Think about a memory from childhood: something you did or something that happened to you.
That was you, right?
But what makes you the same person as the kid you remember being?
This question concerns personal identity, what makes someone one and the same person over time.
Some answers appeal to biological sameness: being, in some sense, the same organism over time.
Other answers appeal to psychological sameness: having, in some sense, the same mind over time. Personal identity is explained in psychological terms: having conscious experiences and memories of these experiences.
This essay explores such psychological approaches.
1. Psychological Theories
We sometimes think, “I’m not the same person I used to be.” Many of our characteristics change; we are not what’s called qualitatively identical over time.
But, literally, each of us is the same person we used to be. Personal identity is a matter of being what’s called numerically identical to who we used to be—being one and the same person, a single individual.
Philosopher John Locke (1632–1704) famously argued that persons are conscious, intelligent beings, capable of rationality and reflection, including self-reflection. See for yourself: you are such a being!
Locke then theorized that personal identity is a matter of consciousness:
“as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person.”
This is sometimes understood as a memory theory of personal identity:
person A is numerically identical to some past person, B, if and only if A remembers experiences had by B.
According to memory theory, you are numerically identical to the kid you remember being because you remember their experiences.
Locke’s theory presumably does not require perfect memory: we forget things we’ve experienced.
Rather, it requires that a continuous series of changing conscious states connects someone to their past and future selves. Today you remember what you did yesterday; yesterday you remembered what you did the day before, etc. Each conscious moment directly connects to those that precede and follow it.
So, Locke’s theory is sometimes alternatively understood as a psychological continuity theory of personal identity:
person A is numerically identical to some past person, B, if and only if A is psychologically continuous with B.
Although you’ve psychologically changed since childhood—you’ve had new experiences, formed memories and forgotten others, gained preferences, abandoned interests—you’re still the same person. These changes contribute to the continuous flow of consciousness, connecting your childhood self to you now. According to psychological continuity theory, that’s why you are the kid you remember being.
Psychological continuity theory reflects an important philosophical (and mathematical!) principle, the transitivity of identity.
For example, I am the author of this essay, and the author of this essay is Kristin Seemuth Whaley. You can therefore conclude that I am Kristin Seemuth Whaley because identity is transitive.
We can see such transitivity in this scenario:
Kelly, a high-school student, thinks back to childhood and remembers dropping an ice cream cone on the ground. Later, in college, they remember being in high school but have forgotten the ice cream.
That college-age-Kelly doesn’t remember dropping the ice cream doesn’t mean she didn’t do it. Psychological continuity, too, is transitive. The high-schooler is psychologically continuous with the child, and the college student is psychologically continuous with the high-schooler. Therefore, the college student and the child are psychologically continuous with each other, through the high-schooler. Through many years of changing but continuous psychological states, each is Kelly.
2. The Duplication Case
Philosophical thought experiments demonstrate an interesting implication of psychological theories: that multiple people could be psychologically continuous with a single past person! Imagine this:
Kelly is diagnosed with an untreatable illness. Doctors propose a cutting-edge procedure to download Kelly’s consciousness and then “transplant” it, uploading it into another body.
Kelly reports to Room A; the doctors perform the procedure on Kelly and a donor body. Afterward, the patient in Room B with Kelly’s consciousness wakes up and thinks, “Amazing! I have a new body and no illness!”
In case anything went wrong, the doctors had a backup plan: duplicating Kelly’s consciousness into a second donor body in Room C. This patient, too, wakes up and thinks, “Amazing! I have a new body and no illness!”
Each patient is psychologically continuous with Kelly. So, it may seem that the patient in Room B is Kelly and, for the same reasons, the patient in Room C is Kelly. If each patient is Kelly, then the transitivity of identity entails that the patients are numerically identical to each other. But Kelly cannot be both; the patients are two different people.
Because of these implications, some philosophers have proposed revised psychological theories.
3. Revised Psychological Theories
Some propose that personal identity is a matter of being psychologically continuous with at most one person. If so, the doctors’ backup plan causes Kelly’s demise; a patient psychologically continuous with Kelly is Kelly as long as uploading happens only once.
Others suggest that numerical identity over time isn’t what matters when we consider our pasts and futures: what matters is whether our interests will continue to be pursued, regardless of how many persons who are psychologically continuous with us pursue them. If so, the doctors’ backup plan actually doubles the likelihood that Kelly’s interests will be satisfied; two distinct patients who share the same past can continue living a life Kelly cared about.
Psychological theories are ethically significant. They suggest that we never were embryos, which had no conscious experiences. And we will never be patients on life-support with no possibility of regaining consciousness. Without the capacity for consciousness, these beings cannot be psychologically connected to us; they couldn’t be us, for we are conscious beings.
Psychological theories may also provide hope for an afterlife, supernatural or technological: once someone physically dies, they can survive as long as their consciousness does. While theories of personal identity cannot guarantee the realization of this possibility, they prompt us toward further inquiry about ourselves, our pasts, and our futures.
 To see the distinction between qualitative and numerical identity, consider the claim: Laurel’s book is identical to Lenny’s. If we mean qualitative identity, then it’s possible that Laurel has one copy of the book and Lenny has another. Being copies of the same book, they are identical with respect to their qualities. But if we mean numerical identity, then Laurel and Lenny are sharing one and the same copy of the book. There is just one book there. Likewise, mathematical expressions like 1=1 are expressions of numerical identity. They express the idea that each side of the equation is the very same thing, not that each side of the equation is merely similar to the other.
 See Locke (1690, II.xvii.9).
 Philosophers recognize that such beings are persons, but there is debate about which beings who lack these features are also persons. For discussion, see Jonathan Spelman’s Theories of Moral Considerability: Who and What Matters Morally? and Nathan Nobis’s The Ethics of Abortion.
 See Locke (1690, II.xvii.9).
 Here ‘A’ and ‘B’ each refer to a person. These labels remain neutral with respect to whether each refers to the very same person or different persons. If, in this case, A remembers experiences had by B, then ‘A’ and ‘B’ would, indeed, refer to the same person.
 Thomas Reid (1785) discusses this issue, which elaborates on concerns from Butler (1736).
We also sometimes falsely seem to remember things that we haven’t done. But a memory theory does not distinguish between false and genuine memories. Plausibly, if it was not you who experienced what you seem to remember experiencing, then this is a false memory; if it was you who experienced what you seem to remember experiencing, then this is a genuine memory. However, this distinction appeals to personal identity to determine whether a memory is false or genuine, and the memory theory appeals to memory to determine personal identity. A memory theorist who appeals to such a distinction is therefore relying on circular reasoning. For further discussion, see Olson (2019, §4).
For debates concerning the interpretation of Locke’s theory and a discussion of objections to them, see Gordon-Roth (2019).
 In general, if identity holds between A and B and between B and C, then by the transitive property, A is identical to C; A is C. Other familiar properties, like being taller than or being a sibling of are also transitive. If Mara is taller than Nelson, and Nelson is taller than Oliver, then Mara is taller then Oliver. If Oliver is a sibling of Nelson and Nelson is a sibling of Mara, then Oliver is a sibling of Mara. For further discussion of the transitivity of identity, see also Graham Seth Moore’s Frege’s Puzzle and the Meaning of Words.
 For the sake of contrast, consider also the brain transplant case in my Are we Animals? Animalism and Personal Identity. Analogous implications arise in considering the possibility of an afterlife. If surviving into an afterlife depends on psychological continuity between the person who physically died and the person existing in an afterlife, what happens if two afterlife people are psychologically continuous with the person who died? The possibility of duplication would arise here, too.
 If the patients do not have all their features in common, then they are not numerically identical to each other; see Leibniz’s Law (Forrest 2010). Since they are in different places, each patient has at least one feature that the other lacks. Beyond the fact that they are in two different places, even psychological continuity theory entails that they are not identical because they are not psychologically continuous with each other.
 For discussion of “fission” cases (where psychological continuity branches into more than one continuer) and responses to them, see, e.g., Lewis (1976), Parfit (1984), and Shoemaker (1984).
 This is referred to as a non-branching requirement, which has the counterintuitive consequence that your identity through time depends on something completely external to you: if someone makes a copy of you, then you would cease to exist even if there is no interaction between the original and the copy, according to a non-branching view. For more discussion of such consequences, see Olson (2019, §5).
 See Parfit (1984).
 Especially relevant are applied issues in bioethics, including abortion, euthanasia, and the use of advance directives. See Nathan Nobis’s The Ethics of Abortion and Euthanasia, or Mercy Killing and DeGrazia (1999) respectively. Shoemaker (2019) also discusses the relevance of personal identity to ethics.
 Such consequences might prompt us toward consideration of biological approaches to our identity instead, according to which we were embryos and could be patients with no possibility of regaining consciousness. See my Are we Animals? Animalism and Personal Identity for discussion.
Are we Animals? Animalism and Personal Identity by Kristin Seemuth Whaley
Possibility and Necessity: An Introduction to Modality by Andre Leo Rusavuk
Descartes’ Meditations 1-3 by Marc Bobro
Frege’s Puzzle and the Meaning of Words by Graham Seth Moore
Is Death Bad? Epicurus and Lucretius on the Fear of Death by Frederik Kaufman
Hell and Universalism by A.G. Holdier
Is Immortality Desirable? by Felipe Pereira
Theories of Moral Considerability: Who and What Matters Morally? by Jonathan Spelman
The Ethics of Abortion by Nathan Nobis
Euthanasia, or Mercy Killing by Nathan Nobis
Many thanks to my Fall 2021 PHI101 class at Carroll University for their insights and comments on this essay.
Download this essay in PDF.
About the Author
Kristin Seemuth Whaley teaches philosophy at Carroll University in Wisconsin. She specializes in metaphysics and philosophy of religion, and she is a recipient of the AAPT Grant for Innovations in Teaching. KristinSeemuthWhaley.com