Ship of Theseus

One of my favorite philosophical ponderings is the paradox known as the ship of Theseus. The oldest known written version of the story was presented as a brief interlude in Plutarch's The Life of Theseus. A much more interesting version was written by Thomas Hobbes.

Basically, Theseus was a Greek hero who did all sorts of great things like slaying the Minotaur and becoming king of Athens. The ship which he had sailed upon (ship A) was preserved by the Athenians for many years. In order to keep the ship in pristine shape, older planks were replaced with new identical parts as needed.  Eventually, no part of the ship had been left unreplaced.  Unbeknownst to the Athenians, the ship repairman had kept every old plank and had been slowly building another ship using these parts. The parts were arranged exactly as they were on the original ship. This ship (ship B) was eventually revealed and docked next to the other.

The questions now posed is, which is the ship of Theseus?

The primary issue here is one of identity. More precisely, it is how identity relates to the passage of time and the degree of change it may bring.

One answer would be to say that ship B is the ship of Theseus since it is composed of all the original parts. This solutions brings up an interesting question. If the identity of the ship relates to it's material parts, then at what point in time did ship A stop being the ship of Theseus? Remember that the parts of ship A were only replaced little by little over the course of many years. With this in mind would we say that ship A stopped being the ship of Theseus after that first single plank was replaced? Most would think not. Say another year passes and ten more planks are replaced. At this point there is still only one ship with eleven old planks sitting in a shed. Wouldn't most people say that ship A is still the ship of Theseus?

Ok, so lets say that ship A is the ship of Theseus. After all, it is the original ship and as we demonstrated above changing a single small part at some point in time shouldn't change it's identity. But this of course forces us to confront the fact that ship A in it's current state is not composed of a single part of the original ship of Theseus.

Of course there is no easy answer to the question but it does force one to think about some pretty deep questions. The following links provide some followup material for those who are interested.



Wikipedia: Identity and Change
Stanford Encyclopedia of Philosophy: Identity Over Time
NeuroLogica: The Continuity Problem