---
title: Identity and Persistence
section-id: metaphysics
sort: 110
author: Prof. James Okafor
created: 2026-02-05 09:00
modified: 2026-03-12 10:00
language: en
description: Personal identity over time, the Ship of Theseus, and criteria for persistence.
---

# Identity and Persistence

What makes you the same person you were ten years ago? Your body has changed; many of your beliefs, memories, and preferences have changed. And yet there seems to be a fact of the matter: you are the same person. What does this fact consist in?

## Numerical vs Qualitative Identity

Philosophers distinguish **numerical identity** (being the very same thing) from **qualitative identity** (being exactly alike). Two identical twins are qualitatively identical but numerically distinct. Leibniz’s **Law of the Indiscernibility of Identicals** states that if A and B are numerically identical, they share all the same properties. This seems uncontroversial. Its converse—the **Identity of Indiscernibles** (if A and B share all properties, they are identical)—is more contentious.

## The Ship of Theseus

The ancient puzzle: The Ship of Theseus has its planks replaced one by one until no original plank remains. Is it still the same ship? If yes, at what point did it stop being the original? Now suppose the removed planks are collected and reassembled into a ship. Which is the original—the continuously repaired ship or the reconstructed one?

This puzzle illustrates the tension between two criteria for identity over time:
- **Spatiotemporal continuity**: the ship persists by continuous physical presence through space and time
- **Material composition**: the ship persists only while it has (most of) its original matter

The puzzle has no universally accepted solution. It suggests that identity over time may not be a fully determinate matter—or that our ordinary concept of identity is not precise enough to handle these cases without stipulation.

## Personal Identity

### Psychological Continuity
Locke proposed that personal identity consists in **memory**: you are the same person as some past individual if and only if you can remember their experiences as your own. This grounds personal identity in psychological rather than bodily continuity.

Thomas Reid objected with the brave officer paradox. An old general remembers being a young officer who was flogged as a boy. The young officer remembered the flogging. But the general, by hypothesis, does not remember the flogging. By Locke’s criterion, the general is the same person as the officer, and the officer is the same person as the boy, but the general is not the same person as the boy. Identity is not transitive—a contradiction.

Derek Parfit refined psychological continuity theories. He distinguished **connectedness** (direct psychological links: memories, intentions carried out, beliefs persisting) from **continuity** (overlapping chains of connectedness). Continuity does not require connectedness at every step, only that the chain holds. Parfit also argued that personal identity may not be what matters: what we care about in survival is psychological continuity, not strict numerical identity.

### Bodily Criteria
Other philosophers ground personal identity in bodily or biological continuity. You persist as long as the same living organism persists. This handles cases where psychological continuity breaks down (severe amnesia, persistent vegetative state) by keeping the answer simple: if the organism continues, so does the person.

### No-Self Views
Hume could find no impression of a self persisting through time. Introspecting, he found only a bundle of perceptions: impressions and ideas following one another rapidly. The self, on this view, is not a further entity but a bundle or a construction. Buddhist philosophy developed similar no-self (anātman) views independently, arguing that the belief in a persistent self is both philosophically mistaken and a source of suffering.

## Four-Dimensionalism

**Four-dimensionalism** or **perdurantism** holds that objects persist through time by having temporal parts (stages) at different times, just as they have spatial parts at different locations. You are the mereological sum of your temporal parts. On this view, the question of what makes you the same person over time is answered: there is no further fact beyond the existence of a series of temporally ordered stages connected in the right ways.

Opponents (**endurantists**) hold that objects persist by being wholly present at each time they exist. Identity over time is not a matter of having stages; it is a matter of the same whole object existing at different times.

## Summary

- Numerical vs qualitative identity: two things can be perfectly alike without being the same thing
- The Ship of Theseus: tensions between spatiotemporal continuity and material composition criteria
- Personal identity: Locke’s memory theory, Parfit’s psychological continuity, bodily criteria, and Humean no-self views
- Four-dimensionalism treats identity over time as a matter of having temporally ordered stages
