To begin with, then, some exception may be taken to the statement that the student who accepts a scientific doctrine, has the full evidence before him from the beginning. That it is not altogether so is evident from the simple consideration, just mentioned, that his work is a progressive one; and that the whole course of his experience tends, and will tend, to deepen the certainty of his first principles. But in so far as the proof of any leading principle is being deepened and strengthened by the student's daily work, so far it is clear that the amount of certainty about his principles with which at first he began, must be less than that with which he ends at last; and therefore that the proof presented to him at the beginning, however much it may have been adequate to the purpose, (even though it may have been the completest proof capable of being presented in the way of exposition from the lip to the ear) was nevertheless most incomplete in comparison with the fulness of attainable proof. And further, it may certainly be said also, that in the convincingness of this evidence as at first presented, authority, whether more or less, had an undoubted part. At the very least it had a negative place, as a guarantee to the young mind rejoicing in the ingenuity of the apparent demonstration, that the apparent demonstration was not vitiated by some unseen fallacy, or that there was not a series of other considerations behind, which would rob the lesson just learnt of its practical usefulness. Often, indeed, the degree of authority in the first scientific convictions would be very much higher. Often, however helpful the arguments or illustrations of a principle may seem, the really overruling consideration will at first be this, that the whole scientific world has absolutely accepted the principle as truth. So much is this the case, that if an average student should find himself unable in any point to receive the ascertained truths of his science with intelligent agreement, he would not hesitate to assume that the whole fault lay with himself; he would really be convinced in his soul that the dicta of his scientific teachers were right, and that he himself would see the certainty of them by and by.