A) I know that this is true.
B) I think this is most probably true.
C) This may be true, but it also may not.
D) This is probably untrue.
E) This is absolutely not true.
Let's take something uncontroversial. For instance, whether or not there are polar bears in Antartica. I don't know that there are no polar bears in Antartica. I have not been to Antartica, and I certainly haven't done an exhaustive search of it. I wouldn't think someone to be irrational if they said there are polar bears in Antartica. If someone rational told me s/he saw a polar bear in Antartica, I would move from D to C or even B.
I have not seen any Objectivist treatment of this.