Let's say, what we in the western world would class as a "mad man" has delusions, sees things that no one else around him can, hears voices only audible to himself, to him it is totally real, it is his reality. If his "madness" spread and became visible/audible to others, it would soon become reality to them also. Reality is only what you're experiencing at any given time and can change quite drastically. Like I said it is accepted only because so many experience it at the same time. In some cultures these "mad men" are revered, "shaman/holy men" if you will...just a train of thought...
Yokel