Replace every plank of a ship one at a time — is it still the same ship?

Plutarch records that the Athenians preserved Theseus's ship for centuries by replacing its rotting planks one by one, until none of the original timber remained. Philosophers have used the puzzle ever since to probe what makes something 'the same thing' over time — your body, a river, or a company that has replaced every employee it started with.

— Plutarch, Life of Theseus — 1st century AD
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