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. According to legend, 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 or restoring. No problem. They just found some new parts, shaped just like the old parts and made out of the same materials, and carefully swapped them in where needed. It seems plausible that it was still one and the same ship, even after some replacements 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 that 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’s original ship. Of course, it has undergone some qualitative changes. But that was also true of the ship when Theseus still sailed it. Back in Theseus’s day, the ship was constantly getting wet, having pitch applied to it, and having parts repaired and replaced. That’s ordinary life for a ship.
Now we 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 maintaining and restoring all these years was the ship that Theseus used to sail.
Who is right? Which ship is one and the same as the original ship? The carefully refurbished ship? Or the ship reconstructed 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 dismantled and rebuilt according to its original design? (As the ship reconstructed 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.
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 could and 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 a future 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 that ship will then have been dismantled 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 a future 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 in a file, and then revive you in another body, or as a sentient computer, we also need to distinguish the metaphysical questions:
Could such a future 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 if it were the same person? How would they know?
If some of the crucial evidence gets lost, it could turn out that some future person is the same person as you, even though no one (including you) is in a position to prove or 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 could and should answer the epistemological question. Nonetheless, they are different questions, and it is important not to confuse them.
When we discussed the Ship of Theseus puzzle in class, I introduced it by a number of “stages” where I expected it would, at least initially, seem very plausible to you 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.
After thinking about the puzzle, of course, you may 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 some examples where the changes were more extensive:
Perhaps in Case 5, we may be reluctant to count the future ship as still being the same ship as the original. If there’s a disaster that destroys the original ship, we might build a replica and call it “The Ship of Theseus,” but this won’t anymore be the same ship that Theseus used to sail. It will be a copy.
But many find it plausible that in the other cases, it would still be the same ship. Yes, it’s changed in some ways. But there was a ship that Theseus used to sail and then was temporarily dismantled; or that had this plank removed and replaced with some new wood; and so on. The future ship is that ship.
The full-blown Ship of Theseus puzzle is what we get when we combine Cases 3c and 4b:
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?
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 Case 4b).
Here are a few: