Joe claims: "There is no proof that unicorn exist, therefore unicorns do not exist".

Alice claims: "There is no proof that unicorns do not exist, therefore unicorns exist".

Bob claims: "There is no proof either way, therefore I'm agnostic about the existence of unicorns".

From a formal point of view, Joe and Alice's claims are both arguments from ignorance, hence they are unsound. Bob's argument is sound.

But we all intuitively know that Joe is right (Or at least probably right) despite the apparent formal symmetry between Joe's claim and Alice's claim.

So, there must be something asymmetric that makes Joe's claim stronger. But what is it? Where does the assymetry stem from?

Related term: Russel's teapot