Phil 89: Ship of Theseus

A useful way to begin tackling questions of personal identity is by considering the Ship of Theseus. This is a famous philosophical example, dating back to ancient times. Theseus was an Athenian hero who sailed to Crete, defeated the Minotaur, rescued some Athenian captives, and sailed them back to Athens. In his honor, the Athenians preserved the ship, and used to sail it around regularly on parade. Over time, however, various bits and pieces of the ship needed repairing. No problem. They just found some new parts, shaped just like the old parts and made out of the same materials, and made the repairs. It seems plausible that it was still one and the same ship, even after some repairs of this sort were made. This is just the ordinary sort of maintenance that all well-kept ships experience.

Now suppose that over time, every part of the ship eventually gets replaced in this gradual, piecemeal way. Is it still one and the same ship? Here it seems a bit more problematic than before to say it’s the same ship. But we might persuade ourselves that, since the replacements were all gradual and piecemeal — and since it doesn’t appear like any one of the replacements caused the original ship to cease to exist, and a new ship to spring up in its place — that yes, this is still the same ship as the Theseus’ original ship. Of course, it has undergone some qualitative changes. But that was also true of the ship when Theseus still owned it. Back when Theseus owned the ship, it was constantly getting wet, having pitch applied to it, and also having parts repaired and replaced. That’s ordinary life for a ship.

Now let’s add a new twist to the story. Suppose that one enterprising Athenian collects all the discarded parts of the original ship. As time goes by, his collection accumulates, until eventually he has all the pieces of the original ship. Now he carefully puts these pieces together, according to the design of the original ship. Next time the annual “Ship of Theseus Parade” comes around, he brings out his ship and bills it as the Real Ship of Theseus! The other Athenians are dismayed. They thought that the ship they had been carefully repairing all of these years was the Ship of Theseus.

Who is right? Which ship is one and the same as the original ship? The carefully repaired ship? Or the ship built out of all the discarded pieces?

This puzzle raises interesting issues about the identity of ships (and other human artifacts, and perhaps physical objects more generally) over time. Can an object survive the replacement of some of its parts? Can it survive the gradual, piecemeal replacement of all of its parts? Can it survive being disassembled and rebuilt according to its original design? (As the ship built out of the discarded pieces purports to have done.) And so on. Some of these questions go beyond what we can pursue in this class. But other questions raised by the Ship of Theseus puzzle have counterparts for persons.

Epistemological vs Metaphysical Questions

When thinking about the Ship of Theseus, and about identity conditions for persons, it is important to keep epistemological questions separate from metaphysical questions. The metaphysical question here is: What would make this one and the same ship (or person) over time? The epistemological question is: How can we tell whether or not this is the same ship (or person)?

These questions are related. In particular, the way we answer the metaphysical question will make a big difference to how we should answer the epistemological question. Nonetheless, they are different questions, and it is important not to confuse them. For instance, suppose that we decide to answer the metaphysical question by saying that, if the new ship is built out of all the same planks and pieces as the original ship, then they are one and the same ship — even if the ship has been disassembled in the meantime. In that case, we would still be left with the question, How can we tell whether these are the same ship? How do we go about determining whether or not the new ship is built out of all the same planks and pieces as the original ship?

Similarly, when we’re talking about whether it would be possible to “store” your personality on tape, and then revive you in a new body, or as a sentient computer, we also need to distinguish the metaphysical questions:

Could such-and-such an individual be the same person as you? What would make it the case that he or she was, or was not, the same person as you?

from the epistemological questions:

Who would be able to tell? How would they know?

If some of the crucial evidence gets lost, it might be that some future person is the same person as you, even though no one (including you) is in a position to know this.

As with the Ship of Theseus, here too the metaphysical questions and the epistemological questions are closely related. The way one answers the metaphysical question will make a big difference to how we should answer the epistemological question. Nonetheless, they are different questions, and it is important not to confuse them.

Case by Case

When we discussed the Ship of Theseus puzzle in class, I introduced a number of “base cases” where I supposed it would, at least initially, seem very plausible to everyone that there’s a single ship that undergoes some changes and then is still around. The ship after the change is numerically the same as the one before the change.

Case 1
The ship undergoes no intrinsic change, but drifts across the sea, changing its location
Case 2
The ship raises its flag, changing its shape
Case 3
The ship loses one small part
Case 4
The ship has one small damaged part replaced
Case 5
The ship is separated into two (to go under a bridge), and then reassembled afterwards

After thinking about the puzzle, of course, you might decide to come back and change your mind about some of these base cases. But I expect that even after careful reflection, many of us will still want to say that in these initial examples, it could still be a single ship throughout.

Next we had two examples where the changes were more extensive:

Case 6
One by one, all of the ship’s parts are gradually replaced (as in Case 4). The removed parts are destroyed.
Case 7
One by one, many small pieces of the ship are dismantled (nothing is put in their place) and taken to a different location, where the ship (or anyway a ship which is intrinsically just like the original) is eventually reassembled. This is like Case 5, but with more and smaller pieces.

The full-blown Ship of Theseus puzzle is what we get when we combine Cases 6 and 7:

Case 8
The original ship is gradually repaired, as in Case 6. But the parts aren’t destroyed, they’re set aside in a junkyard. Some entrepeneur collects all the pieces and reconstructs a ship just like the original, as in Case 7. Which of the two ships at the end of the story (if either) is the ship that’s numerically identical to the original ship? Of which of them can we truly say “This ship used to …,” citing adventures the original ship took part in?

Real-life Examples

I don’t know whether Theseus or his ship really existed. But there are some real-life cases that approximate parts of this story (at least as far as Cases 6 and 7).

Here are a few: