I thought I'd try to kick of some discussion with the good old Ship of Theseus.
The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.
Imagine a wooden ship in a warehouse. I'm going to try out this whole egotism thing for once (yeah, right) and call this ship the SS Kraftster. So, I've got a great boat guy I know who I have come down to the warehouse and take a look at my boat. He tells me that he sees two planks that need replaced right away, and that eventually he's probably going to have to do every dang board.
So, the following week, the boat guy comes down and replaces two of the planks on my boat. I ask you, is the boat still the same boat that it was before? Is it still the SS Kraftster?
A year goes by and my boat guy says, 75% of the planks on that boat need replaced. I hire him to do the work, and he does it. Now having replaced 75% of the planks (+2), is the boat still the same boat it was before? Is it still the SS Kraftster?
Finally, last week, the boat guy replaced all of the remaining planks on the boat, so that it is now all new wood. Is it still the Kraftster?
What I just found out, though, is that my boat guy has been taking all of the planks that he removed from my boat, and he's been storing them in a different warehouse. Once he finished all of the work, he put all of the decaying planks back together in the other warehouse in exactly the way they were on my boat. Is this seaworthy vessel in the other warehouse my SS Kraftster?