Personal identity

DOI: 10.4324/9780415249126-V024-1
Version: v1,  Published online: 1998
Retrieved June 13, 2024, from

5. Fission of persons (cont.)

(4) ‘The case of Arnold’s fission has been misdescribed. Lefty and Righty exist prior to fission, but only become spatially separate after fission.’ This theory also has different versions. Some philosophers think that only Lefty and Righty occupy the pre-fission body, and that the name Arnold is ambiguous. Others think that three people (Arnold, Lefty and Righty) occupy the pre-fission body, but that only Lefty and Righty survive fission. The differences between these versions of the theory will not concern us.

This theory is very strange. It involves a tremendous distortion of our concept of a person to suppose that more than one person occupies the pre-fission body. Surely to one body and a unified mind, there corresponds only one person? However, the strangeness of response (4) may depend on one’s general metaphysics. In particular, the degree of strangeness may depend on whether we accept a three- or four-dimensional view of continuants such as persons.

On the three-dimensional view of persons, persons are wholly present at all times at which they exist (much as a universal, such as redness, is said to be wholly present in each of its instantiations). On this view, persons are extended only in space, not in time, and have no temporal parts. On the four-dimensional view of persons, persons are four-dimensional entities spread out in space and time. Persons have temporal parts as well as spatial parts. Hence, at any given time, say 1993, only a part of me is in existence, just as only a part of me exists in the spatial region demarcated by my left foot. (See Time for more on the contrast between three- and four-dimensional views.)

On the three-dimensional view of persons, response (4) is not just strange but barely intelligible. Consider a time just prior to fission. On this view, two wholly present persons (entities of the same kind) occupy exactly the same space at the same time. This ought to be as hard to understand as the claim that there are two instantiations of redness in some uniformly coloured red billiard ball. On the four-dimensional view, however, Lefty and Righty are distinct persons who, prior to fission, share a common temporal part. It ought to be no more remarkable for two persons to share a common temporal part than for two persons (Siamese twins) to share a common spatial part.

Suppose, for present purposes, that we accept the four-dimensional view. Response (4) is still counter-intuitive. It is implausible to hold that two persons (Lefty and Righty) share a common temporal segment in the absence of any psychological disunity. We should be loath to give up the principle that to each psychologically unified temporal segment there corresponds just one person. Second, there is the problem of how we are to account for the coherence and unity of the I-thoughts associated with the locus of reflective mental life that occupies the pre-fission body. How can there be such unity if two persons occupy that body? These objections to response (4) may not be decisive, but they do show that the multiple occupancy view is problematic, and we should avoid it if we can.

(5) ‘What’s the problem? When Arnold divides into Lefty and Righty, Arnold ceases to exist (one thing cannot be two). Lefty and Righty then come into existence, and are numerically distinct, though initially very similar, persons.’ This is the response I favour. When Arnold divides, there are two equally good candidates for identity with him. Since they are equally good, and since one thing cannot be two things, Arnold is identical to neither. And since there is no one else with whom we could plausibly identify Arnold, Arnold no longer exists. This response respects the logic of identity, and does not violate our concept of a person by supposing that one (post-fission) person is composed of two persons or that more than one person occupies the pre-fission body. This is a victory for common sense!

Indeed it is so. But it is important to realize that, in embracing response (5), we are committing ourselves to a quite particular conception of the identity over time of persons. On this view, Arnold is not Lefty. Why is this true? The reason given is not: because Arnold and Lefty do not have the same body, or because Arnold and Lefty do not have the same (whole) brain. (These would anyway be bad reasons – see §1.) The reason is that one thing cannot be two. Arnold is not Lefty because Righty also exists. Whether Arnold continues to exist depends upon whether he has one continuer or two. Since Lefty and Righty are causally isolated from each other, this implies that the identity over time of a person can be determined by extrinsic factors. Theories that allow for such extrinsicness are sometimes called best-candidate theories of personal identity. According to these theories, B at t2 is the same person as A at t1 if and only if there is no better or equally good candidate at t2 for identity with A at t1. If there are two equally good candidates, neither is A.

Are such theories, and hence response (5), acceptable? Some philosophers have thought not, but for bad reasons. It has been thought that best-candidate theories violate the widely accepted thesis that identity sentences are, if true, necessarily true and, if false, necessarily false. Is not the upshot of response (5) precisely that Arnold is not Lefty, but that had Righty not existed (had the surgeon accidently dropped the right hemisphere, for example), Arnold would have been Lefty? Here we have to be careful. The widely accepted thesis is that identity sentences containing only rigid singular terms (that is, terms which do not shift their reference across possible worlds) are, if true, necessarily true and, if false, necessarily false. We can read the term Lefty as rigid or as non-rigid. If it is non-rigid (perhaps abbreviating the definite description ‘the person who happens to occupy the left-hand branch’), then it is true that, had Righty not existed, Arnold would have been Lefty. But this result is consistent with the necessity of identity and distinctness. If Lefty is rigid, then the best-candidate theorist, if he is to respect the necessity of identity, must deny that Arnold would have been Lefty if Righty had not existed. If Righty had not existed, Arnold would then have occupied the left-hand branch, but that person (namely, Arnold) is not Lefty. Lefty doesn’t exist in the nearest world in which Righty does not exist, though an exact duplicate of Lefty – twin Lefty – exists there.

Best-candidate theories do not violate the necessity of identity. However, they do have consequences that might be thought objectionable. Consider again the world in which Arnold divides into Lefty and Righty. According to the best-candidate theory, Lefty can truly say, ‘Thank goodness Righty exists, otherwise I wouldn’t have existed’. Given that Lefty and Righty exert no causal influence on each other, such dependency is apt to seem mysterious.

These consequences are not objectionable. They simply illustrate the fact that properties like being occupied by Lefty (where Lefty is understood to be rigid) are extrinsic properties of bodies. That is, whether the left-hand body has the property of being occupied by Lefty, rather than by Twin Lefty, is fixed by an extrinsic factor (namely, the existence or non-existence of Righty). But this is not counter-intuitive. The property being occupied by Lefty is not a causal property of a body. In contrast with properties of shape and weight, and so on, this identity-involving property does not contribute to the causal powers of any body in which it inheres. (The causal powers of the left-hand body are unaffected by whether Lefty or Twin Lefty is the occupant.) It is typical of a non-causal property that its possession by an object may depend upon what happens to other objects which exercise no causal influence on it. For example, the property of being a widow is not a causal property and, unsurprisingly, whether a woman is a widow may depend upon what happens to someone who, at the relevant time, exercises no causal influence on her. Response (5) teaches us that identity-involving properties (like being occupied by Lefty) are also extrinsic. This is not a counter-example, merely a consequence.

The best-candidate theory provides the most satisfying response to the case of fission. It also reveals something important about our concept of personal identity: its structure is that of an extrinsic concept. This result may be surprising, but it is not objectionable. If we combine this result with the central claim of the last section, we arrive at the following modified sufficient condition for personal identity over time: A at t1 is identical to B at t2 if A stands to B in the relation of psychological continuity with a cause that is either normal or continuous with the normal cause, and there is no better or equally good candidate at t2 for identity with A at t1.

Citing this article:
Garrett, Brian. Fission of persons (cont.). Personal identity, 1998, doi:10.4324/9780415249126-V024-1. Routledge Encyclopedia of Philosophy, Taylor and Francis,
Copyright © 1998-2024 Routledge.

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