The Real Ship of Theseus Wasn't a Thought Experiment

Plutarch's Life of Theseus, written around 100 CE, contains the original passage. The Athenians, he says, kept the thirty-oared galley that Theseus had supposedly sailed home from Crete, and as planks decayed they pulled them out and replaced them with new ones. By the time of Demetrius of Phaleron in the late 4th century BCE, the ship was still in port and still being called the Ship of Theseus. Plutarch notes that philosophers had already started using it as a stock example in debates about identity through change — "one side holding that the ship remained the same, and the other contending that it was not the same."

That's the whole original puzzle: a real maintained relic, and a question about whether continuous repair preserves identity. There is no second ship in Plutarch.

The second ship is Hobbes's. In De Corpore (1655), Thomas Hobbes upgraded the puzzle: imagine someone has saved every original plank as it was discarded, and at the end reassembles them into a ship. Now there are two ships — the maintained one in the harbor and the reassembled original — both with a credible claim to being Theseus's ship. Hobbes thought this was absurd, and used the absurdity to argue that bodily identity over time is a slipperier notion than common sense allows.

The modern "if you replace every part, is it the same thing?" version is really two arguments stacked: Plutarch's question about gradual replacement, and Hobbes's question about competing claimants. Stripped to Plutarch alone, the answer most Athenians would have given is the answer most ship owners give today: of course it's still the same ship.