The Ship of Theseus: the puzzle is about logic

You can answer the Ship of Theseus either way, and you will pay for it. The puzzle is not about wood. Identity obeys a rule of logic, and the two ships that could claim to be the original cannot both be it. Philosophers choose which price to pay, and the choice shows what they think a thing is. When the thing is a person, the choice stops being a game, and the profession has no majority answer.

The ship, and the second ship

Plutarch records that the ship in which Theseus and the youth of Athens returned from Crete had thirty oars, and that the Athenians kept it down to the time of Demetrius Phalereus, taking away the old planks as they decayed and putting in new and strong timber. It became, he says, a standing example among the philosophers, for the logical question of things that grow. One side said the ship stayed the same; the other said it was not the same.

The puzzle in its usual form asks whether an object remains the same when all of its original components are gradually replaced over time. In the 17th century Thomas Hobbes added the twist that gives it teeth. Suppose someone gathered up the decayed planks as they were thrown away and built a second ship out of them. Now there are two ships, and a question: which one, if either, is the ship of Theseus?

Why logic gets involved

The Stanford Encyclopedia names the two ships. Call the gradually repaired ship Replacement and the one rebuilt from the old planks Reassembly. Each has a claim: Replacement has the continuous history, Reassembly has the original wood.

Here the rule enters. Identity is, very plausibly, an equivalence relation, and one consequence is transitivity: if a is the same as b and b is the same as c, then a is the same as c. If Replacement and Reassembly were both identical with the original ship, then by transitivity they would be identical with each other. But they are two ships, standing side by side. So at least one of the following has to go:

1. Replacement is the original ship.
2. Reassembly is the original ship.
3. Identity is transitive.
4. They are two ships.

Nobody wants to drop the last two, so the live options say neither, or one but not the other, and each way of choosing carries a cost.

The main answers, and what each costs

Of the solutions, the Wikipedia article reports that according to the Stanford Encyclopedia the most popular is to accept that the material out of which the ship is made is not the same object as the ship. That is the constitution view, the one that separates identity from constitution. It handles a second puzzle from the Stanford entry as well, the lump of clay that coincides with a statue until it is squashed. David Lewis's temporal parts theory is the other well-known option.

Hobbes's own answer was not to pick a side. For him the answer depended on the name by which we ask the question: we must consider by what name anything is called when we inquire concerning the identity of it. The word "ship" can point to the form, which persists through replacement, or to the matter, which does not.

Where it stops being a game

For a ship you can say it depends what you want the word for. The Stanford Encyclopedia of Philosophy's entry on personal identity makes clear that for people the question is not so easy to defer. It says there is no single problem of personal identity, but a wide range of questions loosely connected. And on what we are, it lists, among the options, partless immaterial souls as Plato, Descartes and Leibniz thought, collections of mental states, "bundles of perceptions" as Hume said, parts of animals, and the claim that there is nothing that we are. Its verdict: there is no consensus or even a dominant view on this.

The profession's own numbers say the same. In the 2020 PhilPapers Survey, 43.65% of professional philosophers accepted or leaned towards a psychological view of personal identity, 19.07% a biological view, and 14.86% a further-fact view. No answer reaches half, and nearly a third chose none of the three. The shape of the Ship of Theseus, a fight over which kind of continuity counts, is the shape of the argument about you.

Three thinkers show how far apart the answers go. In the Stanford Encyclopedia's words, Locke said a person is a thinking intelligent being that has reason and reflection and can consider itself as itself, the same thinking thing, in different times and places. Hume held that we are collections of mental states, bundles of perceptions. And Parfit, on our map, is placed with a reason we wrote beside him: that personal identity is not what matters, since no deep further fact holds a person together. Hume and Locke have pages too. The map's placements are our own, not survey results.

Our view

We think the right way to hold the Ship of Theseus is to separate the logical question from the verbal one. The logical question is real: transitivity forces a loss, and every answer has to say which. The verbal question, which of the planks makes a ship, is where people argue past each other, and it is settled by what the word has to do in a given case. A registry of ships asks about continuous documentation. A museum asks about original material. A sailor asks whether the ship still floats.

That gives a test for any identity puzzle: before you ask whether it is the same, ask what you need same to do. For a ship the answer can change with the job. For a person it cannot, because the jobs that same person does, responsibility, promising, expecting to wake up tomorrow, are the ones we cannot hand to a convention. We suspect that this is why the profession is so evenly divided on persons while it is comfortable giving different answers for ships.

The strongest objection is that this makes the puzzle verbal and so empty. We do not think so. The logical constraint is not about words, and each answer, drawn out, commits you to something definite, such as two objects in one place. Calling that choice a convention does not remove the cost; it only says who gets to decide.

What would change our mind is an argument for one answer that did not depend on the purpose of the question. We have not seen one in the sources we used.

Where to read next

The Stanford Encyclopedia's entry on identity over time works through the options with the logic written out, and its entry on personal identity is the best guide to the fight over persons. On this site, personal identity and permanence and change take up the questions directly. To see where your own answers place you on the map, take the walk.