16 What Matters in Survival
Derek Parfit defends at least two important theses in chapter 12 of (Parfit 1984):
Personal identity may be indeterminate.
There are cases in which it is not determinate whether a person \(X\) at \(t\) is, or not, numerically identical to person \(Y\) at \(u\).
Personal identity is not what matters in survival.
What we should value and regard as important are relations of psychological continuity and connectedness, which accompany personal identity in ordinary cases. There are special cases in which personal identity and psychological continuity come apart and in which we should value the latter over the former.
16.1 Fission
We begin with some evidence for reductionism:
Human beings have a lower brain and two upper hemispheres, which are connected by a bundle of fibres. In treating a few people with severe epilepsy, surgeons have cut these fibres. The aim was to reduce the severity of epileptic fits, by confining their causes to a single hemisphere. This aim was achieved. But the operations had another unintended consequence. The effect, in the words of one surgeon, was the creation of “two separate spheres of consciousness”.
Consider the case of a pair of identical twins and suppose that both my body and my twin’s brain have been fatally injured. We can now imagine that because of advances in neuro-surgery, we can combine my healthy brain with my twin’s healthy body. The psychological criterion seems to suggest that I am now identical with the survivor of the operation, since that survivor will be psychologically continuous with me and the continuity in question is non-branching. In fact, this is a case in which I am the recipient of a new body.
Some proponents of the physical criterion would agree that I am now identical with the survivor of the operation because enough of my brain is spatiotemporally continuous with that of the survivor. Furthermore, one hemisphere would be enough, since many people have survived when a stroke or injury impairs one of their hemispheres.
So, I would survive if half of my brain was successfully transplanted into my twin’s body and the other half was destroyed. What if the other half was not destroyed?
My body is fatally injured, as are the brains of my two brothers. My brain is divided, and each half is successfully transplanted into the body of one of my brothers. Each of the resulting people believes that he is me, seems to remember living my life, has my character, and is in every other way psychologically continuous with me. And he has a body that is very like mine.
What happens to me in that case? There are four main options:
- I do not survive.
- I survive as one of the two people.
- I survive as the other.
- I survive as both.
We may consider each option in turn:
I do not survive.
I would survive if half my brain was successfully transplanted and the other destroyed. So, how could I fail to survive merely because the other half is successfully transplanted. How does twice the success lead to a failure?
Notice that if personal identity is what matters, then 1 is tantamount to death.
I survive as one of the two people.
I survive as the other.
The problem with these two options is that they seem completely on a par, and it is not clear what could make one true and the other false.
I survive as both people.
There are two ways to understand this option.
i. I survive as one of them, and I survive as the other.
The problem with this option is that it violates the transitivity of identity. Two different people emerge from the operation, yet I cannot be identical to one without being identical to the other.
ii. I survive as the sum of the two people.
Only one person emerges from the operation, but it comes with two person-like parts. The new person has a divided mind, two bodies, and two streams of consciousness.
One issue is that if the two person-like parts move on to live very different fruitful lives, we would be inclined to judge that they are two different people.
How should we think of a case in which they both fight in a duel? How many parties are involved? One person-part on each side and one person on both sides? And supposed one bullet kills. Are there two acts, one murder and one (half) suicide? How many people remain? One? Two?
The problem is especially acute if one is not a reductionist about personal identity. For suppose you think that you are a Cartesian Ego. Then there are four different possibilities as to the fate of the Cartesian Ego. It could either cease to be, or it could move to one of the two bodies but not the other, or it could finally somehow inhabit both.
The reductionist may avoid this problem. There are no different possibilities to consider; there is just one outcome in which one person is psychologically continuous with two different people. Furthermore:
The question of whether I will be one person, or the other, or both is empty and lacks a determinate answer.
Compare with a situation in which a political party splits. We may choose to describe the scenario as one in which the party ceases to exist and two new parties come to existence or as one in which the original party survives as one but not the other of the new parties, etc. We could stipulate an answer, but there is, at the end of the day, no fact of the matter as to which is the true answer to this question.
There is, accordingly, no fact of the matter as to which one is the true description of the case at hand. We may ask which one is the best description, and by those lights, it may be better to say that none of the two people will be me.
I bear all the relations that matter in survival to each one of the two people even though I am not identical to either one of them. I survive as each one of them if we use the term in a way in which it no longer entails identity.
16.2 Survival and Identity
The case of fission is important because in brings out the fact that what matters in survival may on occasion come apart form personal identity. Here are two important theses Parfit holds:
Relation R is what matters.
R is psychological connectedness and psychological continuity with the right kind of cause.
The crucial point is that unlike numerical identity, relation R can take a branching form. In ordinary cases, both coincide, but they come apart in special situations such as fission.
Proponents of the psychological criterion will generally take personal identity to come down to the combination of relation R and uniqueness:
\[ PI = R + U \]
Most agree that PI matters or has value, and we may assume that R similarly matters. What should we take the contribution of uniqueness (U) to be to the value of personal identity (PI)? Parfit considers some options:
- R without U has no value.
- U marginally enhances the value of R.
- U makes no difference to the value of R.
- U marginally reduces the value of R.
The thought now is that the presence or absence of U makes no important difference to the intrinsic nature of my relation to a future person to which I’m R-related. So, there is no rational incentive for me to seek to bring about uniqueness.