What then shall he do with this superabundance of mental and physical strength, which he will hereafter need, but endeavor to employ it in ways which will at some time be of use to him, and thus throw this surplus vitality forward into the future? The robust child shall make provision for his weaker manhood. But he will not garner it in barns, or lay it up in coffers that can be plundered. To be real owner of this treasure, he must store it up in his arms, in his brain, in himself. The present, then, is the time to labor, to receive instruction, and to study; nature so ordains, not I. Human intelligence has its limits. We can neither know everything, nor be thoroughly acquainted with the little that other men know. Since the reverse of every false proposition is a truth, the number of truths, like the number of errors, is inexhaustible. We have to select what is to be taught as well as the time for learning it. Of the kinds of knowledge within our power some are false, some useless, some serve only to foster pride. Only the few that really conduce to our well-being are worthy of study by a wise man, or by a youth intended to be a wise man. The question is, not what may be known, but what will be of the most use when it is known. From these few we must again deduct such as require a ripeness of understanding and a knowledge of human relations which a child cannot possibly acquire; such as, though true in themselves, incline an inexperienced mind to judge wrongly of other things. This reduces us to a circle small indeed in relation to existing things, but immense when we consider the capacity of the child's mind. How daring was the hand that first ventured to lift the veil of darkness from our human understanding! What abysses, due to our unwise learning, yawn around the unfortunate youth! Tremble, you who are to conduct him by these perilous ways, and to lift for him the sacred veil of nature. Be sure of your own brain and of his, lest either, or perhaps both, grow dizzy at the sight. Beware of the glamour of falsehood and of the intoxicating fumes of pride. Always bear in mind that ignorance has never been harmful, that error alone is fatal, and that our errors arise, not from what we do not know, but from what we think we do know.[1] The Incentive of Curiosity. The same instinct animates all the different faculties of man. To the activity of the body, striving to develop itself, succeeds the activity of the mind, endeavoring to instruct itself. Children are at first only restless; afterwards they are inquisitive. Their curiosity, rightly trained, is the incentive of the age we are now considering. We must always distinguish natural inclinations from those that have their source in opinion.