This is also the time to train him gradually to prolonged attention to a given object; but this attention should never be the result of constraint, but of interest or desire; you must be very careful that it is not too much for his strength, and that it is not carried to the point of tedium. Watch him, therefore, and whatever happens, stop before he is tired, for it matters little what he learns; it does matter that he should do nothing against his will. If he asks questions let your answers be enough to whet his curiosity but not enough to satisfy it; above all, when you find him talking at random and overwhelming you with silly questions instead of asking for information, at once refuse to answer; for it is clear that he no longer cares about the matter in hand, but wants to make you a slave to his questions. Consider his motives rather than his words. This warning, which was scarcely needed before, becomes of supreme importance when the child begins to reason. There is a series of abstract truths by means of which all the sciences are related to common principles and are developed each in its turn. This relationship is the method of the philosophers. We are not concerned with it at present. There is quite another method by which every concrete example suggests another and always points to the next in the series. This succession, which stimulates the curiosity and so arouses the attention required by every object in turn, is the order followed by most men, and it is the right order for all children. To take our bearings so as to make our maps we must find meridians. Two points of intersection between the equal shadows morning and evening supply an excellent meridian for a thirteen-year-old astronomer. But these meridians disappear, it takes time to trace them, and you are obliged to work in one place. So much trouble and attention will at last become irksome. We foresaw this and are ready for it. Again I must enter into minute and detailed explanations. I hear my readers murmur, but I am prepared to meet their disapproval; I will not sacrifice the most important part of this book to your impatience. You may think me as long-winded as you please; I have my own opinion as to your complaints.