I. Introduction

I am the same person as when I was ten years old—this is common intuition [1]. In another sense, as I have a different body and psychology, it seems that I cannot be the exact same person as I was when I was ten years old [2].  In this paper I ask how exactly we were the same people when we were ten. This question is equivalent to asking what the persistence conditions for “an entity of our kind” to remain the same over time: call these diachronic persistence conditions [3]. In asking this question, we ask two separate questions

(1) What should “entities of our kind” refer to?

(2) What are the persistence conditions for these entities?

First, I review the literature regarding the two most popular theories of personal ontology, or the study of what we are. In doing so, I will then introduce the brain criterion based on egoistic concern—the special concern about one’s future that arises from anticipation of continued existence. Providing justification in ontological coherence and ethical plausibility, I submit that the brain criterion is the superior ontology. In the second half of this paper, I will be responding to objections to the brain criterion, specifically regarding persistence by considering commonplace thought experiments. As a synthesis from these objections, indeterminacy thesis and multiple occupancy will be defended as part of the paper’s overall argument.

 

II. What Should "Entities of Our Kind" Refer To?

To determine the persistence conditions for some entity, one must start by specifying exactly what that entity is. I say this because whether entities can survive events is subsequent to what the entity is. For example, a square cannot survive being stretched in one axis whereas a rectangle can, because a square is defined must have four sides of equal length. This is to say, the persistence conditions of a square are informed by its ontology. In the same way, the persistence conditions for a person depend on personal ontology. Despite this, much of the contemporary literature on personal identity seems to treat personal ontology as a secondary question. Instead, the focus has been placed upon a gamut of thought experiments [4]. To this, in "What Are We”, Eric Olson blames “the unserious air of many discussions of personal identity” on this lack of focus on personal ontology [5]. 

There are two dominant answers to the persistence question: neo-Lockeanism and Animalism [6]. Neo-Lockeanism is the view that we are creatures with certain psychological traits essentially [7] Animalism says we are human animals essentially; we are identical with our human bodies. Each view is associated with at least one ontology. Neo-Lockean theory most popularly claims that we are constituted by our animal bodies or that we are temporal parts of our animal body. Most neo-Lockeans would not say we survive a permanently vegetative state, despite our animal bodies being able to survive, and to a neo-Lockean, this is an example of how we differ from our animal bodies. Animalists simply state that we are our human bodies; if our bodies were to enter a permanently vegetative state, we would enter that state as well. 

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Neo-Lockean views lack the “metaphysical leg up” that ontologically focused views like animalism have [8]. Contrastingly, animalism often ignores normative concerns [9]. Despite this trade off between normativity and ontology, when we refer to others, we do not do so as moral agents and organisms separately, but as one unified forensic unit: otherwise this would be absurd [10]. Parfit also touches on similar ideas when he sets out two requirements that a theory of personal identity must satisfy [11] :

(1) Whether a future person will be me must depend only on our intrinsic properties. It cannot depend on what happens to other people.

(2) Since personal identity is of great importance, whether a future person is me cannot depend on a trivial fact.

Parfit also seems to segregate requirements on the basis of (1) a strict ontological concern and (2), an ethical concern. What idea we have converged upon seems to be this: an account of personal identity (and thus, ontology) must be, as Olson puts it, both “ontologically coherent as well as ethically plausible” [12]

In the following sections I will forward an account of personal ontology which describes entities of our sort as a human animal under two constraints:

(1) That the entity has certain traits that warrant continuous egoistic concern 

(2) That the entity is the spatial part of a human animal in which (1) necessarily and sufficiently obtains (i.e part of the brain)

Evaluating the contemporary two main theories of personal identity, along with our own as delineated above, I will recommend this account of personal ontology to have best fulfilled the need to be ethically plausible and ontologically coherent. 

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A. Ethical Plausibility

I propose that to say an account of personal ontology is ethically plausible is to say that it reasonably includes all entities that our normative and ethical concerns refer to when we use the everyday pronouns of ‘I’, ‘you’ or ‘we’, and reasonably excludes all entities that do not. Further, if it is said that you and I are ‘entities of our sort’, we can expect that normative claims which apply to you would also apply to me. 

We start with cases where the brain-criterion provides for a necessary liberalization of inclusivity. First, in contrast to animalism, brain views allow for entities of our kind to survive after brain transplantation. The common intuition that follows is that after the brain is transplanted into another person’s head, the entity follows with it [13]. Animalists must maintain that because the human organism is left behind and dies during the process of transplantation, we must die along with it. This is a strikingly unintuitive statement to endorse. On the other hand, because physical continuity of the brain is maintained and is presumably enough to warrant egotist function once transplanted, the brain criterion successfully represents the intuition that we would survive after transplantation. 

Second, in contrast to neo-Lockean continuity, we are able to account for individuals who lack meaningful psychological-connectedness, yet we must still include them as one of us. Jeff McMahan illustrates that in cases of Alzheimer’s, neo-Lockeans seem to imply that the individual ceases to exist and becomes a sort of “post-person.” [14] This is because insofar as a case of Alzheimer’s progresses so that almost no function of memory remains, neo-Lockean theories suggest that this is a case where the psychological connection is broken, and individuals cannot survive [15]. When considering the ethical plausibility of this, we suspect that instead, we would still consider the Alzheimer’s patient an entity of our sort, and that our normative claims and duties would still apply to them. Concretely, if the Alzheimer’s patient was my mother, just because she lacks psychological connection does not mean she is not “one of us”. In contrast to the brain criterion based on egoistic concern, as long as there is brain continuity supporting the function of egoistic concern, we may say the entity is one of us. For example an Alziemer’s patient still has egoistic concerns because she considers actions not as disconnected events that will only impact an entity similar but identical to herself (and that she exists only for a brief moment before her psychological connection deteriorates) but that her actions will influence her future. Note the difference here is that egoistic concerns need not be a degree of psychological unity in which even a semblance of qualitative identity is sufficiently obtained [16]. We see this when we consider two statements, that normatively we take as non-mutually exclusive:

1. My mother has not been the same person recently and does not remember me.

2. That woman who does not remember me is my mother.

The first statement expresses our intuition that people can change drastically, even to the point where psychological unity according to a neo-Lockean would be lost. The second statement, however, speaks to our intuition that numerical identity can survive far more liberally, when considering strict psychological relation, than a neo-Lockean claims. Indeed, the exclusionary policy of the brain criterion is the most ethically plausible. 

There are also cases where the brain criterion restricts cases necessarily. The main difference between neo-Lockean views and general brain-views is that the brain-criteria explicitly requires a physical contingency. This is to say, a sufficient part of the brain which is necessarily part of a living being must remain continuously. From this, we can locate cases that should be excluded, such as that where an entity can survive a total loss of body. Neo-Lockeanism generally endorse teletransportation as an event in which we can survive. Despite whatever prima facie intuitions we may have, consider you were being transported, but a replica of an entity at some point B was created while you were still alive. Thus, you are not the entity at point B, and for there to be one entity, you must have been destroyed at point A [17]. Moreover, Animalist views consider a fetus and an individual in a permanently vegetative state to also fall under our general normative conventions, as they are simply stages in the human animal’s development. Extreme views notwithstanding, common ethical norms tell us otherwise: we have intuitions that it is permissible to kill an early stage fetus, for example, where we do not for toddlers [18]. This is a difference that Animalists do not account for. This difference is crucial in ontology: we say it is permissible to kill a week-old fetus because at that point it more closely resembles an unconscious collection of cells than the entity we normatively refer to we say “you” or “I”. The ethical norms about “entities of our sort” that Animalism implies do not match our commonly held ones; thus, we cannot say an Animalist conception of persons is ethically plausible. Considering both comparisons to neo-Lockean and Animalists views, it is only by using the definition of brains with egoistic concern that we can arrive at a superior ontology.

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B. Ontological Coherence

To say that a theory is ontologically preferable to any other is to say that it answers key issues concerning personal ontology at as little cost in way of unfortunate implications that one must accept. In evaluating the seven main personal ontologies, Olson generally considers one issue as most important: the thinking animal problem.

The thinking animal problem is the following argument:

(P1) Presently sitting in your chair is a human animal.

(P2) The human animal sitting in your chair is thinking.

(P3) You are the thinking being sitting in your chair.

(C) Therefore, the human animal sitting in your chair is you.

The crux of the thinking animal argument is that insofar as rejection of P1-P3 requires us to accrue the cost of, as Olson puts it, an “impenetrable” [19] ontology, we must conclude that we are human-animals. By proving that we can escape this conclusion, we can prove the brain criterion is preferable to neo-Lockean theories which fall victim to the argument. 

I will now defend the second constraint using a generalization of the thinking animal problem: 

 

(P1) There is a spatial part of a human currently located where you are.

(P2) The spatial part currently located where you are is thinking.

(P3) You are the thinking being located in your chair.

(C) Therefore, the spatial part of a human where you are is you.

 

Note that this argument is analogous to the thinking-animal-argument so that we may adopt its conclusion. The difference is that the conclusion is such that we must be spatial-parts of the brain, some that we can be essentially reduced to a part of the brain. Note that any spatial part of an animal with greater inclusivity than what is necessary for a thinking part to think will fall trap to this argument, given the animal with greater inclusivity contained a non-essential part was incorrectly considered essential. This is to say that the argument implies “you” are identical to infinite smaller spatial-parts unless “you” refers to the smallest possible spatial-part of an animal which thinks. This smallest possible part is the only part that is not affected by the argument since, any less inclusive and the animal loses the property of thinking, so P2 falls, making the argument inapplicable. Dualist theories notwithstanding, this smallest spatial-part of a thinking animal must refer to some part of the brain, and so we have proven our second constraint on animalism.

 

To conclude, this makes animalism and the brain criterion at least equal in ontological coherence, which combined with a brain criterion advantage in ethical plausibility allows us to recommend over the other theories. 

 

III. What Should “Entities of Our Kind” Refer To?

Given we have answered first question of this paper, there are two main objections specifically to how the brian criteria persist : 

1. That the brain-criterion is unnecessary and insufficient 

2. That the brain-criterion is necessary but insufficient

In this section, I will deal with both of these objections, and in doing so maintain that Brain Criterion is both sufficient and necessary.

 

A. Unnecessary and Insufficient

Parfit’s “combined spectrum” shows that any account based on “empirical fact” will have cases of indeterminate identity [20][21]. This is because any empirical criteria, such as psychological or physical continuity operate on a spectrum of absolute similarity to no similarity. If that is true, then there are cases on that spectrum where it seems that the connectedness between two entities is indeterminate to whether they are the same entity. Consider our brain criterion: existence is guaranteed in the case of 100% paradigmatic brain-function, and guaranteed false in the case of no function. However, there are cases in-between whereby it is indeterminate that consciousness is present: it is hard to see an argument for consciousness given 2% function, but what of 12% or 24%? There are two possible conclusions we can make of this:

1. That indeterminacy cannot exist, and so some “further-fact” must be considered [22].

2. Or, we must allow for cases where indeterminacy arises. 

If we accept the consideration of a “further fact” in indeterminate cases, this implies the same further fact could determine the answer to the persistence question in any other case. What rejecting cases of indeterminacy entails is accepting a “further fact” ontology, such as immaterialism. I will comment that even if we cannot assuage the issue of indeterminacy, it may be preferable than to contend with the burden of proving dualism and other theories associated with immaterialism.      

In our paper’s defense of the brain criterion, indeterminacy would not mitigate claims that it should be recommended over animalism or neo-Lockean views –– both rely on empirical criteria. Yet as a foundational argument, I will contribute a defense of indeterminacy. Note that indeterminacy in things other than the existence of people is uncontroversial and common, for example, given a tallness spectrum where 7ft is guaranteed to be tall and 4ft guaranteed not to, there must be indeterminate cases of tallness in between. However, indeterminacy seems to be unreasonable when it comes to issues of persistence. Bernard Williams provides a thought experiment where one has to imagine that entity X, which is indeterminately identical to me, will be tortured tomorrow if indeterminacy is true. Does it follow that the feeling of great pain will be indeterminately felt by me? Noonan points out that this merely illustrates the “very great unnaturalness of this way of thinking” that is present in these cases, not that the cases themselves are unnatural [23]. Note then because it must be accepted that indeterminacy exists in other contexts, we must simply prove that indeterminacy in persistence is also acceptable. Many metaphysical arguments have been offered to this end; I will propose a practical one: to assume that issues of persistence must have determinate answers where other reducible substances do not is to assume there is something irreducible about selves. This begs the question on whether there selves are reducible in the first place, and thus we have no reason to reject indeterminacy in persistence.  

 

B. Necessary but Insufficient

A hemispherectomy is a procedure where one half of the cerebrum is removed. Despite having half their brain removed, patients that undergo hemispherectomy expect to survive the operation. Our intuitions indicate they have good reason to make this assumption: we treat postoperative entities as the same people, and indeed, as their brain hemisphere adapts to serving the role of two, often cognitive function is returned as well [24]. In other words, if I receive a hemispherectomy, theoretically there seems to be enough brain continuity so that the resulting person is me. However, the reality is that whether you survive is indeterminate. If the brain criterion is necessary and sufficient in the light of indeterminacy, we must prove that for all conclusions that could be made, but are unobservable, there is still ethical plausibility and ontological coherence. If I end up in surgery or even if it is indeterminate that surgery kills me, the discussion ends here. Things are more complicated if you survive. Given that we can accept the transplantation of the cerebrum while maintaining continuity, it can be said that transplanting half of a brain also continues the entity. However, in a case there are two candidates, both sharing physical continuity of a human animal in which egoistic concern is retained, it seems that the brain criterion is insufficient to prove persistence which entity persisted. There are three interpretations to this case [25].

(a) I do not survive. 

(b) I survives as either candidate-A or candidate-B  

(c) I survive as both

First consider (a), commonly referred to as the “non-branching view” [25]. Notice that I would survive if one half was destroyed, but in the case of both being preserved, I die. This seems immediately strange: How is double success considered a failure? Given the symmetry of the problem, (b) is incoherent as well, considering facts about both candidates are equal. We must then turn to (c), the only case in which brain continuity is sufficient. To avoid implicating that candidates A and B are the same, I forward that candidates A and candidates B are distinct entities that were once spatially coincident within the original, or multiple occupancy[26]. Given that either (a) or (b) are both untenable, the implication is that if one wants to reject “further fact” accounts, multiple occupancy must be endorsed [27]. Two things must be proven for us to adopt this: 

1. It does not affect the ethical plausibility of the theory

2. It does not affect the ontological coherence of the theory

If these two requirements are met, we will have a theory that sans fission preserves our original account and considering fission, will have the most realistic account in approaching it. 

 

C. Ethical Plausibility

Let us first consider ethical plausibility. Firstly, given that pre-fission agents are unified, there is no change from our original theory. Moreover, the fact that an entity undergoes fission later down the road would not retrospectively change the normative considerations we give to the pre-fission entity. Does our criterion provide the most ethically plausible account of entities post-fission? Consider your spouse undergoing the fission operation. We may measure the ethical plausibility by considering how each post-fission theory affects your duties to your spouse. If (a) is true and whether your spouse lives if there is no second transplant but dies if fission takes place, if there is a gap in time between when half your spouse’s cerebrum is removed, and when it is transplanted into a host body, do you have marital duties toward your spouse during that gap where the second transplantation did not occur, but that these duties disappear the second the operation is successful? That our duty to people should be as arbitrary as the existence of another person seems strange.

Strange conclusions are also reached when (b) is considered – why would you have the martial duty to love and be faithful to one of your spouse candidates and no qualms abandoning the other? If we maintain that it is immoral to abandon our duties on arbitrary facts, so (b) is also not a viable conclusion. (c) is the only scenario which is compatible with our conventional moral ideas. Yet it is also true that our commitments to our spouse are not exactly the same: they are, in a way, inflated. I must now commit to caring and providing for two bodies instead of one, being affectionate and loving to two bodies instead of one. However, while our duties to our spouse now split between two people is a commitment that is inflated, it is inflated based upon ideas we already accept as a posteriori moral—compare this to duties suddenly appearing and disappearing based on arbitrary facts. Consider a situation where your spouse is experiencing a mental health crisis. As a result of going through this situation, it is your duty to be more sensitive around them, spend more time and energy tending to their care etc—in other words, your commitment has been inflated. However, we accept this as a natural part of our duty because we hold a duty to a loved one in a difficult situation, despite having our commitments inflated. In the same way, our duties to a spouse do not change on account of this strange situation happening to them; duty is not situational. This is the principle that only multiple occupancy can reach, given all other seniors change duty based on the arbitrary details.  Thus, being that it would be most accurate to say you have a duty to both, multiple occupancy is the most ethically plausible interpretation. 

What may be suspect is the impact on personal ontology. Multiple occupancy does not affect the arguments for ontological coherence we have laid out before if committing to its thesis does not require committing to additional burdens. We may prove this by considering each ontological assumption that our original theory could operate under and prove how multiple occupancy is compatible with the original metaphysical assumptions. Thus, if each assumption that is compatible with our original theory is also compatible with multiple occupancy, we can say that the original theory’s ontological coherence was not affected. 

There were two metaphysical assumptions we could make in which our account of personal ontology retained: that four dimensionalism was true, and that four dimensionalism was false. Under the assumption of four dimensionalism, the two separate entities after the fission operation are just temporal parts of the original entity that simply stand spatially distinct. This stands unproblematic among thinkers who accept four dimensionalism [29]. However, we require an account with the original metaphysical assumptions free of four-dimensionalism. Note that multiple occupancy seems absurd because common sense counting would suggest that 1 person becomes 2 people. We may resolve this by suggesting that it is possible to count 2 people before the fission operation as well. Before the operation, a singular entity is counted because counting was done by “spatio-temporal coincidence”  rather than counting by identity [30]. This itself is also acceptable: if we can say that we can be identical to some entity that is not spatio-temporal coincident with us, as we do in everyday language when we say “my past self" or my “future self,” we are saying that identity and spatio-temporal coincidence represent two different things. 

Thus, given multiple occupancy can be integrated within either framework without necessitating a revision of our fundamental metaphysical assumptions, we can say it has not impacted the ontological coherence of the original theory. When we consider that both the ethical plausibility and ontological coherence has been preserved, while multiple occupancy is highly counter-intuitive, it must be accepted.

 

IV. Conclusion

In answering how we are the same person as when we were ten, I have considered two important questions in personal identity: what “we” are, which is a question of personal ontology, and how "we" persist. I have evaluated the merits of the brain criterion based on egoistic concern against both Animalism and Neo-Lockeanism, arguing that it is this paper’s variant of the brain criterion which best encompasses both ethical and ontological considerations when answering what “we” are. Then, I have argued that the persistence criteria which follow from the proposition that we are brains is necessary and sufficient, on the basis that one rejects a further fact ontology. What follows is the question of multiple occupancy, which seems quite counter intuitive when considering prima facie. However, multiple occupancy as I have proven, remains the only solution to deal with the cases of fission satisfyingly. 

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Footnotes

[1]  Francisco Muñoz et al., “Spatio-Temporal Brain Dynamics of Self-Identity: An EEG Source Analysis of the Current and Past Self,” Brain Structure and Function 227, no. 6 (2022): 2167–79, https://doi.org/10.1007/s00429-022-02515-9.

[2] There is quite a lot of unintended semantic conflation in discussions of personal identity. Even the label by which we refer to it almost assumes person essentialism. When I use the term “person”, I refer to the colloquial usage, not the neo-Lockean kind, unless explicitly stated. Moreover, in usages where “person” may be easily conflated, I have substituted the more neutral “self” or “selves”. This is why I refer to the persistence questions with the set of <S1, T1> rather than <P1, T1>. Most clearly neutral is the term “entities of our sort”, which I have tried to use most often, but selves serves the same purpose with less of a word count cost.

[3] Harold W. Noonan, Personal Identity (London: Routledge, 2019), 85-86

[4] David Shoemaker and Kevin Tobia, “Personal Identity,” The Oxford Handbook of Moral Psychology, 2022, 542–63, https://doi.org/10.1093/oxfordhb/9780198871712.013.28, 9. 

[5] Eric Todd Olson, What Are We?: A Study in Personal Ontology (Oxford: Oxford University Press, 2007), v.

[6] David Shoemaker and Kevin Tobia, “Personal Identity”

[7] Eric T. Olson, “Personal Identity,” Stanford Encyclopedia of Philosophy, https://plato.stanford.edu/entries/identity-personal 

[8] David Shoemaker and Kevin Tobia, “Personal Identity,”

[9] Ibid

[10] Marya Schechtman, Staying Alive: Personal Identity, Practical Concerns, and the Unity of a Life (Oxford: Oxford University Press, 2017), 49-56

[11] Derek Parfit, Reasons and Persons (Oxford: Clarendon Press, 1987), 267.

[12] Gendler, Tamar Szabo and Olson, Eric T, The Human Animal. (Philosophical Review, 1999)

[13] Nichols, Shaun, and Michael Bruno. “Intuitions about Personal Identity: An Empirical Study.” Philosophical Psychology 23, no. 3 (2010): 293–312. doi:10.1080/09515089.2010.490939.

[14] Jeff McMahan, The Ethics of Killing: Problems at the Margins of Life (New York: Oxford University Press, 2003), 47 

[15] Lukas J Meier. “Memories without Survival: Personal Identity and the Ascending Reticular Activating System” The Journal of Medicine and Philosophy, no.5 (2023):  478–491. https://doi.org/10.1093/jmp/jhad028 

[16] Two things are qualitatively identical if they share all their properties, and numerically identical if they are not two, but one.

[17] McMahan, The Ethics of Killing: Problems at the Margins of Life, 57

[18] Steinbock, Bonnie. “Abortion.” The Hastings Center, February 22, 2024. https://www.thehastingscenter.org/briefingbook/abortion/. 

[19] Olson, What Are We?: A Study in Personal Ontology, 214.

[20] Noonan, Personal Identity, 18

[21] Parfit, Reasons and Persons, 236

[22] Parfit, Reasons and Persons, 210

[23] Noonan, Personal Identity, 191

[24] Noonan, Personal Identity, 5

[25] Parfit, Reasons and Persons, 248 - 253

[26] Noonan, Personal Identity, 18

[28] Noonan, Personal Identity, 140

[29] Olson, “Personal Identity”

[30] Noonan, Personal Identity, 139