You can only prove what is, not what isn't
This argument is used in the debate Does God exist?.
This argument is an objection to Nothing proves the non-existence of God.
Summary
Quotes
“At best, we can only prove what is, not, on an infinite scale, what is not. Nothingness, by definition, is without effect. How could it not be without proof? I can certainly prove, with a little luck, that I didn't commit the act of which I'm accused: all I have to do is make it impossible, for example by proving that I was a thousand kilometers away when the crime was committed. That's what we call having an alibi. An outside witness is enough. But there is no alibi for nothingness, and no external witness for everything. How can we prove non-existence? Try, for example, to prove that Santa Claus doesn't exist, nor vampires, fairies, werewolves... You won't succeed. That's no reason to believe it. On the other hand, the fact that their existence has never been proven is a strong reason for refusing to believe in them. The same applies, all things considered (I admit that the stakes are higher, the improbability less), to the existence of God: the absence of proof is an argument against any theistic religion. If this is not yet a reason to be an atheist, it is, at the very least, a reason not to be a believer.”
