Ship of Theseus - Theseus' paradox

Greek legend

According to Greek legend as reported by Plutarch,

The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.
—Plutarch, Theseus

Plutarch thus questions whether the ship would remain the same if it were entirely replaced, piece by piece. Centuries later, the philosopher Thomas Hobbes introduced a further puzzle, wondering: what would happen if the original planks were gathered up after they were replaced, and used to build a second ship. Which ship, if either, is the original Ship of Theseus?

Various variants, notably grandfather's axe, George Washington's axe, and (in the UK) Trigger's Broom is a paradox that raises the question of whether an object which has had all its component parts replaced remains fundamentally the same object.