Let us consider that we are all partially insane. It will explain us to each other; it will unriddle many riddles; it will make clear and simple many things which are involved in haunting and harassing difficulties and obscurities now.

Those of us who are not in the asylum, and not demonstrably due there, are nevertheless, no doubt, insane in one or two particulars. I think we must admit this; but I think that we are otherwise healthy-minded. I think that when we all see one thing alike, it is evidence that, as regards that one thing, our minds are perfectly sound. Now there are really several things which we do all see alike; things which we all accept, and about which we do not dispute. For instance, we who are outside of the asylum all agree that water seeks its level; that the sun gives light and heat; that fire consumes; that fog is damp; that six times six are thirty-six, that two from ten leaves eight; that eight and seven are fifteen. These are, perhaps, the only things we are agreed about; but, although they are so few, they are of inestimable value, because they make an infallible standard of sanity. Whosoever accepts them him we know to be substantially sane; sufficiently sane; in the working essentials, sane. Whoever disputes a single one of them him we know to be wholly insane, and qualified for the asylum.

Very well, the man who disputes none of them we concede to be entitled to go at large. But that is concession enough. We cannot go any further than that; for we know that in all matters of mere opinion that same man is insane — just as insane as we are; just as insane as Shakespeare was. We know exactly where to put our finger upon his insanity: it is where his opinion differs from ours.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]
Christian Science, Book 1, ch. 5 (1907)
    (Source)