It's almost like intangible social constructs don't really exist or something. As long as you do not know the thing in itself, how do you know it's really there?
Identities exist and are associated with the material world, but are not directly tied to it. "The king is dead! Long live the king!" illustrates this perfectly. The mortal person who has been embodying the identity of "king" has died, but the identity of "king" persists and will be embodied by a different person soon.