Parfit on What Matters

Parfit's Division1Part Three of Derek Parfit’s Reasons and Persons is titled “Personal Identity”.  One of its central claims is what Parfit calls the Reductionist View: that persons are not “separately existing entities” over and above their brains and bodies.  What is important about being the same person at different times consists primarily in psychological continuity and connectedness.

Another, related claim is that being the same person is not in itself very important.  In particular, it is not a rational justification for self-concern.  If I know that someone in the future will not be myself, that is not a good reason not to anticipate having that person’s experiences.  What is important are the underlying, real relations of psychological continuity and connectedness.  And even they do not have exactly the same importance that we tend to believe personal identity has.

Part Three of Reasons and Persons contains 150 pages of closely-reasoned arguments which are by and large original, compelling, and illuminating.  I will not try to restate all of Parfit’s arguments, or to comment on them all; instead, I strongly recommend his book to anyone interested in this subject.   In this post, I will review one of Parfit’s more important lines of argument in Chapter 12, “Why Our Identity is Not What Matters.”


Parfit begins this chapter by making a refreshing break from the philosophical practice of thought-experiments, building instead on actual cases documented in medical literature.  These are the famous ‘split-brain’ cases, in which surgeons severed the corpus callosum, the main bundle of nerve fibres connecting the left and right hemispheres of the human brain, as a treatment for epilepsy.  Cutting the connection reduces the severity of epileptic attacks by preventing seizures from spreading from one hemisphere to the other.  But there are side-effects.

The effect, in the words of one surgeon, was the creation of ‘two separate spheres of consciousness’.  (p 245)

This radical conclusion was strongly based on clinical evidence.  The hemispheres of the brain have long been known to show strong left-right differentiation.  Images from the left side of the visual field flow to the right hemisphere, and vice versa.  The right hemisphere primarily controls the left hand, and vice versa.  Parfit describes split-brain patients being presented with two colours: red in the left half of their visual field and blue in the right half.  The patients are questioned about what they saw.  Answers originating in the left hemisphere are given by the right hand, and vice versa.  A patient is asked how many colours he can see.  The response from each hand is, “One.”  When asked to identify the colour, the left hand responds, “Red” and the right hand responds “Blue”.

Differences in visual experience associated with left and right hemispheres in split-brain patients are an established fact, documented in many studies.  Similar clinical evidence shows that the divided hemispheres can have different emotions and attitudes.  “One of the patients complained that sometimes, when he embraced his wife, his left hand pushed her away.”  The complaint, in this example, originated in the patient’s left hemisphere, which dominates linguistic behaviour in most people; the left hand, of course, is controlled by the right hemisphere.

I will not argue further for the conclusion that cutting the corpus callosum can result in two separate centres of consciousness.  If we are willing to speak of consciousness at all, the clinical evidence makes this conclusion inescapable.

It is interesting that the split-brain studies have not caused more consternation among Non-Reductionists – those who believe that personal identity consists in something over and above facts about the physical brain and body.  Many Non-Reductionists believe in the soul, which is often held to be created by God, indestructible, and indivisible.  What would a believer in immortal souls say about split-brain surgery?  That the surgeon’s knife divides a soul, creating two from one?  Or that a single soul can have two centres of consciousness?

Parfit also refers to clinical cases in which human beings survive the loss of a hemisphere.  Our brains have much redundant capacity.  A single functioning hemisphere – either one – is enough for survival.  “With his remaining hemisphere, such a person may need to re-learn certain things, such as adult speech, or how to control both hands.  But this is possible.” (p 254)

We do not say that a person who has lost one brain hemisphere has ceased to exist, any more than we say that a person who has lost a leg has ceased to exist.

Parfit proceeds to launch into a new thought-experiment, made plausible by the fact of split-brain surgery coupled with the fact of survival with a single hemisphere.   He supposes that he is among the minority of people who have little differentiation of ability between left and right hemispheres. (Although in most people, the left hemisphere is much more proficient than the right in linguistic and numerical abilities, and the right hemisphere is better than the left in geometrical reasoning, face recognition, and musical abilities, this is not true of everyone.)  He then supposes that surgeons have implanted a clever switch that enables him to disconnect his two hemispheres, and reconnect them, at will, by raising or lowering an eyebrow.   “This ability would have many uses,” says Parfit.  “Consider…”

My Physics Exam. I am taking an exam, and have only fifteen minutes left in which to answer the last question.  It occur to me that there are two ways of tackling this question.  I am unsure which is more likely to succeed.  I therefore decide to divide my mind for ten minutes, to work in each half of my mind on one of the two calculations, and then to reunite my mind to write a fair copy of the best result.  (p 247)

Given what is known about hemispheric specialization, a geometry exam might have been a better example.  However, we can accept Parfit’s stipulation that both his hemispheres are about equally as good at physics.  Parfit describes the case very clearly.  While communication between his hemispheres is blocked, he has two sets of experiences, each focussed on a particular solution to the physics problem.  Each centre of consciousness has control of the corresponding arm and hand, which writes out the results.   As he works at the calculation ‘in his right-handed stream,’ he may wonder how the other stream is getting on.  He could get an idea, by glancing across at his left hand.  “This would be just like looking to see how well my neighbour is doing, at the next desk.”  When the time is up, he lowers an eyebrow to reunite his mind.  As he does so, the two streams converge.  He remembers working at both approaches to the problem.  He remembers working at each one while being then unaware of working at the other one.

This technology promises to open a new dimension to multi-tasking, and is therefore likely to attract teenagers and adults who juggle multiple concurrent texting and VOIP sessions, homework, music and videos.  Is Parfit’s brain-switch far-fetched?  Not very.  Conservatives on this subject may be disquieted by the fact that dividing the cerebral hemispheres was described in 1860 by Gustav Fechner, who predicted that the result would be two centres of consciousness.  Fechner considered his account a “thought experiment”, impossible to achieve in reality.


Permanent brain-splits, temporary brain-splits, and hemispherectomies are all a prelude to Parfit’s central thought-experiment, which he calls My Division.   In Parfit’s story, he is one of ‘identical’ (that is, monozygotic) triplets.  Several cases are described.

In the first case, Parfit has been mortally injured, perhaps by a car accident.  At the same time, one of his brothers has an inevitably fatal brain disease.  To save what can be saved, surgeons transplant Parfit’s brain into his brother’s body.  The brain transplant allows Parfit to survive.  Parfit points out that this could be done with existing techniques at the time of writing in 1984.  A drawback of the procedure is that, without means of connecting the transplanted brain to the remainder of his brother’s nervous system, the resulting person would be paralyzed.  But a paralyzed life may be better than death, especially if the paralyzed person has some means of communicating with others.  And with today’s technology, some such means is possible.  Functional MRI scans can be used to crudely interpret conscious mental states.  Crude interpretation is enough for communication.  All that’s really needed is a binary signalling capability.  Jean-Dominique Bauby suffered a stroke resulting in complete paralysis except of his left eyelid.  He and his caregivers were able to develop his ability to blink into a channel of communication that resulted in Bauby’s remarkable book, The Diving Bell and the Butterfly.  And prospects for recovery from paralysis are improving.  In 1998, J.W. McDonald used stem cell therapy successfully to repair spinal cord lesions in rats.  Perhaps such therapies will be used to cure human paralysis within a few years.

The philosophical point is a simple one, and relatively uncontroversial.  If his brain is transplanted to his brother’s body, Derek Parfit, not his brother, is the survivor.  Even Aristotle, who thought the heart to be the seat of the rational soul, might have come round on this point after learning that Derek Parfit’s memories, intentions, and attitudes were transplanted along with his brain.

In the second variant of this case, only one hemisphere of Parfit’s brain is transplanted into his brother’s body.  As we have seen, a person can survive if one hemisphere is destroyed.  So in this case too, Derek Parfit survives.

There is a complication about the ‘lower brain’, which is not so neatly divided into hemispheres.  This part of the brain evolved earlier than the cerebral cortex, and is found in ‘lower animals’ which lack hemispheres.  In human beings, the activities of the ‘lower brain’ are not directly associated with consciousness.  I will assume that only the cerebral cortex of Derek’s brother is removed because of disease; his lower brain is allowed to remain. And only one of Derek’s hemispheres, not his lower brain, is transplanted to his brother’s body, where stem cells are used to connect it to his lower brain.  This, I think, would also be enough to justify saying that Derek Parfit survived the operation.

We come to the third case, in which both of Derek’s brothers have incurable brain diseases.  The surgeons transplant one of Derek’s hemispheres to each of his brother’s bodies.

If we agree that Derek survives in the case where one hemisphere of Derek’s brain is transplanted to his brother’s body, it is hard to deny that he would survive if both hemispheres are separately transplanted to both brothers’ bodies.  A double success cannot be failure.

But of course, there are two survivors in this case.  They are two different people, who will lead separate post-operative lives.  And by the now-familiar argument, they cannot both be the same person as the original Derek Parfit, because “being the same person as” is a transitive relation.  Let us call the post-op Parfit twins Lefty and Righty.  If Lefty is the same person as the pre-operative Derek, and Righty is the same person as Derek, then Lefty is the same person as Righty.  Since Lefty is not the same person as Righty, and both have essentially the same relation to Derek, neither of them can be Derek.

It makes no sense to say that original Derek survived the single transplant case, but that in the double transplant case, he died.   Yet it does not seem right to conclude that Derek did not survive the single transplant case either.  The argument that, in the single transplant case, Derek died on the operating table because he could have been doubly transplanted, is not credible.

Ordinarily, we count persons by counting heads.  We count two persons following Parfit’s Division, where before we counted just one.  If our ordinary way of counting is correct, Parfit’s argument convincingly separates personal identity from what matters in survival.  It is not surprising that we tend to conflate the two, since historically they have coincided very closely.  But the actual split-brain cases have driven a wedge between the two concepts, because they clearly show the physical possibility of human fission.

Perhaps our ordinary way of counting persons is wrong for this sort of case.  In describing the double transplant case, we have the following choices:

  1. Contrary to appearances, there is just one post-split Derek Parfit.  That single person has two bodies, indeed two independent lives.  What makes what appear to be two people one is their shared past.
  2. Contrary to appearances, there were two Derek Parfits all along.  Before the split, they shared one body and one life.  At that time, what made them two people is the fact that they would, in future, be split.  (David Lewis argued for this view.)
  3. Pre-split, there was one person; post-split there were two.  Since the pre-split person has essentially the same relation to both post-split persons, the pre-split person cannot be the same person as either one.  Therefore, when the split occurred, the pre-split person ceased to exist and two post-split persons came into being.

The third choice best fits the way we ordinarily use the term “same person.”  It allows us to continue to count persons by counting heads.  If we choose (3), we must make a further choice:

3.1 When Derek Parfit was divided, he died.  If, before the split, he anticipated waking up as either or both of the post-split survivors, he was sadly deluded.  Rationally, he should have regarded his division as death – because, by the ordinary criteria of personal identity, he ceased to exist.

3.2 When Derek Parfit was divided, he ceased to exist.  But funeral arrangements are not appropriate, because division is not like death.  Everything important about Derek Parfit’s continued survival continues in the post-split Parfits – continues doubly, in fact.  Double survival is not death.  The pre-split Parfit was quite right to anticipate waking up in both bodies – or at least, no more wrong than anyone is to anticipate waking up tomorrow morning.  It doesn’t matter that the original Derek Parfit ceased to exist, because being the same person (that is, personal identity) is not what matters.

All Parfit’s arguments in Part Three of Reasons and Persons tend to the conclusion that 3.2 is the correct description of the case.  Parfit’s solution to the dilemma of the double transplant is that personal identity is not, after all, what matters in survival.  In the single transplant case, Derek’s life is preserved in the successfully transplanted hemisphere.  In the double transplant case, his life is no less preserved.  Only his identity is lost in the double transplant case, and that is because there are two survivors.

Parfit’s arguments are by no means universally accepted by philosophers who have considered them.  The most stubborn objections seem to turn on an insistence that being the same person is what matters in our continued existence.  Some think that the story of Parfit’s division is correctly described by 3.1.  When Derek divided, he ceased to be; and if, pre-split, he anticipated having post-split experiences, he was mistaken.  At least one, David Lewis, has argued that choice 2 is the correct description of the case.

Does it matter to me whether I – the same person – will be alive tomorrow?  Parfit says no: what matters is whether someone will be alive who is psychologically connected and continuous with me.  For me to survive in the way that matters – to be justified in expecting to wake up tomorrow morning – it is enough for someone to inherit enough of my psychological attributes (and if we like, we may add physical attributes which are important to me, such as my appearance).   If two or more people inherit my attributes, that is almost as good as ordinary survival.

Or it might be slightly better than ordinary survival.  People commonly say they wish they could ‘clone themselves’ in order to get all their work done.  No doubt some people would sincerely regard their personal division as a benefit.


Bauby, Jean-Dominique (1997) The Diving Bell and the Butterfly

Lewis, David (1983) “Survival and Identity”, in Martin and Barressi, ed. (2003) Personal Identity

Parfit, Derek (1984) Reasons and Persons

Springer, Sally P., and Deutsch, George (1981) Left Brain Right Brain


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6 Responses to “Parfit on What Matters”

  1. […] Whether this is a good or bad thing is hard to say. It might in fact just be a matter of opinion. One thing’s for sure, though: if concentrating hard on something really is like resource locking in software terms, then I hope we never learn to use both hemispheres of our brains independently. […]

  2. Louise Kelly says:

    Hi there, I just found this post and referenced it in my own post about Derek Parfit (I appreciated finding a resource online that so nicely summarized the various scenarios Parfit describes in part 3 of Reasons and Persons). If you ever get a chance, I’d be curious what you think of the points I make in this post:

    It seems to me that our unique existence in space and time, along with the relationships developed through that existence, create a kind of real self that transcends the ‘brain and body’ conception of self as defined by Reductionism, and create a continuity that challenges some of Parfit’s arguments you outline here. But I am not a philosopher, so I’d love to hear how I’m either misinterpreting Parfit and/or simply exhibiting flawed logic.


  3. […] feels as if they’re you, Now there are two people claiming to be you. Are there now two yous, or have you been destroyed by the creation of two […]

  4. […] feels as if they’re you, Now there are two people claiming to be you. Are there now two yous, or have you been destroyed by the creation of two […]

  5. […] feels as if they’re you, Now there are two people claiming to be you. Are there now two yous, or have you been destroyed by the creation of two […]

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