The Ship of Theseus

The Ship of Theseus (but which one?)

The wandering adventurer of Greek myth, Theseus, has had many an epic voyage in his time, but has decided to call it a day. Among the possessions to be stored away is his magnificent ship, which has seen better days. Theseus asks his followers to repair the ship, and restore it to its former glory by replacing all the planks of wood out of which it is made. So one by one, the old planks are replaced by new planks, and then piled up in a heap. Finally, the work is done, and Theseus’ followers invite him to survey the restored ship. But something is not right; Theseus ponders a while, and then says ‘well, it looks very nice and all, but I don’t really see in what sense this is MY ship at all - for this is an entirely new ship, which has no material in common with my old, beloved vessel.’

The philosophical problem raised by this story is the following: if an object is replaced, part by part, until it is composed of a set of entirely new parts, is it still the same object? Here, we are using the example of a ship, but the problem is similar to the famous question of personal identity - since the majority of our bodily cells are completely replaced in the course of 15 or so years (let’s assume), in what sense am I the same person as I was 15 years ago? (Personal identity, it should be noted, is more complicated, since matters concerning consciousness and psychological continuity muddy the waters).

The issue in cases such as the Ship of Theseus is about persistence and parthood - that is, what it takes for the same object to exist over time, and retain its identity if its parts are changed. In the Ship of Theseus example above, the problem can be sharpened when we add to the story that Theseus’ followers took the old, replaced planks, and rebuilt the old Ship of Theseus exactly as it was before replacement. Now we have two ships - but which one is the ship of Theseus?

1. Consider the original ship. It was replaced, plank by plank, by new wood, and then rebuilt from the old planks. Perhaps this is the ship of Theseus, which simply disappeared out of existence, and then re-appeared again as soon as it was rebuilt. But this is absurd - for this leads to the acceptance that as soon as one plank was replaced, the Ship of Theseus lost its identity - either that, or it lost its identity arbitrarily at some point in the process, on account of one particular plank being replaced. This is obviously not true. If an object loses a part - for example, if a table loses a leg - then it is still the same table as it was before it lost the leg.
2. Consider the new ship. Perhaps this is the ship of Theseus. However, this cannot be, as we cannot say for definite at what point this new ship came into existence. Was it after the replacement of one plank? Two planks? The object that is made up of half old planks, half new planks (i.e. the ship halfway through repair) has a claim to be both the old and the new ship. And the new ship when completed shares no material with the old ship at all, which was the source of Theseus’ original concern.

This paradox arises because of the intuition that no single physical object can have two spatial locations at once. For this reason, we cannot simply say that both ships are the Ship of Theseus. It also seems odd to say that the original ship popped out of existence as soon as it began to be replaced. Metaphysical problems concerning the nature of an object’s identity, and to what extent that identity depends on physical make-up, are raised in consideration of this problem.