The Ship of Theseus

In his Life of Theseus, written around 100 CE, the Greek biographer Plutarch describes a ship that the Athenians kept for centuries. It was the thirty-oared galley in which, by legend, Theseus sailed back from Crete after killing the Minotaur. The Athenians preserved it as a relic, and as the old timbers rotted they replaced them with sound ones. Plutarch says that philosophers used the ship as an example in the dispute about growth: some said it was still the same ship, others said it was not. He adds that the ship stayed in Athens down to the time of Demetrius of Phalerum, around 300 BCE.¹

The story is a puzzle about identity over time. If every plank is eventually replaced, what makes it the same ship: its shape, its history, its continuous use? In 1655 the English philosopher Thomas Hobbes added a twist in De Corpore. Suppose someone collects the discarded planks and rebuilds the original. Which of the two ships is Theseus' ship? The first has continuity of repair, the second has the original matter. Heraclitus had already raised the problem with his river, and the issue extends to people, whose cells are mostly replaced over time. Locke tied personal identity to memory and consciousness rather than matter, and Derek Parfit's thought experiments in the twentieth century pushed these questions further. The ship also bears on law and ownership, for example in deciding whether a restored building is the same building.²¹

Plato's Phaedo records that the ship made a yearly voyage to Delos in honor of Apollo, and the city could carry out no public execution until it returned, which delayed the death of Socrates by about a month.¹