Not to get all psychobabble on you, but mental health experts have established that compulsive, pathological liars do tend to believe the stories that they tell, which is what makes them so effective at lying, at least when it comes to bamboozling their disciples. They don't hold two opposite thoughts in their head—as in, (1) I'm saying something I know is wrong, and (2) I know the truth in my head is different from what I'm saying—because it creates all kinds of mental stress from which they would eventually break down before long as a result. On the other hand, they have no trouble replacing an old inconvenient truth with a new convenient truth, and then discarding the old one while embracing the new one completely. The key is having only one story, one "truth", in their head a a time. Calling it a form of task-switching, I suppose. But I have no doubt that Trump believes whatever truth he has in his head at any given time, and that he couldn't reason or logic his way out of it even if he wanted to, which he doesn't, because he's mentally ill rather than evil.