Much madness is divinest sense
To a discerning eye;
Much sense the starkest madness.
’T is the majority
In this, as all, prevails.
Assent, and you are sane;
Demur,—you ’re straightway dangerous,
And handled with a chain. (Emily Dickinson, "Much Madness")
Since One Flew over the Cuckoo's Nest, it's been something of a cliche that the crazy people are sane and the sane crazy-- but I do think there's a truth there that, as cliche as it has become, could be better understood. Because what makes us supposedly sane is a filtering system that blocks out the real, because none of us can bear too much reality. I am quite content to live in a consensus reality in which I, for instance, don't see or hear dead people. But I have no problem believing that some people do. Whether they're sane crazy or not depends on how they handle it.
Sanity has always been in large measure measured by one's capacity to accept and work within the consensus reality of one's time and place. Wisdom among the sane is that Tom Sawyerly ability to understand how the consensus reality works, to accept it for what it is without much thinking about it, and to work it to one's advantage. Mental illness is measured by one's incapacity to work within the consensus reality. I'm not here to tell you that there is some virtue in mental illness, but simply to acknowledge that the more ordinary syndromes we call mental illness--compulsive disorders, obsessive thinking, severe levels of anxiety and depression--are a function of breakdowns in the filtering system the consensus reality establishes, the psyche's freaked-out response to the leak, and its struggle to patch it for fear of the flood that will drown it.