Which is the real ship?
Continuity of form, original matter, both, or neither?
I
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.
- 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.
- Defended by
- Aristotle, John Locke
II
Original matter wins
The reassembled ship, made from the original planks, is the real ship of Theseus.
- 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.
- Defended by
- Thomas Hobbes (as one option)
III
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.
- 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.
- Defended by
- David Lewis, Derek Parfit
IV
Four-dimensionalism
Objects are extended in time as well as space. The two ships share some temporal parts, and the puzzle dissolves.
- 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.
- Defended by
- David Lewis, Ted Sider, W. V. O. Quine
For the full background, variations and open questions, read the entry: The Ship of Theseus.