That may be the border of my english-understanding, but i can prove non-existence of certain things (are they entities?).

Since this is currently my seminar-research topic: In the Zermelo–Fraenkel set theory there is only one Urelement.

I can prove the non-existence of another Urelement not equal to the Urelement.

Since i only care about Zermelo–Fraenkel, i can define Urelemente as things inside the Zermelo–Fraenkel set theory that are not sets themselves but elements of a set. And there is only one element that satisfies this definition. Every other thing is not a Urelement, since it does not satisfy the definition.