Returning to the Beginning
“I should believe this” is not a property of a statement or an argument. It’s a label that we apply to statements or arguments, a categorical container that we put them in, once we’ve evaluated them according to certain criteria.
The whole point of having those criteria is to distinguish between correct and incorrect belief, to the best of our ability.
Whether we already believe or disbelieve the statement is irrelevant. The evaluation is supposed to help us determine whether we’ve chosen correctly; it would be absurd to presume that our choice was right before we evaluated it.
The fact that we believe (or disbelieve) something cannot reasonably be part of the criteria, because their purpose is to answer a question, and part of the act of questioning is admitting to uncertainty about its answer. We want to check our judgment, not presume that it’s correct.