The Ship of Theseus
Replace a ship's planks one at a time until none of the originals remain. Is it the same ship? And if someone builds a second ship from the old planks, which one is the original?
- Reading time
- 5 min
- Difficulty
- Introductory
- Branch
- Identity
- First posed
- Plutarch, Life of Theseus, c. 100 CE; extended by Thomas Hobbes, De Corpore, 1655
The setup
The Greek writer Plutarch records that the Athenians preserved the ship in which the hero Theseus had returned from Crete. As its old timbers decayed, they replaced them with new ones, so that, as Plutarch put it, the ship became a standing example among philosophers of the logical question of growth: some held that it remained the same, others that it did not.
The puzzle is simple to state. Replace one plank, and surely it is the same ship; people repair ships all the time. Replace another, and another. Each change is tiny. But eventually not a single original plank remains. Is it still the ship of Theseus?
In 1655 the English philosopher Thomas Hobbes added a twist. Suppose someone had kept every old plank as it was removed, and at the end reassembled them, in the original order, into a ship. Now there are two ships. One has continuity of form and history, kept in service the whole time. The other has all the original material. Which is the real ship of Theseus? They cannot both be, since they are two.
Interactive
Replace the ship, plank by plank
The original ship. Every plank is the one Theseus sailed on.
Why it matters
The puzzle is about identity over time: what makes something the same thing through change. It applies to everything that persists: rivers, cities, companies, bands that have replaced every member, and above all ourselves. Your body replaces most of its cells over the years. Is it the same body? Are you the same person? See personal identity.
It also has practical stakes. Courts decide whether a restored classic car may be sold as the original. Museums decide whether a heavily restored painting or building is authentic. Companies argue over whether a brand that has changed every product is the same company.
Key terms
- Identity over timeWhat makes something at one time the very same thing as something at another time, despite changes.
- Numerical identityBeing one and the same thing, as opposed to being exactly alike. Twins may be qualitatively identical; only you are numerically identical to you.
- Qualitative identityBeing exactly alike in every property, without necessarily being one and the same thing.
- MereologyThe study of parts and wholes: when several things make up one thing, and whether a whole is anything over and above its parts.
- Four-dimensionalismThe view that objects persist through time by having temporal parts, just as they extend through space by having spatial parts. A person is a 'spacetime worm'.
The positions
Continuity of form wins
The repaired ship, kept in continuous use and form, is the ship of Theseus. Gradual replacement of parts does not end a thing's existence.
Original matter wins
The reassembled ship, made from the original planks, is the real ship of Theseus.
Both are partly right
There is no single correct answer. 'Same ship' is vague or has several meanings, and both ships have a reasonable claim.
Four-dimensionalism
Objects are extended in time as well as space. The two ships share some temporal parts, and the puzzle dissolves.
Arguments and objections
Continuity of form wins
- Best argument
- Living things replace their materials constantly; a river's water is always new. What makes something the same over time is continuity of structure and function, not of stuff. Every repair was minor, and at no point did anyone build a new ship. The Athenians were right to call it Theseus's ship.
- Strongest objection
- Hobbes's reassembled ship has a powerful claim too: it is made of exactly the material Theseus sailed on, in exactly the same arrangement. If a museum displayed it, many people would say it is the authentic ship.
Original matter wins
- Best argument
- A ship is a particular collection of timbers arranged in a certain way. The reassembled ship has all and only the original timbers in the original arrangement. It is as if the ship were disassembled for storage and put back together, which does not destroy a ship.
- Strongest objection
- This implies that, during the replacement process, the ship was gradually moving to a pile of planks in a warehouse, while the sailors on the working ship were, unknown to them, sailing a new one. That seems strange.
Both are partly right
- Best argument
- Our concept of a ship was never designed for this case. In ordinary life, continuity of form and continuity of matter go together, so we never had to decide which matters more. When they come apart, the question 'which is the real ship?' may have no determinate answer, just as there may be no exact moment when a person becomes bald.
- Strongest objection
- It seems there must be a fact about which object Theseus sailed in. Identity seems all or nothing, not vague.
- The reply
- Some philosophers accept vague identity; others hold that the vagueness lies only in our words, which fail to pick out one object rather than another.
Four-dimensionalism
- Best argument
- On four-dimensionalism, a ship is like a road that stretches through time. The ship in service and the reassembled ship are two four-dimensional objects that overlap in their early stages, like two roads sharing a stretch before they fork. Asking which is 'the' ship of Theseus is like asking which fork is 'the' road.
- Strongest objection
- It seems to multiply objects: before the fork, there were already two ships occupying the same place, sharing the same planks. Many find this counterintuitive.
Who argued what
- Continuity of form wins Aristotle, John Locke
- Original matter wins Thomas Hobbes (as one option)
- Both are partly right David Lewis, Derek Parfit
- Four-dimensionalism David Lewis, Ted Sider, W. V. O. Quine
Variations
Grandfather's axe
'This is my grandfather's axe. My father replaced the head, and I replaced the handle.' The joke works because we feel both that it is and is not the same axe. Versions appear in many cultures; in France the same joke is told about 'Jeannot's knife'.
The cells of your body
It is often said that every cell in your body is replaced every seven years. That is a myth: neurons in the cerebral cortex and the lens of the eye mostly last a lifetime, while gut lining cells last days. But the atoms are mostly replaced. If the ship's identity survives replacement, so, presumably, does yours.
The band
A band forms in 1970. By 2020, every original member has left and been replaced. Is it the same band? Courts have had to decide cases like this when band names are disputed, and they usually rule by legal ownership of the name, which says nothing about metaphysics.
Go deeper
Heraclitus and the river
Long before Plutarch, Heraclitus is reported to have said that you cannot step into the same river twice, for new waters are always flowing on. Plato quotes the idea in the Cratylus. One interpretation is that a river is the same river precisely because its waters change: a river is a pattern of flow, not a body of water. The ship of Theseus and the river of Heraclitus raise the same question from opposite sides.
Sorites and the ship
The puzzle has the same structure as the sorites paradox. Replacing one plank never turns one ship into another. But replacing all of them seems to. Where is the line? If there is no sharp line, does that mean there is no fact about whether the final ship is the original?
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: Identity Over Time
- WikipediaShip of Theseus
- BookLife of Theseus, Plutarch
- BookMetaphysics: A Very Short Introduction, Stephen Mumford
- BookFour-Dimensionalism, Ted Sider
Open questions
- If every part of you were replaced, slowly enough, would you notice?
- Is a restored building still the original building?
- Do things really persist through change, or do we simply give them the same name?