On Derek Parfit and Personal Identity

The problem with double survival is that it does not fit the logic of identity. Like certain other Reductionists, I claim

Relation R is what matters. R is psychological connectedness and/or psychological continuity, with the right kind of cause…

In the imagined case where I divide R takes a “branching” form. But personal identity cannot take a branching form. I and the two resulting people cannot be one and the same person. Since I cannot be identical with two different people, and it would be arbitrary to call one of these people me, we can best describe the case by saying neither of these will be me.

Which is the relation that is important? Is what matters personal identity, or relation R? In ordinary cases we need not decide which of these is what matters, since these relations coincide. In the case of My Division these relations do not coincide. We must therefore decide which of the two is what matters.

If we believe that we are separately existing entities, we could plausibly claim that identity is what matters. On this view, personal identity is a deep further fact. But we have sufficient evidence to reject this view. If we are Reductionists, we cannot plausibly claim that, of these two relations, it is identity that matters. On our view, the fact of personal identity just consists in the holding of relation R, when it takes a non-branching form. If personal identity just consists in this other relation, this other relation must be what matters.

-Derek Parfit

This excerpt by Derek Parfit is part of a response to the psychological continuity theory of personal identity seeking to answer whether it is personal identity or psychological continuity that ultimately matters.

The psychological continuity theory states that the perpetuation of personal identity is psychological connectedness and continuity, referred to here as relation R. Two people are numerically identical if and only if the two are linked through a sequence of stages each of which is psychologically continuous and connected with its predecessor. Being psychologically connected entails having a certain psychological state because of a previous one (such as remembering something because of having experienced it), and being psychologically continuous requires an actual chain of connections. For example, this theory entails that a person does not survive complete psychological “erasure,” or a complete reprogramming of psychology with a new set of beliefs, desires, memories, despite having the same body. A scenario can arise in which nothing dies biologically but a person ceases to exist (for example, if Peter were to be psychologically “erased,” the person Peter is destroyed but the animal Peter, object Peter etc. continues to live).

In the first paragraph, Parfit raises an interesting point in that in cases of fission (also referred to as double survival, division, or branching), a person does not survive. Fission is the bizarre scenario in which one person is divided into two duplicates, each psychologically continuous and connected with the common predecessor. Parfit makes it clear that relation R is what ultimately matters, but makes the case that whereas psychological connectedness and continuity persists (or branches) in the case of fission, personal identity is instead lost and the person undergoing fission does not survive. Since one person cannot be identical with two separate people in the future, neither of the two copies after fission will equal the original. This can be clarified using an example. Suppose Person A is physically cut into two halves, and by some feat of science, each half is restored back into a full person, labeled Person B and Person C, each completely identical and, most importantly, psychologically continuous and connected with the original Person A. By the psychological continuity theory, Person B numerically equals Person A and Person C with Person A. However, Person C does not equal Person B because Person B and Person C are not psychologically continuous and connected with each other as they now each have separate experiences. Since the end result of fission is A=B, A=C, and B=/=C, which is impossible, by reductio ad absurdum, psychological continuity and personal identity must not be the same and the original theory falls apart.

David Lewis challenges Parfit’s take and provides a different resolution to the conflict. Using the same fission example as earlier, Person B and Person C are indeed different people, but according to Lewis, it only appears that Person A = Person B and Person A = Person C. Lewis would argue that if fission is in the future of Person A, then Person A is a stage in the life of two people who just happen to overlap. There aren’t two distinct minds or people in Person A, two people just happen to coincide as one during this time. An analogy can be made with two roads that share a segment together but eventually branch off into separate destinations. It would be improper to ask where the combined segment leads since it is actually two roads merely appearing as one; similarly, in the case of a person to undergo fission in the future, asking “what will this individual be doing in the future” is inherently a nonsensical question given that in the future, he or she will actually be multiple people.

Another way to interpret Lewis’s theory is to assign time to the numeric personal identity of an individual. In mathematical terms, the theory entails that personal identity is actually a function of time. Applying this to the previous example, it is lacking to simply state A=B, A=C, C=/=B, which appears impossible, but instead, defining t1 as time before splitting and t2 as time after splitting, A(t1)=B(t1), A(t1)=C(t1) (in which case B(t1)=C(t1)), and C(t2)=/=B(t2), which is intuitively and mathematically reasonable. As such, Parfit’s claim that personal identity is lost through branching is not upheld and the original psychological continuity theory remains valid if we accept Lewis’s proposition.

After tackling the psychological continuity theory, Partfit proceeds to put forth an interesting question. Given his conclusion that personal identity and psychological connectedness are separable, does it matter if we lose personal identity and not survive if we remain psychologically connected?

If we are to accept Lewis’s argument, then Parfit’s initial claims regarding the psychological continuity theory is subverted, and this question is in turn undermined as well. It is unnecessary to ask if personal identity is more important than psychological continuity if it cannot be established that personal identity and psychological continuity are two separate and distinct entities. However, an interesting point is nevertheless raised here, in that if we for sake of argument assume personal identity and psychological continuity to be separable things, and that fission does lead to loss of personal identity but perpetuates psychological continuity, then what is truly important in such a case?

Parfit takes the firm stance that psychological continuity is what ultimately matters. Parfit makes the case that if humans are “separately existing entities,” which refers to beings whose persistence through time is not determined by physical and psychological continuity, then personal identity does matter. This is a rather obvious claim, essentially stating that if one’s continuation does not depend on psychological continuity, then identity is more important than just psychological continuity. Both Parfit and Lewis agree that personal identity depends on psychological continuity, and it would be difficult to suggest otherwise.

In Parfit’s view (deemed the Reductionist view), personal identity consists of maintaining psychological continuity, but the reverse is not necessarily true. It is possible to lose identity with psychological continuity intact. In a non-branching form, the question is irrelevant because both are preserved; however, when branching occurs, one does not survive because personal identity is lost. Parfit makes a clear distinction between survival and what actually matters in survival, claiming that it doesn’t matter whether one survives into the future, even if driven by self-interest, as long as psychological continuity preserved. What matters ultimately is whether there are individuals in the future who are psychologically continuous with the original person. The difference between ordinary survival and fission is only that there is a single, unique psychologically continuous individual in the ordinary case there are multiple in fission.

Lewis, when presented with the same question, would immediately dismiss the question as nonsensical. According to his theory, psychological continuity and personal identity are inseparable, and there is no conceivable situation in which one can be preserved but not the other. However, Lewis agrees with Parfit that psychological continuity is what is ultimately important, personal identity just inevitably accompanies it.

With this insight, the conflict between the theories of Parfit and Lewis is reduced to a disagreement over lingual technicalities. Parfit believes that fission causes an individual to lose identity, but this is not problematic since psychological continuity is maintained. Lewis believes that fission does not cause one to lose identity, but fission is still not an issue because psychological continuity is sustained by default. Lewis and Parfit are disagreeing over the best way to extend our ordinary concepts and intuition to bizarre concepts, but the underlying metaphysical laws governing the fission situation are identical under both theories. Such a debate still has important implications but only in terms of offering guidance on how we interpret and comprehend complex situations in which common sense is no longer serviceable.

Parfit and Lewis do not take the concept of personal identity any deeper, which is reasonable in the case of fission; after all, being divided into two does not intuitively seem to be of any major inconvenience and is certainly preferably to death or mental erasure. However, this conclusion appears incomplete when applied generally. It is important whether a future person would still be me, regardless if said person is psychologically continuous (we will assume here that personal identity without psychological continuity is impossible). For example, if I and everybody I knew were to be replaced overnight with a physically identical and psychologically continuous copy, I would feel a tremendous unease that would be unwarranted if, as the theories of Parfit and Lewis would imply, psychological continuity were the sole factor of significance in all cases. There remains legitimate concerns regarding personal identity that reach beyond psychological continuity, revealing additional elements to personal identity that Parfit and Lewis do address (though they may perhaps agree that these elements exist). If such elements are important, then the loss of personal identity while retaining psychological continuity would still produce a significant problem. If this is truly the case, personal identity takes a much deeper metaphysical meaning that Parfit and Lewis dismiss, leaving their theories incomplete and unsatisfactory.