Thanks Leeneia!

Bill D, As I understand it, only a small percentage has to be original for a piece to be considered an "authentic antique". In the case of that ax, the part of the axe that is original is the "essence" part. The replacement parts still occupy the same space that the old parts would have continued to occupy had they not worn out. And since the laws of physics tell us that two things cannot simultaneously occupy the same space, it must be so.

Or maybe it's just "if I can think it I can believe it".

I can see it now -- a sign over an Inn door, "George Washington essentially slept here because he would have if it had been built in colonial times and he had passed this way and he might have needed a night's sleep.

That's essentially the same as "George Washington slept here".