“World A: Contains nothing but a single being.
World B: Contains nothing but a single being, that is a different being than the one in World A.
Do you have any objection to either of these worlds? Each seems entirely possible.
Unless you can come up with some sort of objection to THEM, we’ve got a proof. They contain nothing in common, so there cannot be any necessary being.” Smullyan-esque, source.
Where “world” means possible world. This is an interesting counter-example because it goes through even if people find the idea of an empty world contradictory/meaningless. The only part that I find disagreeable is the unnecessary (heh) “cannot” in the last sentence but I am extremely careful with modalities.