Miss Martineau in her Autobiography tells us that as a child of ten she entirely and unaccountably failed to see a comet which was visible to all other people; but, although her own senses were at fault, the evidence for the comet was so conclusive that she may be said to have _known_ there was a comet in the sky. On sufficient evidence we can know anything, just as we know there is a great water-fall at Niagara though we may never have crossed the Atlantic. But we cannot be so certain simply on the evidence of our senses. If we trusted entirely to them we might take the earth for a plane and “know” that the sun moved round it. § 8. But Locke probably considers as the subject of knowledge not so much physical facts as the great body of truths which are ascertained by the intellect. It is the eye of the mind by which alone knowledge is to be gained. Of these truths the purest specimens are the truths of geometry. It may be said that only those who have followed the proofs _know_ that the area of the square on the side opposite the right angle in a right-angled triangle is equal to the sum of the squares on the other sides. But even in pure reasoning like this, the tiro often seems to see what he does not really see; and where his own reason brings him to a conclusion different from the one established he _knows_ only that he is mistaken. § 9. It must be admitted then that first-hand knowledge, knowledge derived from the vision of the eye or of the mind, is not the only knowledge the young require. Every learner must take things on trust, as even Lord Bacon admits. _Discentem credere oportet._ To use Locke’s own words:—“I do not say, to be a good geographer that a man should visit every mountain, river, promontory, and creek upon the face of the earth, view the buildings and survey the land everywhere as if he were going to make a purchase” (C. of U., iij, _ad f._). So that even according to Locke’s own shewing we must use the eyes of others as well as our own, and this is true not in geography only, but in all other branches of knowledge.