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| title | sort | section-id | description | language |
|---|---|---|---|---|
| Identity and Persistence | 110 | metaphysics | The Ship of Theseus, personal identity, and four-dimensionalist theories of persistence. | en |
Identity and Persistence
What makes a thing the same thing over time? The ship that returned to Athens was rebuilt plank by plank during the voyage; no original material remained. Is it the same ship? The person who wakes tomorrow morning shares your memories and psychology — but not your matter, since your cells replace themselves over years. Are they you?
These are not idle puzzles. They bear on personal survival, moral responsibility, and the metaphysics of change, and their answers connect to fundamental questions about what kinds of things exist.
The Ship of Theseus
The Ship of Theseus is one of philosophy's oldest thought experiments, recorded by Plutarch ^[Plutarch, Theseus, ch.23, c.75 CE]. Its philosophical interest lies not in the historical case but in the structure of the puzzle it reveals: ordinary objects persist through gradual material change, but taken to the limit, material continuity seems to dissolve.
Hobbes added a further twist: suppose someone keeps all the original planks and reassembles them. Which is the original ship — the continuously maintained vessel or the reconstructed one? ^[Hobbes, T., De Corpore, 1655, II.xi.7]. The puzzle reveals that "same ship" may be determined by different identity criteria (material continuity, spatial-temporal continuity, functional continuity) that can diverge.
This connects to a broader metaphysical debate about constitution: when a lump of clay is shaped into a statue, are there two objects (the lump and the statue) that share matter but have different persistence conditions? Or only one? Defenders of constitution theory say two; critics say one (and find the view of two things in the same place implausible) ^[Wiggins, D., Sameness and Substance, Blackwell, 1980; Gibbard, A., "Contingent Identity", Journal of Philosophical Logic 4, 1975].
Personal Identity: The Classical View
John Locke offered the first systematic philosophical analysis of personal identity ^[Locke, J., Essay, II.xxvii]. He distinguished the identity of substance (a material thing, continuous in matter), organism (a living thing, continuous in life), and person (a thinking conscious being, continuous in consciousness).
Persons, for Locke, persist through psychological continuity — specifically, memory. What makes the person who did action A the same person as me is my memory of having done A. This allows persons to come apart from both bodies and souls: the prince and the cobbler might "swap" if their consciousnesses were somehow exchanged.
Objections: Bishop Butler accused Locke of circularity: memory presupposes personal identity, so it cannot constitute it ^[Butler, J., The Analogy of Religion, 1736, Appendix I]. Thomas Reid's brave officer paradox sharpened this: an old general remembers his younger self's bravery as a junior officer, but the officer (flogged as a boy) no longer remembers the boy's actions. By transitivity, the general is not the same person as the boy — but this seems absurd ^[Reid, T., Essays on the Intellectual Powers of Man, 1785, III.6].
Neo-Lockean Theories
Derek Parfit developed the most sophisticated neo-Lockean account ^[Parfit, D., Reasons and Persons, Oxford UP, 1984, Part III]. He replaced memory with psychological continuity — overlapping chains of psychological connections (memories, intentions, beliefs, desires) — and argued that what matters for survival is not strict identity but this continuity relation.
Parfit's striking conclusion: personal identity is not what matters in survival. In cases of fission (where your psychology is duplicated in two people), neither resulting person is strictly you — but both have what matters as much as survival does. We should care about psychological continuity and connectedness, not about identity itself.
This has radical implications for ethics: if personal identity does not matter in itself, many concerns about future persons — including concerns about one's own future self — are impersonal, and the boundaries between persons may be less sharp than we normally assume.
The Biological Criterion
Eric Olson has argued for animalism: we are human animals, and our persistence conditions are those of biological organisms ^[Olson, E., The Human Animal, Oxford UP, 1997]. Personal identity is not constituted by psychological continuity; you persist as long as your body's metabolism continues. Brain transplants, on this view, are body transplants.
Animalism avoids neo-Lockean puzzles by refusing to split the person from the organism, but it faces difficulties with cerebral bisection, personal survival, and cases where psychological continuity intuitively tracks identity better than biological continuity does.
Four-Dimensionalism
The four-dimensionalist (or perdurantist) view holds that objects persist by having temporal parts at different times, just as they have spatial parts at different locations ^[Lewis, D., On the Plurality of Worlds, Blackwell, 1986, pp.202-204; Sider, T., Four-Dimensionalism, Oxford UP, 2001]. The person who existed yesterday is a temporal part of a four-dimensional object extended through time as well as space.
On this view, there is no problem of persistence through change: different temporal parts can have different properties. The Ship of Theseus problem dissolves: the ship at time t1 and the ship at time t2 are different temporal parts of the same four-dimensional whole, even though their material composition differs.
Four-dimensionalism is technically elegant but counterintuitive: it implies that we never change — what we ordinarily call "my change" is two different temporal parts having different properties. And the multiplication of temporal parts raises concerns about parsimony.
Three-dimensionalists (or endurantists) hold that ordinary objects persist by being wholly present at each moment of their existence. They accept genuine identity through time and must therefore give an account of how the same thing can have different properties at different times (through temporally modified property ascription, or relativisation to times).
The persistence debate connects to the metaphysics of time: presentists, who hold only the present exists, are naturally endurantists; eternalists, who hold past, present, and future equally exist, can more naturally accommodate perdurance.