I have known three "mad" people, i.e. people with diagnosable medical disorders which at times unfit them for life outside an institution, well; two of them I have given house room to during difficult periods. I have worked in a building on a corner with a resident homeless man who talked to ghosts. I was in an intimate relationship with an individual who I now believe, based on his behavior during and after the relationship, to be diagnosable, though he never lost his capacity to function at a socially acceptable level. And I know a lot of functional people - myself included - whose behavior patterns fit into clear diagnostic categories, but are not extreme enough to call madness.
Madness is not a function of belief. We all believe a combination of what we are raised to believe, what is convenient to believe, and what seems to us to present convincing evidence. Since we process evidence differently and have different experiences informing our notions of how the world works, most of us believe something that appears nonsensical to the person next to us. Objective truth is not obtainable, due to the limitations of our sensory and interpretive apparatus. We process things subjectively and do the best we can, which, most of the time, is good enough.
Madness occurs when your experience of the world diverges so far from objective reality that it interferes with your ability to cope; when you start to experience yourself as a metaphor, talk to ghosts in preference to talking with flesh-and-blood humans, cannot distinguish between yourself and the exterior world. It can manifest through your belief system, but many people with exactly similar belief systems will do just fine.
Madness frequently is not an intellectual problem at all, but an emotional one. Depression and high intelligence often go together, and a self-aware, intelligent "crazy person" (two out of my three cases cited above) can learn techniques to keep functioning in circumstances that would turn most people into a quivering wreck. Many people, in fact, derive psychological benefit from irrational beliefs, though these beliefs also make them vulnerable to exploitation.
The belief that a neat, tidy generalization will cover all cases, for example, is as irrational, and as demonstrably wrong, as anything in Beckjord. Yet people make these generalizations and stick to them, sometimes to the detriment of themselves and society, without crossing the line into madness.
90% of everything is crap. So?
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