If existence is taken as betokening thisness and thereness, then nonexistence is going to have, speaking informally, this problem: It obliges us to speak of a nothing. If a nonexistent object were always like a footprint in the sand, we might refer to it by its mold, its negative place. But usually the world closes up without much trace around things that have passed their time and ceased to exist, and often there is not even a world left to hold the mold — think of extinct dodos and never existent unicorns; there is no empty niche left in our ‘real’ world for the former and there never was — some say — one for the latter. What kind of focus allows us then to speak of things that are definitely and determinately nowhere and not now and not ever? What, if anything, is it we are referring to when we say: This does not exist?
— Eva Brann, The Ways of Naysaying, 2001