The Original Puzzle

The ancient Greeks kept the ship of the hero Theseus in their harbor as a monument. Over the years, as planks rotted, they replaced them with new wood. Eventually every single piece had been swapped out. Was it still the ship of Theseus? The biographer Plutarch put this question in writing nearly 2,000 years ago.

The puzzle deepens with a twist from Thomas Hobbes: suppose someone collected all the old rotted planks and assembled them into a second ship. Now you have two vessels — the continuously repaired one in the harbor and the reassembled original material.