Ship of Theseus

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The Ship of Theseus problem is one expression of a central problem in the metaphysics of identity and change. To introduce it, we could do worse than cite the great Greek biographer of the ancient world, Plutarch:

The ship wherein Theseus and the youth of Athens returned 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, Life of Theseus, trans. John Dryden)

The problem is also sometimes referred to by the example of "a knife that has had two handles and three blades."