No, no, nothing is proved, nothing is known. And if I were to get up at this very moment and ascertain that the mark on the wall is really—what shall we say?—the head of a gigantic old nail, driven in two hundred years ago, which has now, owing to the patient attrition of many generations of housemaids, revealed its head above the coat of paint, and is taking its first view of modern life in the sight of a white-walled fire-lit room, what should I gain?—Knowledge? Matter for further speculation? I can think sitting still as well as standing up. And what is knowledge?