In what sense are you the same person today that you were when you were ten?
 
Introduction
Personal identity has sparked philosophical debate for centuries. Specifically, this question concerns whether we can be considered the same person from one time to another. The answer to this question consists of two layers: what constitutes continuity of a person and our mental perception of this continuity.1 This essay will focus on the second layer.
Regardless of whether the ten-year-old version of myself is metaphysically the same person, it is hard to deny the apparent continuity and identity that I currently experience with the ten-year-old girl I once was. This continuity can be attributed to the powerful role of autobiographical memory.2 Learning to ride a bicycle for the first time or celebrating birthdays, helps me build my self-awareness and establish psychological connections with my past self, thus forming a consistent personal identity.
This essay investigates personal identity from two aspects: bodily and psychological continuity. Bodily continuity refers to the physical aspect of our identity. By contrast, psychological continuity refers to the idea that personal identity is maintained over time through a series of psychological connections, such as memory, beliefs, desires, personality traits, and emotional experiences. Although both are significant in personal identity, this essay argues that bodily continuity is a necessary but insufficient condition for personal identity. Bodily continuity should always combine with psychological continuity to maintain an individual's identity.
First, this essay will explain Derek Parfit's thought experiment and Andrew Ward's criticism. Then it will discuss the limits of memory and narrative theory and the criterion of psychological continuity. In this regard, Marc Slors's theory of the diachronic self is compelling despite its challenges.
 
Bodily Continuity
In contemporary philosophy, Derek Parfit, a prominent advocate for psychological continuity, has introduced a well-known thought experiment called teletransportation.3 In this experiment, he invites us to imagine that our brain can be divided into two independent parts and transplanted into two different bodies. When each body wakes up, they will have memories of only one half of our original brain, but both will believe that they are the "real" us.4
According to Parfit, this experiment demonstrates that our sense of identity is closely related to the continuity of our mental states. In other words, if both versions of ourselves have the same psychological experiences after waking up, then both individuals can be considered a continuation of ourselves.5
Parfit's psychological reductionism, despite its influential impact, has faced criticism. One main critique is that Parfit underestimates the importance of bodily continuity or numerical identity. Numerical identity refers to something counted as one entity, having a numerical identity relationship with itself, whereas qualitative identity means two objects are completely identical in all properties. Two things that are qualitatively identical do not possess numerical identity because they are separate entities. On this basis, in "The Survival of Persons: A Reply to Parfit's Psychological Reductionism", Andrew Ward argues for a crucial distinction between a person's identity (personhood) and the identity of their lives.6While personhood stresses the psychological components that define an individual, the identity of lives encompasses the sensory experiences, events, and narratives that shape a person's existence. The latter aspect holds significant weight in discussions of identity. According to Ward, our bodily experiences, such as the sensory perceptions we encounter, directly contribute to our psychological aspects, such as intentions, desires, and beliefs.7 In his perspective, if there were another version of myself with a different body but possessing my memories, that person's body would not have physically undergone the same experiences as I did. Rather, they would only have the memories of those experiences.8
I agree with Ward's argument that Parfit fails to adequately consider the importance of numerical identity for personal identity, and it is questionable to suggest that one person could have two distinct lives while preserving identity. To my mind, there is a substantial difference between direct experiences and the memory of those experiences. The human body serves as a complex repository of experiences, acting as a carrier and container that encapsulates the encounters and events an individual undergoes throughout their existence. The actions and sensations we undertake and perceive imprint upon our bodies, which develops our mental capacities and accounts for our consistent self-perception.
While memories and psychological understandings of the body may persist, they do not establish genuine continuity. The uninterrupted existence of the body is a fundamental prerequisite, as any interruption or replacement of the corporeal form gives rise to profound difficulties. For instance, a person suffering from a severe psychological illness experiences changes to their brain, indicating a bodily alteration. This individual possesses memories of how they developed the illness. Imagine transferring this person's memories to a replica that retains all the inexplicable feelings of low mood and anxiety with a completely healthy brain. In such a scenario, if the replica were to process their memories, how would they contemplate their current situation and reconcile the contradiction between their brain and their memories? The replica might experience a sense of discord, perceiving their brain as implanted with someone else's memories.
Many psychological changes stem from alterations in our bodies and sensory perceptions, therefore, disrupting the continuity of these experiences and our sense of identity. In this sense, preserving bodily continuity is essential for maintaining personal identity.
 
Memory, Narrative, and the Quest for Continuity
Although numerical identity is necessary for maintaining personal identity over time, it is incomplete in capturing the full complexity of our identities. As time passes, our bodies change, we grow, develop, and our body structure and cells are constantly changing. Yet despite these changes, we still maintain a sense of identity, which may stem from our psychological continuity.
One of the most popular theories of psychological continuity can be traced back to John Locke's memory theory. Memories allow us to connect our current experiences with our past selves and create a consistent understanding and consciousness of ourselves. Locke famously argued that personal identity is based on memories.9 According to Locke, to make you be you over time, there must be a continuity of memory fragments of past experiences.10 Locke argues that one's identity is lost if memories are lost.11
Locke's thought has profoundly influenced contemporary philosophers, including Derek Parfit and Marya Schechtman. While Parfit is a radical supporter of psychological continuity, Schechtman believes it is insufficient to unify an individual's life experiences throughout their lifespan; at most, it can only achieve similarity.12 In what sense, then, can we find support for our identities? To better support the existence of personal identity, Schechtman proposed a new theory called the narrative theory.13 According to this theory, individuals must have a consistent and unified understanding of their past to have a personal identity. Schechtman's narrative theory offers a more subjective perspective on personal identity, emphasizing an individual's self-awareness rather than relying solely on memories.14
While memories may become distorted or abruptly disrupted, the narrative theory posits that as long as one possesses a cohesive set of narratives that can be woven into a consistent whole, their personal identity endures.
The narrative theory can be viewed as a refinement of psychological reductionism. However, its inherent subjectivity posits ethical problems. For example, a criminal, intentionally or not, may absolve themselves of guilt by presenting a highly distorted self-narration, thus evading legal repercussions.
Although one's memories and self-narratives seem to be a solid foundation for psychological continuity and, thus, the sense of personal identity, they could be unreliable and even self-deceptive. The sense of personal identity based on completely misleading memories or self-narratives is meaningless and can cause serious confusion. Individuals with false memories or fictional life stories may face an identity crisis as their self-perception clashes with reality. It is also challenging for others to assess the authenticity and continuity of such a person's identity. So it is worth exploring a stricter, more objective criterion for psychological continuity, which could account for the sense of personal identity more reliably.
 
The Diachronic Self as a Criterion for Psychological Continuity
As I have mentioned, psychological continuity faces difficulty establishing clear criteria. In recent years, Marc Slors has put forth a compelling perspective that sheds new light on this issue. Slors offers a metaphysical account of our ongoing mental state, positing that our mental status is essentially diachronic.15
Slors's diachronic self theory suggests that our sense of self-identity is built on a continuous chain of causal relationships between our mental states.16 In other words, if there is a causal relationship between our current and past mental states, we are the same entity as our past selves.
For example, if we hold a certain belief today, and we can trace it back to a seed of that belief that was formed when we were ten years old, then we can say that we have maintained our identity over time. What is essential is not that we remember holding this belief but that there is a causal relationship between our past and present beliefs.
This theory establishes a solid criterion for psychological continuity, and the sense of personal identity based on that is a reliable and meaningful one. However, it raises pertinent questions concerning the metaphysical relation between current and past mental states. When forming current mental states, we must first consider the disruption of external factors. Also, the metaphysical existence of isolated mental states warrants examination, as our mental states can undergo sudden transformations. For instance, cravings,17 intense and sudden desires for specific objects or experiences, can spontaneously arise without a clear causal event. The existence of this spontaneous desire provides a conclusive evidence that our motivation can be influenced by factors beyond diachronic mental states.
In summary, Marc Slors's self theory provides valuable insights into personal identity and psychological continuity. One of its strengths is its emphasis on the plausibility of psychological continuity under an objective criterion. While Slors may be on the right track, it is more reasonable to conceive of loosely connected relations that allow for isolated mental states within the framework of psychological continuity. Within this framework, personal identity is not solely reliant on experiences and memories. Instead, it allows for independent, disconnected, or fragmented mental states. This framework recognizes that personal identity is shaped by a combination of interconnected elements, allowing for a richer understanding of the self.
 
Conclusion
To conclude, I have argued that bodily and psychological continuity contributes to the sense of personal identity in different aspects - bodily continuity is indispensable as many of our mental states are attributed to our sensory experiences. In contrast, psychological continuity is fundamental to our personhood. However, psychological continuity is more complicated, especially considering its criterion. To obtain a more objective criterion for psychological continuity, I introduced Slors's theory. Based on that, I further attempt to expand the psychological continuity framework by acknowledging spontaneous mental states' existence.
In essence, personal identity is a multifaceted construct that goes beyond the mere physicality of our existence. It encompasses the intricate web of psychological connections of memories and life experiences that shape our unique identity and provides a sense of continuity throughout our lives.
Therefore, I believe there is an apparent fact that I am the same person as the ten-year-old.
 
Endnotes
[1] Michael Bamberg, Carolin Demuth and Meike Watzlawik, “Chapter 3: Challenges in Research on Self-Identity” in The Cambridge Handbook of Identity (Cambridge University Press, 2021), 58.
2 Endel Tulving, "Episodic and Semantic Memory," in Organization of Memory, ed. Endel Tulving and Wayne Donaldson (New York: Academic Press, 1972), 381-403.
3 Carsten Korfmacher, “Internet Encyclopedia of Philosophy,” Parfit and the Unimportance of Personal Identity,  accessed May 2023, iep.utm.edu/person-i/#SH4b.
4 Derek Parfit, Reasons and Persons (Oxford: Oxford University Press, 1984), 205-209.
5 Ibid., 207.
6 Andrew Ward, “The Survival of Persons: A Reply to Parfit's Psychological Reductionism,” Argumenta, no.09 (November 2019):115.
7 Ibid., 114.
8 Ibid., 117-18.
9 John Locke, An Essay Concerning Human Understanding (London: Penguin Classics, 1997), Book II, Chapter XXVII, Section 9.
10 Ibid.
11 As Locke said: “The consciousness of past actions or thoughts makes up personal identity, for it is by that consciousness that a man is to himself what he is, and thereby a man is one and the same thing to himself throughout the whole course of his life.” John Locke, An Essay Concerning Human Understanding, ed. Peter H. Nidditch (Oxford: Clarendon Press, 1975), 335.
12 Marya Schechtman, “The Story of My (Second) Life: Virtual Worlds and Narrative Identity,” Philosophy & Public Affairs 33, no.3 (2005): 211-238.
13 Ibid.
14 Marya Schechtman, The Constitution of Selves (Ithaca: Cornell University Press, 1996), 102.
15 Marc Slors, “The Diachronic Mind,” Philosophical Studies Series 86 (2001): 4.
16 Ibid., 2-5.
17 Emily S. Barkley and Roy F. Baumeister, "Spontaneous Craving and Motivated Action," in The Oxford Handbook of Human Motivation, ed. Richard M. Ryan (Oxford: Oxford University Press, 2012), 210.
 
Bibliography
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Barkley, Emily S., and Roy F. Baumeister. "Spontaneous Craving and Motivated Action." In The Oxford Handbook of Human Motivation, edited by Richard M. Ryan, 207-225. Oxford: Oxford University Press, 2012.
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Locke, John. An Essay Concerning Human Understanding. Edited by Peter H. Nidditch. Oxford: Clarendon Press, 1975.
Locke, John. An Essay Concerning Human Understanding. London: Penguin Classics, 1997.
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Schechtman, Marya. "The Story of My (Second) Life: Virtual Worlds and Narrative Identity." Philosophy & Public Affairs 33, no. 3 (2005): 211-238.
Slors, Marc. "The Diachronic Mind." Philosophical Studies Series 86 (2001): 2-5.
Tulving, Endel. "Episodic and Semantic Memory." In Organization of Memory, edited by Endel Tulving and Wayne Donaldson, 381-403. New York: Academic Press, 1972.
Ward, Andrew. "The Survival of Persons: A Reply to Parfit's Psychological Reductionism." Argumenta, no.09 (November 2019):114-18.