Identity & change

Theseus had a wooden ship. Over the years, the Athenians replaced its planks one at a time as they rotted. Eventually, not a single original plank was left. Is it still the ship of Theseus?

It gets harder. Suppose someone collected every old plank and rebuilt a ship from them. Now there are two ships. One has continuity: it was always in the harbour, repaired bit by bit. The other has all the original material. Which one is Theseus's ship?

This puzzle is about identity over time: what makes something the same thing at two different moments, even though it changes. You have the same problem. Almost every atom in your body is replaced over the years. What makes you the same person you were as a child?

Some say identity follows continuity of form. Some say it follows the material. Some say the question has no fact of the matter, and “same ship” is just a practical decision.