Key Arguments

• He defines the Combined Spectrum as 'another range of possible cases' that 'involve all of the possible variations in the degrees of both physical and psychological connectedness', with the near end being 'the normal case' and the far end a total discontinuity where 'the scientists would destroy my brain and body, and then create, out of new organic matter, a perfect Replica of someone else.'
• He emphasizes that neighbouring cases differ only trivially in which cells are replaced and in small psychological changes, e.g. 'If the surgeons replace slightly fewer than these cells, and produce one fewer psychological change, it will be me who wakes up. If they replace the few extra cells, and produce one more psychological change, I shall cease to exist, and the person waking up will be someone else.'
• Assuming identity must always be determinate forces us to claim 'Somewhere in this Spectrum, there is a sharp borderline' such that one small extra replacement and 'one more psychological change' makes 'all the difference' between my survival and my ceasing to exist, which he calls 'hard to believe'.
• He argues it is also 'hard to believe' that such a sharp borderline must exist 'though we could never have any evidence where the borderline would be', and notes that 'Some would claim that, if there could never be such evidence, it makes no sense to claim that there must somewhere be such a line.'
• He concludes that these claims, taken together, are 'extremely implausible', and that they are 'even more implausible than the only other possible conclusion, which is the Reductionist View', so 'We should therefore now conclude that the Reductionist View is true.'
• On the Reductionist View, in 'the central cases of the Combined Spectrum, it would be an empty question whether the resulting person would be me', because all the underlying physical and psychological facts are given and nothing further could settle the identity question.

Source Quotes

86. THE COMBINED SPECTRUM Consider another range of possible cases. These involve all of the possible variations in the degrees of both physical and psychological connectedness. This is the Combined Spectrum. At the near end of this spectrum is the normal case in which a future person would be fully continuous with me as I am now, both physically and psychologically.

This is the Combined Spectrum. At the near end of this spectrum is the normal case in which a future person would be fully continuous with me as I am now, both physically and psychologically. This person would be me in just the way that, in my actual life, it will be me who wakes up tomorrow.

This person would be me in just the way that, in my actual life, it will be me who wakes up tomorrow. At the far end of this spectrum the resulting person would have no continuity with me as I am now, either physically or psychologically. In this case the scientists would destroy my brain and body, and then create, out of new organic matter, a perfect Replica of someone else. Let us suppose this person to be, not Napoleon, but Greta Garbo.

We might continue to believe that our identity must be determinate. We might continue to believe that, to the question ‘Would the resulting person be me?’, there must always be an answer, which must be either and quite simply Yes or No. We would then be forced to accept the following claims: Somewhere in this Spectrum, there is a sharp borderline. There must be some critical set of the cells replaced, and some critical degree of psychological change, which would make all the difference. If the surgeons replace slightly fewer than these cells, and produce one fewer psychological change, it will be me who wakes up.

There must be some critical set of the cells replaced, and some critical degree of psychological change, which would make all the difference. If the surgeons replace slightly fewer than these cells, and produce one fewer psychological change, it will be me who wakes up. If they replace the few extra cells, and produce one more psychological change, I shall cease to exist, and the person waking up will be someone else. There must be such a pair of cases somewhere in this Spectrum, even though there could never be any evidence where these cases are.

There must be such a pair of cases somewhere in this Spectrum, even though there could never be any evidence where these cases are. These claims are hard to believe. It is hard to believe (1) that the difference between life and death could just consist in any of the very small differences described above.

Some would claim that, if there could never be such evidence, it makes no sense to claim that there must somewhere be such a line. Even if (2) makes sense, claims (1) and (2), taken together, are extremely implausible. I believe that they are even more implausible than the only other possible conclusion, which is the Reductionist View. We should therefore now conclude that the Reductionist View is true. On this view, in the central cases of the Combined Spectrum, it would be an empty question whether the resulting person would be me.

We should therefore now conclude that the Reductionist View is true. On this view, in the central cases of the Combined Spectrum, it would be an empty question whether the resulting person would be me. This Spectrum provides, as I claimed, a strong argument for this view. There are some people who believe that our identity must be determinate, though they do not believe that we are separately existing entities, distinct from our brains and bodies, and our experiences.

Key Concepts

• Consider another range of possible cases. These involve all of the possible variations in the degrees of both physical and psychological connectedness. This is the Combined Spectrum.
• At the near end of this spectrum is the normal case in which a future person would be fully continuous with me as I am now, both physically and psychologically.
• At the far end of this spectrum the resulting person would have no continuity with me as I am now, either physically or psychologically. In this case the scientists would destroy my brain and body, and then create, out of new organic matter, a perfect Replica of someone else.
• Somewhere in this Spectrum, there is a sharp borderline. There must be some critical set of the cells replaced, and some critical degree of psychological change, which would make all the difference.
• If the surgeons replace slightly fewer than these cells, and produce one fewer psychological change, it will be me who wakes up. If they replace the few extra cells, and produce one more psychological change, I shall cease to exist, and the person waking up will be someone else.
• These claims are hard to believe.
• Even if (2) makes sense, claims (1) and (2), taken together, are extremely implausible. I believe that they are even more implausible than the only other possible conclusion, which is the Reductionist View. We should therefore now conclude that the Reductionist View is true.
• On this view, in the central cases of the Combined Spectrum, it would be an empty question whether the resulting person would be me. This Spectrum provides, as I claimed, a strong argument for this view.

Context

Section 86 ('THE COMBINED SPECTRUM'), where Parfit unifies the earlier Psychological and Physical Spectra into a two‑dimensional range of cases, and uses its structure to argue that insisting on always‑determinate personal identity without Reductionism leads to an unbelievable sharp borderline, making Reductionism the most plausible view.