The Ship That Gets Replaced

Plutarch reports that the ship in which Theseus returned from Crete was kept by the Athenians as a memorial, and that they maintained it by removing the rotted planks and putting in new timber, so that the ship became a standing example for philosophers on the question of things that grow. Some said it was the same ship. Others said it was not. He does not take a side, and neither has anyone else since.

The version everyone knows was sharpened by Hobbes, who added one detail that turns a puzzle into a trap. Suppose somebody collects the discarded planks as they come off, stores them in a yard, and eventually assembles them into a ship. There are now two vessels. One has unbroken use, unbroken location and unbroken name. The other is made of every original plank. Both have a claim, the claims rest on different criteria, and the criteria had never been distinguished because until that moment they had never come apart.

That is exactly what a thought experiment is for. In ordinary life material continuity, functional continuity, spatial continuity and continuity of name travel together, and because they do, we have one concept where we might need four. Hobbes pulls them apart deliberately, and the discovery is not about ships. It is that the word 'same' was doing four jobs at once and we had not noticed.

One response is to say the question is badly formed. On the four-dimensionalist view, objects are extended in time as well as space: what we call the ship at a moment is a thin slice of a long object that stretches from launch to breaking-up. The two ships then share early stages and diverge, the way a road can be one road and then fork. There is no hidden fact about which fork is the real road, only a question about how to carve. This is elegant, and its critics say it answers a different question from the one asked.

The deflationary response is blunter. Once you know the history of every plank, there is nothing left about the world that you do not know. What remains is the question of which vessel to attach a word to, and words are attached by practices. A museum cares about original material because its business is authenticity. A navy cares about continuous service because its business is capability. A ritual cares about unbroken use because its business is unbroken use. Each is right about its own question, and the appearance of a single deep question is produced by using one word for three.

Before concluding that this dissolves the problem, notice how much of ordinary life depends on persistence conditions that have nothing to do with matter at all. A company can replace every employee, every product, every building and every shareholder and remain the same legal person: the contracts still bind, the debts still stand, the pension liabilities do not evaporate. Nothing physical has persisted. What has persisted is an unbroken thread of registration and obligation, and no court finds this puzzling.

Or take a file. You copy it to a new machine, the original disk is destroyed, and every bit is in a different physical location on different hardware. It is the same file, and nobody thinks otherwise. Here identity is pattern, and the matter is explicitly irrelevant, so much so that we have built an entire industry on the assumption.

Which suggests a modest conclusion. The ship is hard not because persistence is mysterious but because ships sit awkwardly between kinds of thing. For artefacts that are essentially physical, the matter matters. For artefacts that are essentially functional or institutional, it does not. A ship preserved as a memorial is being used as both at once, which is precisely why the Athenians could not agree, and why two and a half thousand years of argument have not settled it either.