Definition
The ship of Theseus is a classic puzzle about identity through change. A wooden ship has its planks replaced one at a time during repairs. After enough repairs, no original plank remains. Is the fully repaired ship still the same ship? Most people say yes — gradual repair preserves identity.
But now suppose someone collects every discarded plank and reassembles them into a ship. There are now two vessels. Which one is the original ship of Theseus — the continuously repaired one, or the one built from all the original parts? Both answers have a strong claim, and they cannot both be right.
Why it matters
How it works
The paradox works by pitting two reasonable criteria of identity against each other. One says identity is preserved by spatiotemporal continuity — staying the same continuously functioning thing — which favours the repaired ship. The other says identity is preserved by sameness of matter — being made of the same stuff — which favours the reassembled ship.
In ordinary life the two criteria agree, so we never notice they are distinct. The puzzle is a contrived case where they come apart, exposing that 'the same ship' was never as sharp as we assumed. Responses vary: some deny that the original ship is identical to either successor; some hold that 'same ship' is partly a matter of convention; some accept that identity over time can be indeterminate. Like the sorites, it reveals a concept with no sharp boundary.