The Ship of Theseus Was a Real Boat the Athenians Maintained for Centuries

Plutarch wrote his Lives sometime around 100 CE. In the chapter on Theseus, he mentions that the ship in which Theseus had returned from Crete was preserved by the Athenians as far down as the time of Demetrius of Phalerum — that is, for nearly a thousand years. The ship had thirty oars. Whenever a plank rotted, the Athenians took it out and put a new one in its place. By Plutarch's time, no original timber was left.

He then notes, almost in passing, that the case had become a 'standing example among the philosophers' for the question of whether a thing remains the same thing when its matter has changed. He does not say which philosophers. By the late 1st century the question was already hoary; the Stoics and the Peripatetics had both written on growing things and persisting identity, and the Athenian ship was the convenient illustration.

Thomas Hobbes, in his 1655 De Corpore, sharpened it. Suppose, he said, that as the Athenians took out each old plank, someone else collected the discarded planks and reassembled them. Now there are two ships: the continuously maintained Athenian one, and the reconstructed original. Both have a claim. The first has unbroken historical continuity; the second has every original component. Which is the ship of Theseus? Hobbes thought the question turned on what 'same' was for. A grammarian, a lawyer, and a craftsman would each give a different answer.

The puzzle has not been solved because it is not the kind of puzzle that has a solution. It is a tool for noticing that 'identity over time' is one word doing several jobs.