The Ship of Theseus

Plutarch reports, in his Life of Theseus, that the Athenians preserved the ship in which Theseus returned from Crete, replacing the timbers as they rotted. The case became the standard example because the replacement is gradual, ordinary, and entirely reasonable, and the question it raises has no comfortable answer.

Why the first version is not yet hard

Taken on its own, gradual replacement is manageable. Nobody thinks a car stops being that car when the tyres are changed, or that a person stops being that person as cells are replaced. Whatever makes a thing the same thing over time, it is not sameness of matter.

What can be offered instead is a condition that the repaired ship satisfies easily.

The ship in the harbour has this. It sat in one place while a plank was swapped, then another, and at no point did it vanish or leap. The reasoning is the same reasoning applied to the car and to the person, and by itself it looks like a satisfactory answer.

Hobbes's addition

Thomas Hobbes, in De Corpore, made the case genuinely difficult by supposing that someone kept the discarded planks. Once every original timber has been removed, they are assembled in their original arrangement, and there are now two ships.

The argument is valid, so at least one premise is false, and the difficulty is that all three have something going for them. The third is the least negotiable: identity is transitive, and two things in different places cannot be one.

The available answers

Four responses cover most of the literature, and each gives up something.

Continuity wins. The repaired ship is the original and the reassembled one is a replica, however good. The cost is explaining why the original matter, correctly arranged, does not carry identity with it. The cost is sharper if the reassembly is imagined happening first, with the repairs following, since the verdict would then feel different for no principled reason.

Matter wins. The reassembled ship is the original and the repaired one is a successor. The cost is that the same reasoning applied to a person makes you cease to exist as your material is replaced, which almost nobody accepts.

Neither wins. Once there are two equally good candidates, neither is the original, and the ship of Theseus ceased to exist at some point during the process. The cost is that whether the ship survived the repairs now depends on what someone did with the discarded planks in a shed elsewhere, which makes the ship's continued existence turn on something entirely external to it.

The question is defective. There is no fact about which ship is the original, and the case shows that our concept of a ship was never determinate enough to settle it. The cost is admitting that ordinary object concepts run out, which is a real cost even though many philosophers are willing to pay it.

What the case is used for

Its importance is as a test rather than as a problem to be solved. Any account of persistence can be run against it, and the account's verdict shows what it is actually committed to.

An account on which objects are extended through time gives a clean-looking answer: there are two four-dimensional objects overlapping in their earlier parts, and the question of which is the original is a question about which of two overlapping things a name was tracking. An account on which objects are wholly present at each moment has to choose among the four options above. A view on which ordinary objects are not part of the fundamental inventory at all can say that the question concerns how we group particles and has no answer beyond our conventions.

Each of these is a substantive position with consequences well beyond ships, and the case is valuable precisely because it forces them into the open.

The next article leaves identity and turns to the relation that has been assumed throughout without examination.