It turns out St. Augustine wrote a long essay on the subject of lying in 395. So George Costanza’s “it’s not lie if you believe it!” actually goes back a little further.

For not every one who says a false thing lies, if he believes or opines that to be true which he says. Now between believing and opining there is this difference, that sometimes he who believes feels that he does not know that which he believes, (although he may know himself to be ignorant of a thing, and yet have no doubt at all concerning it, if he most firmly believes it:) whereas he who opines, thinks he knows that which he does not know. Now whoever utters that which he holds in his mind either as belief or as opinion, even though it be false, he lies not. For this he owes to the faith of his utterance, that he thereby produce that which he holds in his mind, and has in that way in which he produces it. Not that he is without fault, although he lie not, if either he believes what he ought not to believe, or thinks he knowswhat he knows not, even though it should be true: for he accounts an unknown thing for a known.

So if you believe the thing you say even though there may be incontrovertible evidence out there in the world that it is false, and you just aren’t aware of that evidence or consciously ignoring it, you are not a liar. You may still be an arrogant idiot of course.