In what sense are you the same person today that you were when you were ten?

Few of us are able to recall our childhoods in vivid detail, and in fact many would declare themselves to have changed profoundly through the courses of their lives. Yet we consider it evident that we share the same identity as our ten years old selves. But what exactly is it that persists through time and establishes a continuation of our identity? This question is known to philosophers as the persistence question. The most famous formulation of the persistence question is found in the puzzle of the Ship of Theseus, recorded by Plutarch. As the planks of the ship wore down, they were gradually replaced to the point that the ship contained none of its original parts, leading philosophers to debate whether it remained the same ship. Whilst considering personal identity, we encounter much of the same situation. The average human replaces 330 billion cells daily, such that every eighty to a hundred days, we effectively replace our entire body with new cells [1]. Thus, if we were to tie our identity to our current material constitution, we would arrive at the perplexing conclusion that we become completely new persons every three months. There must be, then, some essential parts of a person that persists through time and establishes a continuity of identity. This view is known as essentialism, and poses two theories in response to the persistence question, of psychological continuity and of physical continuity. In this essay, I will evaluate arguments both for and against psychological continuity and physical continuity, then illustrate that both cannot fully solve the persistence question. Instead, I will advocate for the four-dimensionalist doctrine of temporal parts as a solution to the continuity of identity.

“Identity” in this discussion refers to “numerical identity” as opposed to “qualitative identity”. To be numerically identical is to remain the same object; to be qualitatively identical is to share all the properties of an object [2]. In order to answer how identity persists between a person and their ten years old self, I will be examining under what conditions the identity of the ten years old remain numerically the same as that of the present-day person. That is to say, I am concerned not with how a person differ in qualities with their ten years old self, but whether they even are the same person. To ask whether they are qualitatively the same presupposes that they are numerically identical, and is therefore beyond the scope of this essay. Taking this into account, let us consider the two essentialist theories of psychological continuity and physical continuity.

Psychological Continuity

The most popular explanation of the persistence of identity is through psychological continuity. On this view, to be the same as a being from the past is to inherit their mental features. Locke argued that a person Y at t2 is identical to a person X at t1 so long as Y's consciousness “can be extended backwards” to X. This is typically taken to mean that Y remembers X’s experiences.

This view has a certain prima facie intuitive appeal, but faces three serious objections. I. Human memory are faulty and by no means objective. Following logically from this view, a person remembering themself to be someone else would in fact be that person, and a person remembering to have performed an action they had not would in fact be identical to the person who had. This objection is particularly significant considering that scientists have found that human memory may become unreliable after just a few seconds [3]. II.

Memory continuity is not transitive like personal identity. Suppose a person A existing at t1 remembers performing some actions at t2 as person B. Person B likewise remembers performing some actions at t3 as person C. Suppose also that the memory of acting at t3 as person C has faded from the consciousness of person A. According to the memory criterion, person A is identical to person B, just as person B is identical to person C, but person A is not identical to person C. This leads to the impossible conclusion that A=B, B=C, but A≠C, violating the transitive property of identity. III. Statements based on the memory criterion appear uninformative. For a person to remember performing an action is to remember themself acting. Thus to claim that person A is identical as person B, person A not only has to remember an experience as person B, they have to remembering themself acting in that experience as person B. This reasoning is uninformative as the conclusion that person A is person B is already assumed as a premise in the argument.

Updated theories of psychological continuity respond to the above objections by redefining personal identity in terms of overlapping chains of psychological connections that are appropriately caused [4]. In response to objection I, psychological theorists specified that psychological connections like memories must be based on appropriate causes. Personal identity are to be connected by genuine memories, which are based on true perceptions. Locke wrote that memory is a power of the mind "to revive Perceptions, which it has once had, with this additional perception annexed to them, that it has had them before” [5]. Faulty memories or delusions are not based on perceptions that the mind had, so they are not genuine. In response to objection II, psychological theorists modified the memory criterion from direct to indirect psychological connections.

Overlapping chains of psychological continuity could indirectly connect person A with person C via person B. Hence this allows for the view to say if A=B and B=C, then A=C. Indirect psychological connections also pertain to not just memory but also other mental features like beliefs and intentions, to further resolve the problem of fading memories. In response to objection III, psychological theorists defined the term “quasi- memories” to refer to appropriately caused memories that do not presuppose identity. In theory then, a person could have quasi-memories of experiences they did not have [6]. A psychological continuity theory in terms of quasi-memories (as well as quasi-intentions, quasi-beliefs, etc.) therefore avoids circularity.

Physical Continuity

Opponents of the psychological criterion argue that our biology rather than psychological states are the essence of our identity. The continuity, then, of a person’s physiological characteristics and processes determines the persistence of their identity.

This view avoids certain objections made against the psychological view. With psychological continuity, identity could not persist for persons who lack psychological capacities, like humans in vegetative states and fetuses. The physical criterion, however, could easily described the continuation of identity through biological processes without appealing to psychological states. Psychological theories also have difficulties addressing the existence of human animals. As persons are said to persist by virtue of psychological connections, persons are not their human bodies at all, leading to three awkward implications. First, for every human being, there is a person and a human animal distinct from it. Second, the human animal is indistinguishable from the person and releases neurotransmitters to give rise to intelligent thoughts. And third, we could not tell whether we ourselves are the human animal or the person. This is known as the “too- many-thinkers” objection. Physical continuity theories argue that due to such absurd implications, our persistence criterion could not be psychological. Rather, we are simply the human animals.

By considering the perspective of physical continuity, we can see the inadequacies of a psychological theory, with which it is mutually exclusive. Physical continuity, nonetheless, lacks intuitive appeal. Suppose person X’s cerebellum was transferred into person Y’s cerebellum-less body. The resulting person would retain all of person X’s memories, character traits, beliefs, and other psychological characteristics. The physical continuity theorist, nevertheless, is forced to contend that this resulting person is actually person Y, deluded into thinking that they are person X, as their physical body persists, just with a different life-sustaining organ. In addition, dicephalic twins appear to constitute as a counterexample to a physical criterion of identity [7].

Dicephalic twins are decidedly two individuals, and yet they share one body. Thus, it is inaccurate to claim that our identity consists of only our animal properties.

Four-Dimensionalism

Consider the following thought experiment. Suppose that both of X’s hemispheres are transferred, each onto a different body, call them Y1 and Y2. According to essentialist views, both Y1 and Y2 are continuous with X, whether by means of psychological states or a physical brain. However, we reach the impossible conclusion that X is numerically identical with Y1 and Y2, two evidently separate persons existing at the same time. This is known as the fission problem. In the paradox of the Ship of Theseus, suppose also that a custodian gathered up the original, decaying planks and used them to reconstruct a second ship. We are thus confronted with two ships, one replaced with new planks and one restored with old planks, both of which could claim to be the same as the original Ship of Theseus. No criteria of identity, based only on either psychological or physical continuity, could resolve this contradiction.

Four-dimensionalism, though, has a ready response. Four-dimensionalism supports the thesis that the world is composed of temporal as well as spatial parts [8]. Just as they exist in three dimensions by having distinct spacial parts, material objects exist through time by having distinct temporal parts. Objects, then, can be partly identical, if they occupy the same points in space for some period of time. Therefore, we can understand that the persons Y1 and Y2 had overlapped as X and are now separate, but they were always two temporally-extended, distinct (though partially identical) persons. Persons, in this view, are thought of as the sums of all their spatial and temporal parts.

Essentialism makes the mistake of reducing objects to what is considered their intrinsic properties. These intrinsic properties are certainly essential, but also temporary. An object’s shape, for example, is an intrinsic property of its current temporal slice, but could easily change over time. A person is not forever intrinsically their psychological states or their bodies, as shown in the cases of fetuses and people in vegetative states, so the persistence of their identity could not be based on simply psychological or physical criteria. Theories of psychological and physical continuity still have their merits, but only between temporal slices in which they are intrinsic.

To return to the paradox of the Ship of Theseus, the four-dimensionalist would say this. The restored ship and the replaced ship are two distinct identities that shared the same space for the period of time before the first plank was removed. Then in what sense is someone the same person today as when they were ten? A person has their ten year old self as one of their temporal slices, and even if they had lost all of their original parts, they are still linked in personal identity.

Endnotes

[1] Fischetti, M. (2021, April 1). Our bodies replace billions of cells every day. Scientific American. https:// www.scientificamerican.com/article/our-bodies-replace-billions-of-cells-every-day/

[2] Cleave, Matthew Van. “Personal Identity.” Introduction to Philosophy, pressbooks.online.ucf.edu/ introductiontophilosophy/chapter/personal-identity/ #:~:text=Numerical%20identity%20is%20a%20term,the%20same%20properties%20or%20qualities. Accessed 8 July 2023.

[3] Otten, M., Seth, A. K., & Pinto, Y. (n.d.). Seeing Ɔ, remembering C: Illusions in short-term memory.

PLOS ONE. https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0283257 [4]Personal identity. Personal identity - Routledge Encyclopedia of Philosophy. (n.d.). https://

www.rep.routledge.com/articles/thematic/personal-identity/v-2/sections/psychological-continuity-theories

[5] University of Pennsylvania. (n.d.-d). https://www.sas.upenn.edu/~cavitch/pdf-library/ Locke_Essay_Bk2_Ch10.pdf

[6] A defence of quasi-memory - JSTOR. (n.d.-a). https://www.jstor.org/stable/4127438

[7] Animalism and the varieties of conjoined twinning. (n.d.-b). http://www.philosophy.rutgers.edu/ joomlatools-files/docman-files/ Animalism%20&%20the%20Varieties%20of%20Conjoined%20Twinning.pdf

[8] Four dimensionalism - Ted Sider. (n.d.-c). https://tedsider.org/papers/4d.pdf

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