Perhaps a lunatic was simply a minority of one. At one time it had been a sign of madness to believe that the Earth goes round the Sun; today, to believe the past is inalterable. He might be alone in holding that belief, and if alone, then a lunatic. But the thought of being a lunatic did not greatly trouble him; the horror was that he might also be wrong.
George Orwell, 1984
insanity, lunacy, lunatic