Solution to the Ship of Theseus

A refresher on what the Ship of Theseus is:

The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, in so much that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.
Plutarch thus questions whether the ship would remain the same if it were entirely replaced, piece by piece.

Having read Rationality: From AI to Zombie, the solution seems trivial to me. And yet, when I take a look at the Wikipedia article I don’t see the correct solution there. The discussion there of the definition of “same” goes in the right direction but falls a bit short. I guess the solution is just too boring to be considered acceptable for such a classic philosophical problem.

So what is the solution? First you have to realize that it’s only a language problem. What do I mean by that? I mean that both parties, the one saying it’s the same and the one saying it’s not the same, agree on the state of reality. If I ask them, “Does this ship look the same as the Ship of Theseus when it was new?”, they will give the same answer (well, hopefully they do; the look of the ship was never the point of the debate). If I point to a specific plank and ask, “When was this added?”, they will also give me the same answer (assuming they know when it was added). So, both parties completely agree on the physical reality. Their disagreement is only about which words to use.

So, why is it that the choice of word (which has no effect on physical reality) sparked such a strong debate? I think the reason is something which the statistician E.T. Jaynes called the Mind Projection Fallacy.

From Wikipedia:

[The mind projection fallacy] occurs when someone thinks that the way they see the world reflects the way the world really is, going as far as assuming the real existence of imagined objects.

People in the ship debate think that if they think of the ship as “the same” or not, this corresponds to different states of reality. But actually, it’s all in their head. Both ways of thinking correspond in the end to the same configuration of atoms.

The debate is on the same level as the question of whether or not Dolphins are fish. On the one hand they look like fish. On the other hand they breath and give live birth. There are good arguments on both sides but people generally agreed it’s easier to not call them fish. One reason for that is that you can now make the statement “the reproduction of all species of fish involves eggs”. But you can’t make the statement “all mammals live on land”. Calling Dolphins fish or not, doesn’t change them. They’re still dolphins. But not calling them fish is generally more practical.

So, should we call the Ship of Theseus “the same”? Well, which one is more useful? For taxation purposes it’s the same (because the owner and the purpose didn't change). For inspection purposes by a structural engineer it probably shouldn’t be considered the same (because the replacement could have introduced structural issues that weren't there before).

The general answer is: “the current ship shares most of the design with the original, but nearly all of the materials have changed.” And that is all there is to say. I know, it's boring.