I’m assuming that this discussion does not relate to the legal definition of insane.
Maybe it is comformity with what the majority perceives to be reality. But imagine for a moment that the Theory of Everything is absolutely true. Those people who could actually perceive all 11 (?) demensions would still seem kind of crazy to the average fellow. But what if you could perceive only of three demensions plus time and most people perceived of 11 demensions? You would be considered impaired but the majority would not be considered insane.
The unsighted person cannot perceive of sight and doesn’t understand how you know, from a distance, who has possession of the football. You may even sound a little bit crazy to her when you try to explain how you know. It is outside of reality for her. Yet she knows that your ability is not a sign on insanity because the majority of people seem to have this same ability.
Who is to say that schizophrenics aren’t perceiving of a different level of reality also? (I am asking – not making a judgment call.)
I can tell you this much about mental illness. In some forms it can become extremely physically painful. I was misdiagnosed and given the wrong medication once. I assume that the reaction that I had was a psychotic one. It was the most painful episode I have ever endured. I assume that not all psychosis is like that.
And being eccentric isn’t the same as not being able to depend of your perception of reality from moment to moment.