When I, a thoughtful and unblessed Presbyterian, examine the Koran, I know that beyond any question every Mohammedan is insane; not in all things, but in religious matters. When a thoughtful and unblessed Mohammedan examines the Westminster Catechism, he knows that beyond any question I am spiritually insane. I cannot prove to him that he is insane, because you never can prove anything to a lunatic — for that is a part of his insanity and the evidence of it. He cannot prove to me that I am insane, for my mind has the same defect that afflicts his. All Democrats are insane, but not one of them knows it; none but the Republicans and Mugwumps know it. All the Republicans are insane, but only the Democrats and Mugwumps can perceive it. The rule is perfect: in all matters of opinion our adversaries are insane.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Christian Science, ch. 5 (1907)
    (Source)

Often misattributed to Oscar Wilde.