Originally Posted by whswhs
Numerical identity is being, for example, the same physical entity.
When I go to a game, and take out my copy of GURPS Powers, and some of my players take out their copies, and they are borrowed by players who don't own copies, and at the end, I ask, "Is this my copy?"—I'm asking about numerical identity. They're all copies of the same book, so they all have the same qualitatitive identity. But each copy has a different numerical identity.
In some cases, they might have subtle differences that do not change their qualitative identity; for example, my copy is autographed by Steve Jackson. In others, if it mattered, we could create such differences to keep track of which was whose copy: I could put a red tag on mine, and my players could put other tags on theirs. In still other cases, we might not have such signs to refer to, but if we had videotaped the session, a careful examination of the videotape could reveal that each book followed a distinct timelike path through space-time.
But even if we didn't do that, and had no way of telling which book was whose, it would not happen that my book, which was being read by JPS, and JT's book, which was being read by MB, who was sitting next to JPS, would suddenly change numerical identities, so that it would be JT's book in JPS's hands and my book in MB's hands. Numerical identity of macroscopic objects does not hop around like that.
Bill Stoddard
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