The Ship of Theseus
Replace every plank of a ship. Is it still the same ship?
Plutarch, about 100 CE; Thomas Hobbes added the twist in 1655
The snake boat of Kavalam
Every August, the village of Kavalam in Kerala races its snake boat, a chundan more than 30 metres long, rowed by over a hundred people. The villagers love it like a grandparent. They give it a name, a festival, and every year a little repair.
One monsoon a plank near the bow rots, so the carpenter Velu replaces it with fresh wood. The next year he replaces two more. Year after year the old planks go, until one day Velu's granddaughter Anju counts and realises that not a single piece of the original boat is left. The village still calls it the same boat and still cheers it on.
But Velu has been keeping every old plank in his shed. When he retires, he rebuilds them, peg by peg, into a boat. It is made of exactly the wood that won the first race. Now there are two boats on the river, and both claim the same name.
Anju has to write the boat's name on the trophy. Which one does she choose?
The puzzle
If an object has all of its parts replaced, is it still the same object? And if the old parts are put back together, which one is the original?
See it for yourself
What's really going on
The puzzle is about what we mean by 'the same'. One answer says identity follows the material: the rebuilt boat is the real one, because it is made of the original wood. Another says identity follows continuity: the boat on the water has existed without a break, changing slowly, like a river that is 'the same river' even though the water is always new.
A third answer says the question has no hidden fact to discover. 'Same boat' is a label we use because it is useful, and the two meanings of 'same', same material and same history, usually agree. The paradox appears only in the rare case where they come apart.
Still argued about. Your own body replaces most of its cells over the years, so the puzzle applies to you too. Philosophers still disagree about which answer is right.
Think about it
If you replaced every part of your phone one at a time, at which point, if any, would it stop being your phone?