What ideas come in your mind when you read this, "What the majority sees as sanity may in fact be madness?"
This is a very interesting question. It gets to the heart of some important social theory. Many sociologists have already made the point that all knowledge is socially constructed. This is to say that what we deem as right and wrong or sane and insane is colored by our society.
Here is a famous quote by M. Foucault, which gets to the heart of what we are discussing:
"Each society has its régime of truth, its ‘general politics’ of truth: that is, the types of discourses which it accepts and makes function as truth; the mechanisms and instances which enable one to distinguish true and false statements, the means by which each is sanctioned; the techniques and procedures accorded value in the acquisition of truth; the status of those who are charged with saying what counts as true."
In light of these words, we can say that society may shape a person's thoughts so much that what is insane is made to be normal and what is sane seems insane. For example, think of how many people starve themselves to the point of sickness to be thin. Why? Society says being thin is beautiful. Think of how many people work to the point of death to make money! This is sane in our world, but in the end, it is insane. I think you get the picture.