To recall, the Combined Spectrum is an imagined range of operations in which the empirical properties of person p1 (Derek Parfit) are replaced with those of person p2 (Greta Garbo). At the near end of the spectrum, some minutiae of p1’s empirical properties are replaced with those of p2. In the next case along, a trivially larger amount of p1’s empirical properties are replaced with those of p2. This continues until we reach the far end of the spectrum, where we have an operation in which all of p1’s empirical properties are replaced with those of p2.3
It is common sense that persons survive minor physical and psychological changes. If, while I am clipping my fingernails I forget where I put my keys, I do not cease to exist, replaced by an almost-exact replica. So common sense dictates that the first few operations on the spectrum should be regarded as ones in which p1 retains their identity. Thinking now about the far end of the spectrum, it is equally common sense that a person will die if their brain and body are destroyed, and their psychological traits eradicated. So at the far end of the spectrum, where we are to suppose that all of Derek Parfit’s distinctive physical and psychological features are replaced by those of Greta Garbo, we have a clear case of non-identity.
If there is always a ‘deep’ difference between identity and non-identity, then, for any operation on the spectrum, there will be a yes or no answer to the question of whether p1 retains their identity. Towards the near end of the spectrum the answer will be ‘yes’, and towards the far end the answer will be ‘no’; and somewhere on the spectrum there will be a sharp borderline separating the two regions. Parfit argues that such a view is indefensible (1987: 239).4
Firstly, there can be no evidence of such a borderline. If there was a borderline it would lie between a specific pair of adjacent cases. But the difference between any pair of
3 Parfit’s Combined Spectrum thought experiment is an analogue of Chisholm’s (1967). Chisholm
is interested in trans-world identity – that is, what makes person p1 in possible world W1 the same person as p2 in W2. The modified Parfittian argument I develop in this chapter might, with some alterations, be applied to the possible worlds question.
4 The Combined Spectrum can be regarded as laying the groundwork for two related arguments.
One is an argument purporting to show that we should reject Non-Reductionism, and the other is an argument that, assuming the truth of Reductionism, shows that identity can be determinate (and that there is not always a ‘deep difference’ between identity and non-identity). Having argued for Reductionism in the previous chapter, I focus in this chapter on the second of these two arguments.
adjacent cases on the spectrum is just as trivial as any other pair. As such, there would be no empirical reason to favour one pair of cases over another as the borderline’s location. No observation could support the claim that in one particular operation p1 survives, whilst in the next they don’t. Call this the evidence problem.
Secondly, we intuitively believe that there is always a deep difference between identity and non-identity. But positing a sharp borderline entails that this ‘deep’ difference will turn on some trivial physical and/or psychological change. If you believed there was a sharp borderline, you would have to think that a different cell (or molecule, or subatomic particle) and different vague memory (or a slightly stronger desire, or a slightly weaker conviction) would make the all the difference between life and death. But, Parfit asks,
What could make it true that, in one case, the resulting person would be me, and in the next he would not be me? What would the difference consist in? (Parfit 1987: 239)
Call this the truthmaker problem. Parfit claims that only the Non-Reductionist is able to answer these concerns, and so we can only accept the existence of the sharp borderline if we also accept Non-Reductionism.5
Either the Non-Reductionist will be a Cartesian, or they will be a primitivist (including the brute persistence views). If the former, they can hold that in some of the cases p1’s Ego remains throughout the operation, whereas in others it doesn’t; and that between these two cases is where the borderline lies.6 Given that Egos are unobservable, the Cartesian needn’t be worried by the lack of evidence. So the Cartesian Non-Reductionist is untroubled by the evidence objection. And although they couldn’t say where on the spectrum the borderline lies, they are nevertheless able to say why it is true that it lies between whichever cases it does – viz. because the original Ego remains in one case, and doesn’t in the next. This, for them, is what the difference between identity and non-
5 Parfit actually only mentions that the Cartesian Non-Reductionist is able to claim there is a
sharp borderline (1987: 293), but, as shown, the primitivist can also make this claim.
6 That is not to say a Cartesian must hold that there is a sharp borderline. They will if they think
that the existence of an ego is always all-or-nothing. But they might instead think that egos are ontically vague entities (more on that later). The point is that they at least have the option to say that the existence of egos is always determinate; and if all-or-nothing egos did exist, the sharp borderline would sit between two trivially different adjacent cases.
identity consists in. Thus, the Cartesian Non-Reductionist also has an answer for the truthmaker objection.
The primitivist Non-Reductionist is also unruffled by the evidence and truthmaker problems. There is a fact of the matter about which operations p1 survives, and this fact does not consist in other facts.7 So given that the fact of a person’s identity is brute, a lack of evidence is precisely what we should expect. And so it would be missing the point to ask what could make it true that in one case p1 survives and in the next they don’t. For the primitivist nothing makes it true.
The Reductionist, on the other hand, believes that the fact of a person’s identity consists in empirical facts. Given that these facts are empirical, they cannot, as the Non- Reductionist does, accept the lack of evidence for p1’s identity and non-identity. Nor can they avoid the truthmaker problem. Unlike the primitivist, the Reductionist must hold that the difference between identity and non-identity consists in other facts; but unlike the Cartesian, they cannot hide behind any posited unobservable facts to explain the difference. The Reductionist has nothing they can point to that would make it true that the borderline lies between one pair of cases rather than another. And so, given that Reductionism is true, we must reject the idea of a sharp borderline. Our intuitions in that regard would only be justified if Non-Reductionism were true. And so, Parfit concludes, as Reductionists we must accept that there need not always be a ‘deep’ difference between identity and non-identity:
The resulting person would be me in the first few cases. In the last case he would not be me. In many of the intervening cases, neither answer would be true. I can always ask, ‘Am I about to die? Will there be some person living who will be me?’ But, in the cases in the middle of this Spectrum, there is no answer to this question … This question is, here, empty. (Parfit 1987: 232-233)