A friend of mine has a son who is a long time resident of a mental institution. The son hears things that no one else hears, sees things that no one else sees, and, because he believes his senses, acts on the "information content" to the detriment of anyone in his vicinity.
Do you have any way of demonstrating to an objective observer that what you "know" is any different than what he "knows"?
No balrog666. This is something that cannot be demonstrated, only experienced.
But I have observed such a thing as you describe -- people undergoing hallucinations, hearing voices, etc., etc. (I worked two summers as a nursing assistant in a state mental institution when I was attending college.) One such person was convinced he was Napoleon.
I can't prove to your satisfaction (I'm sure) that what I've attempted to (imperfectly) convey is qualitatively any different than what is happening with your friend's son.
You'll just have to take my word on it, and then maybe try to set up the conditions that will permit you to have your own experience of what I'm talking about. You either prove it to yourself, or there's no proof.
In short, it does not appear clear to me that such people had a strong enough self concept to understand their experience as something that could be understood. It was something that just "happened."
Do you think this makes any difference?