Once upon a time — at least according to the ancient Greek writer Plutarch — the hero Theseus sailed from Athens, Greece, to the island of Crete, where he slayed the half-man, half-bull Minotaur before sailing back to rule Athens.
The wooden ship that Theseus sailed on, Plutarch imagined, must have become a national treasure, and he posed a thought experiment that has fascinated philosophers ever since: If you repaired Theseus' ship plank by plank so that no original planks remained, is it still the same ship?
