“In ancient Greece, Plutarch wrote of a wooden ship that Theseus sailed from Crete to Athens. To preserve the ship, as its old planks decayed, Athenians would replace them with new wood. Eventually all the planks had been replaced. It looked like the same ship, but none of its parts was the same. Was it still the same ship? Later, philosophers added a wrinkle: if you collected all the original planks and fashioned them into a ship, would that be the same ship?”
