The universe split in two when the question was raised, and there are now two universes, one with an onion, and one without.

That's a positive claim regarding the content of your bed. There being a "not" in the assertion doesn't make it a negative claim, at least not in the way it is meant here.

@Dietl its not very positive to me!

@Metahuman But maybe for the woman it is

@Diet lol

Proving that something exists (positive assertion) is easy. You find enough evidence of the thing or the thing itself.
Proving that something doesn't exist (a negative assertion) is a little bit harder. And by that I mean impossible. No matter where you look it could be somewhere else or you are not capable to detect it for whatever reason.
(We are only speaking about empirical assertions. There is no even prime number greater than 2 for instance)

@Dietl The question of whether a thing exists or doesn’t exist might be undecidable, but in general there is no problem proving negative assertions.

Proving a Negative:
[departments.bloomu.edu]

@WilliamFleming This is not the same I meant, though. With "a negative assertion" I didn't mean any proposition with a "negative" in it but the claim regarding the existence of a thing. You can't prove that a certain thing does not exist. But you can prove that a thing does exist. We have to agree on how we use words, you know? The link you gave me is a bit imprecise with their definitions.

This exists . . . .

@THHA ?