Monday, October 12, 2015

Truth and the Ship of Theseus


Last week I mentioned the famous puzzle case of the Ship of Theseus. It was first put forward by the Ancient Greeks. Nowadays it is especially used in the debate on personal identity in the analytical philosophy. And indeed, the problem I discussed in my last blog has much to do with the question of group identity; in this case the identity of a group over time. But if a group like a sports team has a continuity over time despite changes in membership, just as the Ship of Theseus remains to exist when its planks are replaced one by one, does this mean that a group exists independent of the members who make up the group? The more I think about the case of the Ship of Theseus, the more intriguing questions come to my mind. It casts even doubt on one of the basic assumptions of classical logic, namely the law of excluded middle and double negation. The first part of this law says that for any proposition either it is true or its negation is true. The second part says that a statement cannot be true and not true at the same time.
To repeat, the case of the Ship of Theseus involves that the vessel is repaired by gradually taking out the old planks one by one and putting in new. Do we then still have the same ship at the end? Of course, Theseus, the owner of the ship who has commissioned the repair, will say “yes, we have”, and we’ll certainly agree if the old planks are destroyed. But suppose that someone has stolen the planks before they could be destroyed, builds a new ship with these planks (and only with these planks) and paints the name “Ship of Theseus” on it, just as on the original ship and on the one repaired by order of Theseus. Which ship is then the real Ship of Theseus? Say that just after the reparation has been finished, a fire completely destroys the ship with the new planks. Then the plank-hoarder appears and says: No problem, I have saved the ship for I have reconstructed the Ship of Theseus with the old planks. I think that it would be absurd to deny the truth of this claim. For if the Ship of Theseus would have been taken apart by taking away the planks one by one, storing the planks for a year, and then rebuilding the ship with the old planks, we would say the same. However, if we had replaced all old planks by new planks and would have destroyed the old planks, we would say that the ship with the new planks was the real Ship of Theseus. So whether the ship made of the old planks is the Ship of Theseus or whether the new ship is depends on the history of the building of the Ship of Theseus and on what has happened with the planks.
Suppose now that the repaired ship hasn’t caught fire and that the old planks haven’t been destroyed but used by an antique dealer to rebuild the Ship of Theseus. Theseus would say then, with right, that the old planks have been stolen and that the repaired ships is the real Ship of Theseus. But the antique dealer maintains that he wanted to save the original Ship of Theseus. Then he can also say with right that his ship is the real Ship of Theseus. As Noonan says about this case: “The identity statement in question is at worst indeterminate in truth-value” (p. 132). The problem can be solved by letting the truth of a proposition depend on who utters it (either Theseus or the antique dealer in my case), but classical logic does not allow this possibility: Either Theseus’ ship or the antique dealer’s ship is the real Ship of Theseus.
But suppose now that Theseus didn’t know that the old planks have not been destroyed and that they were used for rebuilding his original ship. Theseus puts out to sea with his repaired ship and he runs across the antique dealer with his ship. Theseus falls into a rage, when he sees another ship with the name “Ship of Theseus”, and he attacks it with his boat. We get a sea battle and one ship is destroyed and sinks. Then there are good reasons to call the winning ship the real Ship of Theseus, at least from then on. So the result of the battle determines which ship is the real Ship of Theseus. In other words, the truth of the proposition “The Ship of Theseus is identical with the ship constructed of the new planks” depends on the result of the battle. If Theseus wins it is true. However, this doesn’t involve that if the antique dealer wins this proposition is false. Moreover we have then the intriguing question, whether Theseus changes his opinion, in case he loses the battle. Maybe he considers from then on the ship rebuilt with the old planks again the real Ship of Theseus.

Source: Harold W. Noonan, Personal Identity. Second Edition. London: Routledge, 2003

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