What is Ship of Theseus?

In Ancient Greek philosophy, The Ship of Theseus is a thought experiment that asks whether an entity that has had all of its original components replaced remains the same entity. It has applications to the philosophical study of identity, and has inspired a variety of proposed solutions in contemporary philosophy of mind.

The "Ship of Theseus" was first introduced in Greek legend as reported by the historian, biographer, and essayist Plutarch as follows-

"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 places, 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 thus questions whether the ship would remain the same if it were entirely replaced, piece by piece. Centuries later, another philosopher Thomas Hobbes introduced a further puzzle, wondering what would happen if the original planks were gathered up after they were replaced, and then used to build a second ship.