I understand HOW: I do not understand WHY.
He wondered, as he had many times wondered before, whether he himself was a lunatic. 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 that 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.