Personal identity

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

4. Fission of persons

As we have seen, much work in personal identity has made use of various thought experiments or imaginary scenarios. The method of thought experiments in personal identity has recently been subject to criticism. It has been claimed that we should not take our intuitions about thought experiments as guides to philosophical truth, since such intuitions may be prejudiced and unreliable. These criticisms are, I think, misplaced. For one thing, such criticisms ignore the frequent and legitimate use of thought experiments in virtually all traditional areas of philosophy (for example, in theories of knowledge and in ethics). Why is their use in discussions of personal identity singled out for criticism? Second, and more important, thought experiments can be useful in understanding the structure of a concept and the relative importance of its different strands, provided that there is general agreement about the best description of the thought-experiment. There is such general agreement about the counterexamples to the body and brain criteria. Some philosophers have tried to gain mileage from thought experiments in the absence of such general agreement – for example, the case of teletransportation discussed above. But it would be unwarranted to infer from the existence of such abuses that thought experiments can never perform any useful function in discussions of personal identity (see Thought experiments).

One thought experiment which has been much discussed in recent years, and which does not fall into the teletransporter category of thought experiments, is that of the fission or division of persons. This thought experiment is interesting because it shows us something about the nature or metaphysics of personal identity, about what it is to be the same person over time.

Fission is a situation in which one thing splits into two (or more) things. Fission does occur in nature (amoebae, for example). Fission of persons, of course, does not occur – but it might. We can devise a thought experiment to flesh out this possibility. Consider a person, Arnold. Like us, Arnold has a mental life that is crucially dependent upon the normal functioning of his brain. Arnold also has a property which most of us do not have, but might have done: each of his brain hemispheres support the very same mental functions. If one of Arnold’s hemispheres developed a tumour, that hemisphere could simply be removed and Arnold’s mental life would be unaffected.

Suppose that Arnold’s body develops cancer. The surgeons cannot save his body, but they can remove Arnold’s brain and transplant both hemispheres into two brainless bodies, cloned from Arnold’s body many years ago. Arnold agrees to this and the operation is successfully carried out. The fission of Arnold has taken place. We now have two people – call them Lefty and Righty – both of whom are psychologically continuous with Arnold (same character, beliefs, apparent memories of a common past, and so on). They are also physically similar to Arnold, and each contain a hemisphere from Arnold’s brain. There is both physical and psychological continuity linking Arnold with Lefty and with Righty. Suppose also that Lefty and Righty are in different rooms in the hospital, and exercise no causal influence on each other. How should this case be described – who is who? A number of responses have been suggested, which we shall examine in turn.

(1) ‘The case is not really possible, so we can say nothing about it and learn nothing from it.’ This view is implausible. Hemisphere transplants may be technologically impossible; but they are surely not logically impossible. Indeed, hemisphere transplants, like other organ transplants, appear to be nomologically possible (that is, consistent with the laws of nature). If so, such transplants are also logically possible. Response (1) is not a serious contender.

(2) ‘Arnold has survived the operation, and is one or the other of Lefty or Righty.’ Immediately after fission, Lefty and Righty are physically and psychologically indistinguishable. Both stand in the same psychological and physical relations to Arnold. They both believe that they are Arnold. According to response (2), one is right and the other wrong.

Response (2) is implausible for two reasons. First, since Lefty and Righty are symmetrically related to Arnold in respect of physical and psychological continuities, the claim that, for example, Arnold is Lefty, can only be sustained on something like the Cartesian view of persons. If we think of the person as an immaterial ego that typically underlies streams of psychological life, we can suppose that Arnold’s ego pops into the left-hand stream of consciousness, leaving the right-hand stream ego-less or with a new ego. As noted at the beginning of §1, this view of persons is bizarre. The postulation of such an ego is idle, and conflicts with both science and common sense.

Second, the metaphysical absurdity of the Cartesian view has an epistemic counterpart. According to response (2), when Arnold divides, he survives in one of the two streams. So either Arnold is Lefty or Arnold is Righty. But how can we know which? From the third-person point of view, we have no reason to make one identification rather than the other. Nor is appeal to the first-person perspective of any help: both Lefty and Righty take themselves to be Arnold. Nothing in either stream of consciousness will reveal to its bearer that it is Arnold. So if, for example, Arnold is Lefty, this truth will be absolutely unknowable. There may be no incoherence in the idea of unknowable truths, but we should be suspicious of any theory of personal identity which implies that truths about who is who can be, in principle, unknowable. For these reasons, we should reject response (2).

(3) ‘Arnold survives fission as both Lefty and Righty.’ There are three ways in which we can understand this response. According to the first way, Lefty and Righty are sub-personal components of a single person, Arnold. According to the second way, Arnold is identical to both Lefty and Righty (hence, Lefty is Righty). According to the third way, Lefty and Righty together compose Arnold (so that two persons are parts of one larger person, just as Scotland and England are parts of one larger country).

These views are hard to believe. It seems plain common sense that Lefty and Righty are both persons (not sub-personal entities), and that they are numerically distinct. Lefty and Righty both satisfy the normal physical and psychological criteria for personhood. They qualify as persons. And they are two. They may be exactly alike immediately after fission, but exact similarity does not imply numerical identity. (Two red billiard balls may be exactly similar, yet numerically distinct.) Further, they will soon begin to differ, mentally and physically, so that it would be intolerable to regard them as anything but distinct persons.

According to the remaining version of response (3), Arnold exists after fission composed of Lefty and Righty, now regarded as persons in their own right. This is sheer madness. The postulation of Arnold’s existence in this circumstance (in addition to that of Lefty and Righty) does no work whatsoever. It is completely idle. Second, can we really make any sense of the idea that one person might be composed of two separate persons? That one person might be composed of two bodies and two minds? To be a single person is to possess a unified mental life. (This is why we are sometimes reluctant to regard a split-brain patient as constituting a single person.) Yet, supposedly, after fission Arnold is permanently composed of two unconnected spheres of consciousness. How could they possibly constitute a single person? If Lefty believes that Clinton will win the next election, and Righty believes that he will not, does Arnold believe that Clinton will both win and lose the next election? Such problems multiply. It seems that all ways of understanding response (3) skewer our concept of a person to such an extent that they cannot be taken seriously.

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

Related Articles