"And how," asked Uncle Tim, "is all this to be found out?" "By digging down to the first principles," answered the Doctor. "Ay," interrupted Malachi, "there is nothing equal to the spade and pickaxe." "That is true," said my grandfather, going on in Malachi's way, "'tis by digging for the foundation, that we shall find out whether the world exists or not; for, if we dig to the bottom of the earth and find the foundation--why then we are sure of it. But if we find no foundation, it is clear that the world stands upon nothing, or, in other words, that it does not stand at all; therefore, it stands to reason--" "I beg your pardon," interrupted the Doctor, "but you totally mistake me; I used the word digging metaphorically, meaning the profoundest cogitation and research into the nature of things. That is the way in which we may ascertain whether things are, or whether they are not." "But if a man can't believe his eyes," said Uncle Tim, "what signifies talking about it?" "Our eyes," said the Doctor, "are nothing at all but the inlets of sensation, and when we see a thing, all we are aware of is, that we have a sensation of it: we are not aware that the thing exists. We are sure of nothing that we see with our eyes." "Not without spectacles," said Aunt Judy. "Plato, for instance, maintains that the sensation of any object is produced by a perpetual succession of copies, images, or counterfeits, streaming off from the object to the organ of sensation. Descartes, too, has explained the matter upon the principle of whirligigs." "But does the world exist?" asked the Schoolmaster. "A good deal may be said on both sides," replied the Doctor, "though the ablest heads are for non-existence." "In common cases," said Uncle Tim, "those who utter nonsense are considered blockheads." "But in metaphysics," said the Doctor, "the case is different." "Now all this is hocus-pocus to me," said Aunt Judy, suspending her knitting-work, and scratching her forehead with one of the needles, "I don't understand a bit more of the business than I did at first." "I'll be bound there is many a learned professor," said Uncle Tim, "could say the same after spinning a long yarn of metaphysics." The Doctor did not admire this gibe at his favorite science. "That is as the case may be," said he; "this thing or that thing may be dubious, but what then? Doubt is the beginning of wisdom." "No doubt of that," said my grandfather, beginning to poke the fire, "and when a man has got through his doubting, what does he begin to build up in the metaphysical way?" "Why, he begins by taking something for granted," said the Doctor. "But is that a sure way of going to work?" "'Tis the only thing he can do," replied the Doctor, after a pause, and rubbing his forehead as if he was not altogether satisfied that his foundation was a solid one. My grandfather might have posed him with another question, but he poked the fire and let him go on. "Metaphysics, to speak exactly----"