If you replace every plank of a ship, is it the same ship?
The ancient Greeks said Theseus's ship was preserved in Athens, its rotting planks swapped out one by one until none of the originals remained. Philosophers have argued ever since: is it still his ship? The puzzle is also about you. Your body steadily renews its parts.
The puzzle comes to us from Plutarch, writing around the first century AD. He described how the Athenians kept the ship in which the hero Theseus had returned from Crete, replacing decayed timbers with new ones, so that the ship became an example for philosophers of things that grow: some argued it remained the same, others that it did not.
Centuries later, Thomas Hobbes added a twist. Suppose someone collected all the discarded original planks and reassembled them. Now there are two ships. Which is the ship of Theseus: the one that has continuously sailed, or the one built from all the original material?
There is no single accepted answer, which is why the puzzle endures. Some philosophers argue that identity follows continuity of form and function. Others say it follows the original matter. Still others say the question has no fact of the matter at all: 'same ship' is a label we apply for practical purposes, and the puzzle only reveals the limits of the label.
The question turns personal quickly. Many of the cells in your body are replaced over time, though not all, since most neurons in the brain's cortex last a lifetime, and you are not made of the same atoms you were as a child. Your beliefs and memories change too. Yet you feel like one continuous person. Whatever makes you 'you' may have more to do with pattern and story than with parts.
Source: Wikipedia — Ship of Theseus · Text summarised from Wikipedia (CC BY-SA 4.0)
