The Reductionist View of Personal Identity

By Jasmine Pandit

Over the course of several millennia, many great minds have often wondered what precisely makes them them throughout their lives. These individuals have thus engaged in the philosophical question of diachronic personal identity, which asks: what does the continuity of a person over time necessarily involve or consist in? To answer this question, many philosophers have implicitly considered the identity of a person as a sort of deep unshakeable fact, analyzing criterion such as memory, character, and bodily continuity to do so. In his landmark book Reasons and Persons, however, 20th century philosopher Derek Parfit argues for a Reductionist theory of the self. He believes that a person consists just in, and therefore nothing over and above, the holding of certain physical and psychological facts. From this theory, he further concludes that questions of personal identity (such as those asking if two people taken at different times are, in fact, the same person) can be “empty”. In this essay I will attempt to unpack Parfit’s thesis of Reductionism, as well as offer my own critical analysis of it.

Reductionism, taken generally, merely “reduces” a particular fact or entity to the holding of certain, more basic facts, and claims that these facts give an exhaustive account of the overarching phenomena. A reductionist view of geometry, for instance, might claim that every geometrical truth could be boiled down to the truth of a few particular axioms. The Reductionist view we care about, then, claims that (1) the identity of a person over time consists just in a set of facts, and (2) these facts can be described in an “impersonal” way (i.e., they do not reference the identity of the person they seek to describe). Parfit suggests that these more basic facts could involve physical truths (e.g., the continuity of the brain), psychological truths (e.g., psychological connectedness with a past person), or some combination of the two. However, he does not endorse one option over the others, as “Reductionists [should not] decide between the different criteria of personal identity” (Reasons and Persons, 241). 

Parfit believes a core implication of Reductionism is that it allows questions of personal identity to be empty, which he defines as being “neither true nor false” (213). To illustrate this, Parfit uses the example of a club:

Suppose that a certain club exists for several years, holding regular meetings. The meetings then cease. Some years later, some of the members of this club form a club with the same name, and the same rules. We ask: ‘Have these people reconvened the very same club? Or have they merely started up another club, which is exactly similar?’ (213)

Parfit claims that these are empty questions. He asserts that we are reductionists about clubs, meaning we do not believe there is any ‘further fact’ to a club other than what it consists of (its members, its nature, how often it meets, etc.). Thus, if we can provide a complete description of these parts, it logically follows that we know everything we need to know about this club. When asked if this new club is, in fact, numerically identical to the former club, we could argue one way or another. But the label we would put on this club would be a mere matter of semantics; by claiming that this club is or is not the same, the reality of the situation would remain unchanged. Under the Reductionist view, different answers to the question of identity would be “merely different descriptions of [the same] fact or outcome” (214).

I agree with this claim, and reconstruct Parfit’s argument as follows. Say there exists a holistic entity X, which consists of the holding of the more basic facts A, B, C, and D. Further suppose we are reductionists about X, and claim that X involves and consists of just A, B, C, and D. Now say there is an entity not necessarily distinct from X that similarly consists of A, B, C, and D, and call this entity Y. We are given a complete account of the states of A, B, C, and D in Y and are asked: is Y identical to X? Reductionism claims that this is an empty question because what is being asked is not truly a question: that is, we are not trying to choose between different realities. If there were some sort of X-ness to X beyond A, B, C, and D, there would be something to answer. We would have to determine whether what we have been given qualifies as either X or not X. But both X and Y are reducible to merely A, B, C, and D, and we know everything we need to know about these constituents. Thus, we know everything we need to know about Y; claiming it is either X or not X tells us nothing more than we already know. There could arguably be better or worse descriptions of Y in relation to X, but this is not what matters. 

It seems convincing to me that this more general form of Reductionism would apply to persons. I would claim that, under the Reductionist view, all questions of personal identity are technically empty. Parfit does not explicitly make this claim, but I do not think there needs to be controversy over a question of personal identity for it to be empty. Even in the most universally agreed upon cases (e.g., a snapshot of two people physically and psychologically continuous with each other and strongly psychologically connected, the latter of which exists at a time only a second after the former), Reductionism says the identity of a person only consists of the holding of certain facts; thus we can only be sure of a complete description of these facts. Nevertheless, it seems rational to “give” these questions an answer in these cases, if only for practicality purposes. 

Parfit does not merely assume the Reductionist view: he discusses his arguments for Reductionism as well as the notion of an empty question by presenting a series of Spectrums, Physical, Psychological, and Combined. Because we can glean all of Parfit’s arguments from just the Combined Spectrum, I will examine only this one. His thought experiment goes as follows:

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. 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. (236-237)

Let us call this person Barack Obama. We can imagine a series of 1,000 switches that would gradually take us from the near end of the spectrum (a person qualitatively identical to us) to the far end of this spectrum (a person qualitatively identical to Obama). Flipping the first switch would replace a couple of our memories, intentions, character traits, etc. with Obama’s, and so on for every other switch until the person remaining was exactly like Obama. Parfit writes that though there might seem to be a clear answer in the first few or last cases of the Spectrum (as the identity of a person would plausibly be preserved after a few psychological changes), the identity of the person in the middle cases of the Spectrum would be empty. I believe I have explained why this would be true on the Reductionist view. But why should we accept Parfit’s view?

Parfit is a Reductionist, but he also presents the view of Non-Reductionism (which he believes that most of us are more naturally inclined to). A Non-Reductionist, he writes, would reject both the Reductionist claims (1) and (2) and instead see personal identity as involving some “a further fact…not just consist[ing] in physical and/or psychological continuity” (210). To use a common phrase, a Non-Reductionist would view a person as more than just the sum of its parts. 

The Non-Reductionist would also believe that personal identity is always determinate (or at least, Parfit believes that this is the only defensible view). Parfit thus presents two arguments that the Non-Reductionist could make: they could claim that (1) the person that undergoes the Combined Spectrum is always me, even at the farthest end or (2) there is a sharp borderline somewhere in this range of cases. It seems clear that (1) cannot be true, as a person exactly psychologically and physically alike with Barack Obama is clearly not me. The Non-Reductionist would then be forced to accept that a trivial psychophysical change (the replacement of a few cells in the brain, the addition and deletion of a couple memories) in the Combined Spectrum, would mean the difference between life and death. This seems very implausible. Parfit believes this view is more implausible than simply accepting a Reductionist view, and I am inclined to agree. 

There is another alternative that Parfit sees. One can claim that though there is no sharp borderline case, we should draw one “to avoid incoherence” (240). Instead of rebutting this, Parfit accepts it as one version of Reductionism. In this case, we have chosen to “give” an answer to an empty question for the sake of convenience, but our choice is so arbitrary that “we cannot believe that it has, intrinsically either rational or moral significance” (241). 

I shall now briefly discuss two objections I have to Parfit’s arguments. The first concerns the ‘sharp borderline’ view in the Combined Spectrum thought experiment. Parfit claims not only that the existence of this borderline is implausible, but that “there could never be any evidence where [the borderline] cases are” (239). He reasons that when undergoing the Combined Spectrum, the resulting person is inclined to believe they are the person they were at the beginning of the experiment in every case; this perspective is clearly unreliable, and hence we could never discover the true identity of the person undergoing the Spectrum. Nevertheless, this reasoning is flawed, as it assumes that the person undergoing the Combined Spectrum must be the one we ask to judge their own identity. In truth, if this Spectrum existed, any other objective bystander could be the determiner of this person’s identity: the person who originally entered the spectrum, a person very well acquainted with both me and Obama, Obama, some sort of artificial intelligence—anyone or anything that could measure this person’s identity in an unbiased way. 

The second objection concerns Parfit’s discussion of the ‘split-consciousness’ experiment in chapter twelve, which I shall briefly summarize. Parfit notes the real-life examples in certain persons of having “two separate spheres of consciousness,” one controlled by the right hemisphere of the brain, one controlled by the left (245). Parfit stretches this case to form a thought experiment in which a person is capable of splitting into two distinct streams at will, with each stream being totally unaware of the other, and then reuniting these streams so that the resulting person can recall experiences from both streams. He notes that the Non-Reductionist would have to choose and argue for one particular outcome out of a set of seemingly convoluted options: there were two people hidden all along, there are three people, two of which live for only ten minutes, etc. The Reductionist, on the other hand, has the advantage of not having to “choose” an option. They must only describe the facts of the case, and point to each option as being merely a different description of the same set of facts. Nevertheless, there is not an inherent advantage in having a clearer or easier path to the solution. If the Non-Reductionist is able to pick a theory out of many, and provide a painstaking argument as to the truth of that theory, then they are on footing equal to that of the Reductionist. I want to emphasize, however, that this objection is merely for the way that Parfit constructs his argument, and does not apply to his result of the Reductionism. It would be consequential if Parfit provided little to no evidence for Reductionism besides the relative convenience of it, but this is not the argument in which Parfit puts his weight. As I have explained, Parfit provides strong rebuttals to every Non-Reductionist view, and thus this objection does not change my outlook on the validity of his theory itself.

Thus, I believe that Parfit effectively furthers his Reductionist thesis by making an argument that “there is no evidence in favour of [Non-Reductionism], and much evidence against it” (243).