I remember discussing this question in my philosophy class. The argument was "is a key still a key if theres nothing to unlock" first we established that if it is in fact a key, then its sole purpose is to be used to unlock perhaps a door or locket. If neither of those objects existed or if either lock ceased to exist, then the key would have no purpose, therfor would no longer be a key since it had nothing to unlock. I argued this and said that cannot be possible for a purpose to define an object. The key itself would still be a key, because it once had the ability to unlock said locks. If the key was designed to unlock nothing, then it would not be a key, but it is in fact established, that it is already a key. So all thats left in my argument is that the locks that ceased to exist do not change the fact that the key is still a key, for it once was able to unlock a lock.

What do you think. Perhaps you coud extend the theory on this?