A ship can change plank by plank so slowly that nobody marks the moment it becomes a different ship. But has it?
Imagine every board is replaced over the years. The vessel still sails, keeps its name, and carries its familiar crew. Meanwhile, someone saves each old plank and eventually builds a second ship from them. Which one is the original: the continuous traveler, or the ship made of original wood?
The puzzle pulls apart two things that usually travel together: a thing’s history and its material. If both ships seem to inherit the old identity, we face a logical snag: if the first ship is the same as each, then the two new ships should be the same as each other. They plainly are not. Philosophers offer different accounts of what persistence means; none has ended the argument.