Imagine a sea captain called Theseus has a ship constructed out of wooden planks. Give this ship the highly imaginative name of ‘Ship1’. Ship1 sails the seas for many years and subsequently needs to undergo a repair, which involves replacing one of its wooden planks with a more durable metal one. Now, it seems counter-intuitive to suppose that the ship would cease to be the very same ship merely because it’s had one of its planks replaced: we have the same ship with one new part here, not a new ship entirely. Therefore, we have the principle that Ship1 with one changed part is numerically identical to Ship1. But now Ship1 needs to have another wooden plank replaced with a metal one. Since Ship1 with one metal plank is the same ship as Ship1, and we have the principle that Ship1 can have a part replaced and yet still continue to be the very same ship, we have the result that Ship1 with two metal planks is the very same ship as Ship1.
Now, if we continue replacing planks in this manner, we will eventually have a ship which is composed entirely from metal planks. By continuing with the above reasoning, this metal ship will be the very same ship as Ship1. But now suppose that we take all of the wooden planks that originally composed Ship1 and arrange them back into the shape of a ship. Does this wooden ship not have an equally strong claim to being the very same ship as Ship1? After all, it is composed entirely out of the material which once composed Ship 1. The trouble is, though, it cannot be the case that the newly-assembled wooden ship and the metal ship are both identical to Ship1, for a ship cannot be in two places at once, composed of two entirely different sets of materials! So, which one can Theseus claim belongs to him?! This is known as the Ship of Theseus Paradox, and it was first reported by the Greek historian Plutarch (c.45–120 AD). It forces us to think about what kinds of changes material objects can survive, and that, it turns out, is quite a hard question to answer! | |||