This is really a Catch-22, isn't it? I'm referring specifically to what Frater 219 says at the end of his/her writeup: "...'mental illness' is not an objective judgment of someone's ability to perceive or predict reality. It is rather a medical term referring to certain syndromes of socially aberrant thought or behavior."

First of all, we have to determine what is "socially aberrant." To do that you have to establish a norm from which behavior can deviate. An established norm is basically a belief, isn't it? I.e., "I (or we , the society as a whole) believe that if a person does X they are normal, and if they do Y they are mentally ill." If, as a whole, we all believe in these constructs, then, according to this writeup, we are all mentally ill, simply because of our belief.