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.
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