This leads to absurdity. There are infinite non-existances, but finite existences (modulo the infinite of the universe, but the observable universe/the ~13.7 billion lightyear bubble around the Earth or so is finite). To assert that you have a positive belief that things that do not exist do not exists is to assert that you have infinite positive beliefs in the non-existant. To encode these infinite positive beliefs in reality would require infinite storage space, even in something as powerful as the human brain. I instead assert that you have positive beliefs in things that exist and, only upon it being necessary, do you assert the factuality of non-existences in an ad hoc as needed basis as the result of the conjugation of your positive beliefs of existence.