Imagine yourself in ancient Athens. The great Greek hero Theseus died two hundred years before, but his legendary ship is still kept in the harbor, as a memorial. Over the centuries parts of the ship have needed to be replaced. By now virtually every rope, sail, and board on her has been changed.
Is it the same ship? I don't think anyone is prepared to assert that replacing a few portions of something makes it now a different something. Does a kidney transplant or knee replacement make you someone else? Surely you are still the same person. Your car can have major portions of it replaced over your ownership, even the engine, but no one thinks it's the same vehicle only because of the serial number on it. There is something about identity that transcends the sum of the parts.
Where this gets weird, is if you picture the parts of Theseus' ship being gradually replaced, and the original parts (too precious to be destroyed) being stored in a nearby shrine. Once all the parts are replaced, though, some wiseguy steals the old parts and, under cover of a nearby warehouse, assembles the entire ship from the old parts. Which is Theseus' ship? Are they both Theseus' ship? How is that possible? One is the continuation of the original assembly (same space coordinates and a continuum in time) that Theseus sailed. The other, however, is much more accurately the same atoms that Theseus sailed. Before the assembly of the second (or is it the first?) copy, made of the original parts, there would be little argument that the one in the harbor is Theseus' ship, and the other matter was just a pile of original parts. But now that it's been assembled...
We often call a house, rebuilt on a site, even if carefully adhering to every detail, a "reproduction." It's not the same house (even though it DOES occupy the same space coordinates). If it were just repaired (like the ship) over some period of time, but stood in some measure intact the entire time, it would not be a reproduction - it would be the original. It might be called "restored" but it's the same house. So does the replacement have to be gradual? Is the essence of identity about the continuity - the fact that the majority of it existed continuously and was held in all that time to be Theseus' ship?
If that's true, how much of it needs to be the same - and how quickly can it be replaced? If more than half of Theseus' ship burned, and had to be replaced, would it still be Theseus' ship? What if all but the hull burned to the waterline, but it continued to float, and was then restored? The remaining portion of hull could be argued to be far less than half of the total ship, but I think most people would say it was still the same ship. Is there some essential "shipness" that has to be continuous for it to be the same ship? I think many people would argue that if there was a wreck, and the masts and cabin were all that came to shore, and they were used in some act of reconstruction, even if they were more than half of the ship's original matter by weight, that these parts were just used in the building of a "new ship."
So let's assume that people can be gradually replaced in this same manner, and let's grant this idea that the essential portion of an item somehow "carries" the identity. Then humans must have an essential portion, the portion of the human being that embodies the essence of the individual.
What part is it?
And is it physical?
1. (The first steps of this conversation are very old indeed, though I took it where I wanted. Look down the article and see the part about "Ship of Theseus.")
2. (Photo above is Navis, an early watercolor of mine.)
>>>> Appendix de Grenouille #9 <<<<
Bon Voyage Grenouille. This is not Theseus' Ship. For one thing they do not serve baklava mid voyage. Nor is he traveling the wine dark sea, touched by rosy fingered dawn.