Personal Identity: How We Exist Over Time

Author: Chad Vance
Category: Metaphysics
Word Count: 1000

You are a person. Presumably, you used to be a younger person. But, in virtue of what is it the case that you and that younger person are one and the same individual? In other words, how is it that you continue to exist over time?

There had better be something that grounds our continued existence, for we make judgments that presume that people continue to exist every day—for instance, when we assign rewards and punishments to them, or moral obligations, or property rights, and so on:

  • the woman presently being punished for some past crime had better be the same person who committed it, or else her punishment is unjust;
  • if I am obligated to keep a promise, I had better be the one who made it in the first place, or else I have no such obligation;
  • if you presently possess a car, you had better be the one who purchased it, or else you have no legitimate claim to its ownership.

Clearly, the question of what grounds the persistence of one’s identity is an incredibly important one. Let’s look at some proposed answers.

"Freaky Friday," one of many films about personal identity.
“Freaky Friday,” one of many films about personal identity.

1. The Body

Maybe our bodies have something to do with our continued existence; as long as my body continues to exist, I continue to exist.

But, mere sameness of body over time does not seem to be enough to ground the continued existence of a person.1 For, if that were the case, then a brain-dead body in a persistent vegetative state (entirely lacking consciousness) would still be the same person as the one who once talked and thought and laughed with others. That seems wrong.

To illustrate further, imagine this story:

The human body of your next-door neighbor leads two completely different lives—one during the day, and one at night. From 6am to 6pm every day, his body claims to be named “Day Man.” Day Man thinks that his body sleeps every night, but what really happens is that, from 6pm to 6am, his body is awake and claiming to be named “Night Man.” Night Man and Day Man have different jobs, friends, goals, tastes, etc., and, furthermore, have no knowledge of each other.

Doesn’t it seem like what we have here is a case of two persons inhabiting one body? If that is correct, then sameness of body is not what grounds sameness of person; for, in that case, it would turn out that Day Man and Night Man were really just one person and not two (since they share the same body).

2. The Soul

Many people believe that what we are is not something material, but rather something immaterial. They say that each of us has a soul—or rather, is a soul—and that souls are immaterial things that reside in bodies. Maybe our souls ground our continued existence; so long as my soul continues to exist, I continue to exist.

But, if that were so, we would have no way of knowing whether or not people continue to exist over time. Since souls, if they exist, are immaterial, we cannot see or touch or smell them: souls would be completely undetectable by the senses. And yet, it seems that we can know whether or not someone in the present moment is the same person as some earlier one. You do not greet your friends and say, “But, how can I know whether or not you are really my friend? My friend is merely an immaterial thing, and since I cannot see immaterial things, I have no way of knowing whether or not my friend stands before me.”

Furthermore, people who believe in souls typically believe that souls can leave their bodies and go elsewhere (for instance, to heaven). But, then, for all we know, when a soul leaves one body it simply inhabits another. Abraham Lincoln’s soul might have left his body after his death and migrated into mine. If that happened, then I am Abraham Lincoln—one and the same person, such that I was once the 16th president of the United States, who wrote the Gettysburg Address and so on—a result that many find counter-intuitive.

3. Psychology

If Day Man and Night Man are two different people, then it is because they have two different psychologies. If a body in a persistent vegetative state is no longer a person, then it is because that body is no longer conscious. If I am not Abraham Lincoln, despite sharing one and the same immaterial soul, then it is because I cannot remember doing anything that Lincoln did.

For these reasons, we might think that psychology or consciousness is what grounds our continued existence.

But, consciousness alone will not do. Imagine that your body woke up today claiming to be Pope Francis; your body now possesses all of the pope’s beliefs and opinions, and claims to be the pope. In short, imagine your body waking up with the pope’s psychology.2 Imagine also that the actual body of Pope Francis woke up in the Vatican this morning and continued with business as usual, doing pope things. Now you both have the pope’s psychology, so, apparently, you are both Pope Francis. But, that can’t be. You cannot both be the pope (how could one person be in two places at once?). So, psychology alone does not seem to be what grounds identity.

4. Conclusion 

Intuitively, the real pope is the man who wakes up in the Vatican. Meanwhile, your body is merely an imposter pope. Might this be because the man in the Vatican has the same body and psychology as the man elected to the papacy, while your body has only his psychology?

This line of reasoning has led to many hybrid proposals. For instance, perhaps you continue to exist over time only so long as your body and your psychology do. On this proposal, survival of only your body (e.g., as a human vegetable) or only your psychology (e.g., if downloaded into a new body) would not suffice for the survival of you.


For one thing, strictly speaking, we do not even have the same bodies over time. After all, our bodies are constantly taking in new material as we breathe and eat and drink, and constantly shedding material as we perspire and excrete and exhale or lose our hair and skin our knees. So, most body-theorists do not actually believe that exact sameness of material is what grounds identity. Rather, it is only bodily continuity that matters. For instance, even though my body is made of slightly different material today than it was yesterday, there is nevertheless some causal continuity between my present body and my former one which binds them together as one and the same.

Or do you merely seem to have his psychology? E.g., if you seem to remember something the Pope did, do you actually have his memories, or does it just seem like it? What’s the difference between remembering something, and it seeming to you that you remember something? How do answers here make a difference to psychological theories of personal identity?


Locke, John (1689). “Of Identity and Diversity”, in An Essay Concerning Human Understanding, section II.27.

Olson, Eric T. (2019). “Personal Identity”, in The Stanford Encyclopedia of Philosophy.

Parfit, Derek (1971). “Personal Identity”, in Philosophical Review, vol. 80, 3-27.

Parfit, Derek (1987). “Divided Minds and the Nature of Persons”, in Mindwaves, Blakemore and Greenfield, eds. (Oxford: Basil Blackwell), 19-26.

Perry, John (1978). A Dialogue on Personal Identity and Immortality.

For Further Reading

Korfmacher, Carsten. “Personal Identity”. The Internet Encyclopedia of Philosophy.

Related Essays

Psychological Approaches to Personal Identity: Do Memories and Consciousness Make Us Who We Are? by Kristin Seemuth Whaley

Are We Animals? Animalism and Personal Identity by Kristin Seemuth Whaley

The Buddhist Theory of No-Self (Anātman/Anattā) by Daniel Weltman

Origin Essentialism by Chad Vance

The Non-Identity Problem by Duncan Purves

Possibility and Necessity: An Introduction to Modality by Andre Leo Rusavuk 

Modal Epistemology: Knowledge of Possibility & Necessity by Bob Fischer

Hell and Universalism by A.G. Holdier

The Ethics of Abortion by Nathan Nobis

Euthanasia, or Mercy Killing by Nathan Nobis

PDF Download

Download this essay in PDF

About the Author

Chad Vance is a Senior Lecturer in Philosophy at the College of William and Mary. He received his PhD in philosophy from the University of Colorado, Boulder. He also holds degrees in astrophysics and theology. He specializes in metaphysics and is also very interested in normative ethics, applied ethics, philosophy of religion, and early modern philosophy.

Follow 1000-Word Philosophy on Facebook, Twitter and subscribe to receive email notice of new essays at the bottom of