To be insane is to be out of touch with reality. But what is reality?
It is not just the majority opinion. That is the ad populum fallacy. The majority of men once thought the sun moved around the earth. A plurality of Germans voted for Hitler.
Plato’s cave analogy suggests reality is experienced by only a few; most of us may live a delusion. Buddhism asserts the same. As, arguably, does Christianity. “For the wisdom of this world is foolishness in God’s sight.” (1 Corinthians, 3:19) A more familiar concept to many may be that of the Matrix films.
And postmodernists, of course, assert that there is no reality. Everyone just makes things up.
So what is real is not self-evident. We therefore cannot use it as a standard for sanity, or else one man’s sanity becomes another’s madness. The person who experienced the world as it is might be declared insane by the deluded majority. “Men would say of him that up he went and down he came without his eyes; and that it was better not even to think of ascending.” (The Republic, Book 7, Jowett trans.)
Rather, then, than saying that sanity is knowing what is real and what is not, we might say that Sanity is the quest for what is true. Insanity is no longer caring or trying to find out. We all understand the concept of a lie, a denial of the facts or of the evidence. Insanity is believing a lie, while being at least partly aware that it is a lie.
This means that sanity is always at least in part a moral issue.