If you replace every plank, is it still the same ship?

The Athenians kept repairing Theseus's ship until none of the original wood was left. Philosophers have argued about what that means ever since.

In Greek legend, Theseus sailed to Crete, killed the Minotaur and brought the young people of Athens home. According to the story, the Athenians kept his ship for centuries afterwards.

The Greek writer Plutarch tells us how. The thirty-oared ship was preserved down to the time of Demetrius of Phalerum, an Athenian statesman who lived from about 350 to 280 BCE. As its timbers decayed, they were taken out and replaced with new, sound ones. The ship, Plutarch says, became a standing example for philosophers, who argued about it:

some declaring that it remained the same, others that it was not the same vessel.

The trouble is easy to see. Replacing one plank surely doesn't turn a ship into a different ship. But repeat that often enough and nothing original remains. If it is a different ship at the end, which plank made the difference?

Two ships, one name

In 1655, in his book De Corpore, the English philosopher Thomas Hobbes made the puzzle worse. Suppose someone kept every old plank as it was removed and later put them all back together in the same order. Now there are two ships. One has the shape and the unbroken history of repair. The other has all the original material. Both seem to have a claim to be the ship of Theseus, but there can't be two ships that are numerically one and the same, which Hobbes called absurd.

Hobbes saw the two ships as matching two ways of judging sameness, by form and by matter, which give different answers here.

Later philosophers have offered many responses. One says that a thing and the material it is made of are not identical: the ship and the wood that makes it up are two objects in the same place. Another, associated with David Lewis, treats objects as stretched out through time, made up of temporal parts, much as a road is made of stretches. A third, deflationist view says all the facts are agreed and the only dispute is about how to use the word ship.

The question turns up wherever something survives by being repaired, like the old knife that has had a new blade and later a new handle. In philosophy of mind it sits close to a harder one: what makes you the same person you were twenty years ago.