The Ship of Theseus
If every plank of a ship is replaced, one at a time, is it still the same ship? A compact statement of the puzzle and its best-known variant.
The puzzle, in its usual form: a wooden ship is kept in service by replacing its planks as they rot. Eventually no original plank remains. Is it the same ship?
The story is often traced to Plutarch’s Life of Theseus, which describes a ship the Athenians preserved by replacing decayed timbers, and notes that philosophers used it as an example in arguments about things that grow and change.
The reassembly variant
Thomas Hobbes, in De Corpore, sharpened the puzzle. Suppose someone collects every discarded plank and reassembles them into a ship. Now there are two candidates for “the original”:
- the continuously repaired ship, which kept its form and function, and
- the reassembled ship, made of the original material.
Both answers have intuitive pull, which is why the case is useful: it separates criteria for identity that normally travel together — continuity of form, continuity of matter and continuity of use.
Open questions
- Does the answer depend on why we’re asking — legal ownership, museum display, sentiment?
- Is “same” doing different work in “same ship” and “same material”?
See Identity over time for where these questions lead.