerically identical to itself and numerically distinct from the other. Just as two roads may overlap, sharing a bit of pavement for a while before they diverge, so two people may share a stretch of person-stages and then diverge. For the four-dimensionalist the fission case describes two temporally extended people who share a segment, and there is no transitivity problem.
The four-dimensionalist approach does provide a technically viable solution to the problem raised by fission, but this solution comes at a cost. This response implies, among other counterintuitive consequences, that any human person one encounters might in fact be (or, more precisely, be a segment of) more than one person. Whether this is the case will depend upon what happens later. If there is no fission in the future then there is (a segment of) only one person. If there is fission later on then there is, even prefission, (a segment of) more than one. This makes it awkward, for instance, to enforce bigamy laws, or to know how many votes a given person-stage should be allowed. Philosophers differ on how serious the objections to four-dimensionalism are in the end, and the debate between three-dimensionalist and four-dimensionalist accounts of personal identity continues, as do larger metaphysical disputes about whether objects in general should be thought of as three-or four-dimensional.
Parfit’s Response to Fission. Parfit (1984) offers a different and quite radical proposal in response to the fission case. What the fission case tells us, he says, is that ‘identity is not what matters in survival’. He comes to this conclusion because he believes that the best description of what happens in the fission case is to say that neither of the postfission individuals is the same person as the prefission individual. In other words, the prefission individual goes out of existence when he divides and is replaced by two other people, each of whom is psychologically continuous with him but neither of whom is identical to him. To maintain the psychological continuity theory as an account of personal identity in light of this result, a nonbranching clause must be added. Instead of saying simply that personal identity consists in psychological continuity the view must say that a person at time t 2 is the same person as a person at earlier time t 1 provided that the person at t 2 is psychologically continuous in the right way with the person at t 1 and no one else is psychologically continuous with that person. The person who undergoes fission thus ceases to exist through a failure of uniqueness.
There seems, however, to be a vast difference between ceasing to exist by having no one in the future psychologically continuous with oneself, as happens in ordinary death, and going out of existence by having more than one person in the future psychologically continuous with oneself. Whatever practical complications having a double might engender, it is not the same as dying. Each of the continuers in the fission case contains all of what the psychological account deems necessary for survival, so it seems problematic to say that doubling constitutes the end of one’s existence. It is for this reason that Parfit concludes that identity is ‘not what matters’ in survival. What we care about, says Parfit, is psychological continuity. Since identity is a form of psychological continuity (continuity that is nonbranching), identity is important to us. But it is not important to us because it is unique, and so not because it is identity. Since fission does not occur in our world, all instances of actual psychological continuity are also instances of identity, and so it is not surprising that we mistakenly attribute this importance directly to identity. Thinking about a hypothetical world that includes fission, however, helps us to see the deeper source of identity’s importance in our world. This analysis has been extremely influential, and many philosophers have turned their attention from the project of defining personal identity per se to the project of defining the relation that provides what matters in survival.
This conclusion is closely connected with Parfit’s claim that we should take a reductionist approach to personal identity. The reductionist approach says that personal identity is not a deep fact, but instead that ‘the fact of a person’s identity over time just consists in the holding of certain more particular facts’ and that ‘these facts can be explained in an impersonal way’ (Parfit 1984: 210). This means that sometimes the question of personal identity can be an ‘empty question’, meaning that we can know all of the facts and still not have a definitive answer. The relevant information about our survival is information about psychological connections. Claims about ‘identity’ are just shorthand for this more basic information. In the fission case for instance, Parfit says, we know all there is to know. When we are confused about what to say about identity in such a case it is not because we fail to know some fact, but because we are undecided about how to describe what we know.
Responses to fission. Personal identity, 2011, doi:10.4324/9780415249126-V024-2. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/personal-identity/v-2/sections/responses-to-fission.
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