Brian Roberts: Is the concept of personal identity dispensable?

“What thought experiments concerning body-duplication show is that the concept of personal IDENTITY is ultimately dispensable” — Discuss.

[Derek Parfit 1942-2017]

Philosophers have often used thought-experiments as a means of exploring the persistence of personal identity ie: what makes A and B over time the same person? We recognise the importance of this question because of our feelings of subjectivity, our personal identity. The criteria for the persistence of personal identity over time, which include bodily, brain and psychological continuity, have been much debated. To interrogate these issues, a range of thought-experiments has been devised, including those involving body-duplication. This essay, after a brief summary of the background and considering two body-duplication thought-experiments, will propose a possible response to the statement of the title.

Body continuity seems a clear way of establishing personal identity. However, as our body’s cells are replaced on an approximately seven yearly basis, the appearance of our body significantly changes as we age, and the increasingly extensive use of body-part replacement, suggests that bodily continuity may be an insufficient criterion. An alternative criterion would be brain continuity because it seems appropriate that identity is likely to be in some way connected with the brain. Eventually it may be possible however to download all the data in a brain and replace it with an electronic version. This process would retain our memories, beliefs and dispositions and so suggests that our physical brain is not the most significant criterion. If it is not the continuity of the brain that retains survival, but rather its psychological contents, then personal identity is preserved as long as mental states, including memories, beliefs and dispositions, remain connected to the person. But even this criterion does not seem completely satisfactory, as psychological states can change; for example, during aging I do not remember many of my experiences as, say, a child, yet I still consider that I have remained the same person throughout.

In this essay I will refer to two common types of body-duplication thought-experiments. The first is the teletransportation machine, in which a person (let’s call her Tess) is exactly duplicated, teletransported and reformed on Mars (and now called Tessa) by a machine, but because of a fault, instead of destroying the original person as planned, she survives and this gives us both Tess and Tessa who are identical. The second type is the brain bisection and transplant in which a person’s brain is bisected and transplanted into the brain-less heads of two separate people. In both examples there is the fission of one person A to two persons A1 and A2. These exist simultaneously but are separate spatially, causally and each with a separate consciousness, but are both psychologically continuous with A.

In the teletransportation example (the split-brain example has similar results) there arise three possible outcomes:

1. Either A1 or A2 is personally identical to the original person A.

Tess is the person who should have been destroyed, ie person A. But if Tess still exists, what does that make Tessa who is supposed to be the identical teletransported body and carries the psychological contents of Tess? If the machine had worked properly Tess would have been destroyed and Tessa would have presumably been the unique and exact copy of Tess now on Mars. This is an odd result, because without Tess then Tessa would it seems maintain A’s identity, but with Tess still existing, Tessa cannot be A. It is not clear how and why one person rather than the other should be so privileged as to maintain A’s identity?

2. Both persons A1 and A2 are personally identical to A.

But if Tess is still Tess and Tessa also has the identity of Tess, then this result must be false as there cannot be two things identical to each other.

3. If neither A1 nor A2 is identical to A, then A no longer exists.

This seems odd, as we intended to duplicate Tess but due to an error Tess still remained, so how can we now say that Tess no longer exists?

When discussing these issues, Parfit (Parfit, 1984 & 1995) maintained that identity must exhibit uniqueness, which rules out Outcomes 1 and 2 even though they are psychologically continuous, while Outcome 3 implies that Tess has ceased to exist. These outcomes are problematic for personal identity, and so Parfit suggested that we should not place so much significance on it. He considered it dispensable and for our survival we should be concerned about the psychological continuity of our memories, beliefs and dispositions.

Notwithstanding Parfit’s argument, there still remains a persistent feeling of something that is uniquely and identifiably me, my subjectivity. To incorporate subjective continuity without resorting to the supernatural or a Cartesian Ego I would like propose meta-cognition. In brain psychology this function acts as the manager or overseer of the mind’s functioning: its awareness of its own thinking. We see it, for example, in problem-solving. While it may be a part of the brain’s operations, its reflexivity suggests that it may be the process which gives us a sense of personal identity. As meta-cognition arises within a thinking brain, it must be a function of the specific brain in which it occurs, and so must be unique to that brain. Therefore subjective continuity would only be maintained provided that the brain also maintains its meta-cognitive process. Using this argument in the thought-experiment, let us say that Tess has meta-cognitive process M, and Tessa has MA. After teletransportation a newly formed brain (with MA), even though an exact copy, cannot have the same meta-cognition (M) as the original. So for Tess to retain her personal identity one of the two resultant persons must retain meta-cognition M. To do that the unique brain in which M functions must continue, that is, there must be brain continuity.

I will now consider two further points. Firstly, it could be said that meta-cognition is merely a part of psychological continuity and so does not offer any solutions, but this cannot be the case. As the brain’s manager, it operates only in the present moment. It has no span of time and manages exactly those thoughts that are being processed in a given moment of time. It is simply in the present and never of past or future time and so if meta-cognitive processes are being reformed every moment, it is separate from psychological continuity. If subjectivity arises from meta-cognitive reflexivity, and personal identity is essential for persistence of self, then meta-cognition must survive. So I argue that survival in these thought-experiments can only be assured if meta-cognition is maintained. Parfit holds that it is psychological continuity and connectedness that are necessary for survival and that personal identity is dispensable. But I suggest that if the meta-cognitive processes of persons A1 and A2 are different from that of person A there would be no survival even with psychological continuity. For identity survival we need not only psychological continuity but brain continuity too.

My second point refers to the second thought-experiment in which the brain is bisected and transplanted. If half of a split brain is put into another’s head and half is retained in the original head, there are similar outcomes to teletransportation. Again it is difficult to ascertain who would retain identity. There seems to be meta-cognitive continuity because there is brain continuity. However two brains cannot be identical and yet have the same meta-cognition. If both parts of a brain were put into two different bodies again there is brain continuity in both. Clearly there is a problem with meta-cognition and brain continuity. But we must consider the fact that meta-cognition is unique to a specific brain. Therefore it is body/brain continuity that is needed, not just brain continuity alone. Applying this argument to the thought-experiments: the combination of half-brain retained in the original body (A) will retain personal identity. If both halves are transplanted into other bodies (A1 and A2), neither will retain continuity and the personal identity of the A will cease to exist. In the teletransportation example, it is Outcome 1 in which Tess would retain personal identity because of body, brain and psychological continuity, while Tessa would not be Tess.

Parfit has argued that personal identity is over-rated and is dispensable, and argued that it is our psychological continuity that is important. But we do not easily lose the subjective feeling of identity, believing that in some way we have a continuing identifying essence of some sort. I have argued that meta-cognition may be much more than a psychological process arising from the brain’s thought processes because it acts as a self-aware management system. Then I have argued that it is this process that leads to subjectivity and feeling of personal identity. I have further argued that the body-duplication thought-experiments help to show that it is not just psychological continuity, but the unique combination of body, brain and psychological continuity that is required for our survival. On this basis, the concept of personal identity is indispensable to what we know as our unique self to survive through time.


Parfit, D. 1984 Reasons and Persons OUP

Parfit, D. 1995 ‘The Unimportance of Identity in Harris, H. Ed Identity Oxford: OUP

© Brian Roberts 2018