The Ship of Theseus

What it tests What makes a thing, or a person, the same thing over time when every part of it changes?

The case

Plutarch, writing around 75 CE, reports that the Athenians preserved the thirty-oared ship in which Theseus returned from Crete, removing old timbers as they decayed and putting sound new ones in their place. The ship, he says, became a standing example for philosophers arguing about growth: some said it remained the same ship, others that it did not.

Thomas Hobbes, in De Corpore (1655), made the puzzle much harder. Suppose someone kept every plank as it was removed and later reassembled them, in the same order, into a ship. Now there are two ships. One has continuity of form and history with Theseus’s ship; the other has all its original matter. Both have a claim to be the original, and they cannot both be, since there are two of them.

What it tests

The ship asks what the identity of a thing over time consists in. Its parts change, so identity cannot simply be sameness of parts. But if gradual replacement preserves identity, it seems to preserve it however much is replaced, and then the reassembled original looks like an impostor, which is hard to accept.

The main answers

Continuity wins. Identity over time consists in continuous existence under the same form, with parts changing gradually. The repaired ship is Theseus’s ship; the reassembled one is a new ship made of old wood. This matches how we treat organisms, rivers and institutions.

Matter wins. A ship just is its timbers, arranged as a ship. The reassembled ship is the original, and the repaired one gradually became a replica. This explains our intuition that a museum should want the original planks, but it makes nearly every living thing a succession of different objects.

Both, in different senses. “Same ship” can mean same functioning vessel or same material object, and the paradox trades on the ambiguity. On a more radical version, held by some metaphysicians, two objects shared the same planks for a time, and the puzzle is about which object a name picks out.

No fact of the matter. Once we know everything about the planks, the repairs and the reassembly, there may be no further fact about which ship is “really” Theseus’s. The question is then a decision about how to use words, and nothing deep depends on it.

Why it matters for persons

Replace planks with cells, memories and traits, and the ship becomes a puzzle about you. Teleporters that destroy the original, brains split between two bodies, and minds uploaded or copied reproduce the structure of Hobbes’s two ships. Derek Parfit argued in Reasons and Persons (1984) that in such cases identity may be indeterminate, and that what matters in survival is psychological connectedness and continuity, which can hold to degrees and can branch.

Buddhist philosophy reached a related view much earlier. In Nāgasena’s chariot, the chariot is nothing over and above its parts in their arrangement, and the self is no different.

Why it matters for AI

For software, the hard versions of the puzzle are not imagined. A model can be copied onto many machines, paused and resumed, rolled back to an earlier state, fine-tuned into a variant, or merged with another. Each operation is a Hobbesian case. Whether the resumed system is “the same mind”, and whether anything morally important depends on the answer, is taken up in If a mind can be copied, paused and resumed, is it the same mind?, and you can mark the point at which you think the original is gone in the lab’s digital Ship of Theseus.