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.

Hm?

I'm not sure I understand.

I would think that disproving the nonexistence of something would be the same as demonstrating the existence of something. That seems doable, on a sense at least.

And I would think that disproving the existence of something could be done by deriving a contradiction from the assumption that the thing exists. This also seems possible.

I'm guessing I am misunderstanding you in some way. Can you help me understand?

Methinks you had a double-negative too many. Your sentence says it's impossible to prove the existence of something. Unless that's what you had intended, in which case I don't see how that's useful philosophy for this discussion.