Passing from these evidences that the bodily and the mental functions interplay, let us now observe to what extent the mind's own functions touch one another.
We shall see the need, first of all, of knowledge. If one is to think effectively of sugar beets or airplane engines, he must study such beets, such engines. But he will not think effectively upon these if he think of these alone; his interest and his knowledge must widen to the principles of agriculture or of aerodynamics; and beyond, he will need botany or physics, and chemistry. Chemistry, then, is important for a lad uncertain whether he will deal with beets or engines. But what of the boy who does not himself know, and whom no one as yet can tell, whether beets, engines, taxation, tuberculosis, or the Gospel will lie at the center of his thinking in the time to come? Must he give laborious years to all of these and to a thousand things beside, that he may be ready for the day of action? Inevitable and enormous waste is in that direction. He had best be at home in the central studies into which all special subjects lead. These more central studies may be less attractive just because they are more abstract, more remote from some particular work in hand; and for that reason more of art may be needed to make the "practical" youth, hating abstractions, ready to give himself heartily to their forbidding generalities. The skill of the teacher is displayed in conquests of this kind. General truths, when seen and understood, are so much more powerful instruments than are mere particular and detached bits of knowledge, that surrender upon this point will hardly be permitted by any able teacher. Most children prefer to play with an electro-magnet than to ascend from this to the principles of electro-magnetism; prefer to look at striking chemical reactions than to attack with vigor the general truths involved; prefer to draw circles and polygons than to understand geometry. The interest in these general truths is, in a sense, less natural, more a matter of civilization, and has to be imposed upon the child by a kind of contagious interest felt by another who can see the endless applications of what is universal. One has to fortify himself with the stern conviction of this, in order to resist those who see only the attractiveness of an endless list of particular studies of nature and of handicraft and who would urge them to such a pitch that there is no firm grasp of the sciences which deal with principles. Particular and general ideas conjoined is our need; errors of practice are thus avoided; economy of action is reached. Judd found, in striking at a target under water where refraction had to be allowed for, that those who were instructed in the principles of refraction had the advantage over those who merely kept at their interesting target practice without instruction. And in the experiments upon neatness it will be remembered that the neatness spread to other work when there was presented the general idea of neatness and of its value as a universal trait. Ideas, then, are guides, are directors of habit; in them is compacted wisdom, and whoever tries to do without a good stock of them foregoes the advantage which comes from the experience of the race. They permeate the special functions which seem so separate, and bind them into a common plan and use. The organizing effect of such ideas helps one to escape that pseudo-education given by books of ten thousand facts, which is so attractive to scattered wits. But with knowledge, with the ideas, the lad will need certain established habits of mind that are not knowledge or ideas; such as Abraham Lincoln had, who must "bound" every important idea he would use, never at ease until he saw clearly what limited it on north, south, east, and west, with no borders lost in the mist. Such a habit is of use for any idea and for anybody. Because it is not the whole of reason we must not be blind to the part it can play in reason, immensely wide, even universal in its sweep. Then other habits are part of right intellectual equipment: controlled attention to the task in hand; energetic attack upon it; accuracy in interpreting, remembering and reporting what is seen or read or heard; the power to distinguish important and unimportant. These are part of intellectual training; these and other things take the place of the few faculties of the older belief. They stand out significant to an eye bewildered by the endless array of special functions which for some are the only things left. These wide and superior powers call for training, and the lad who has them trained has an incalculable advantage over every lad in whom they remain untrained.
There is cheer at this point for the teacher, the parent, discouraged by the child's talent for forgetting what has been painfully taught. Under the old creed which laid such stress upon memory, and even under the new with its stress upon "contents," there seems here but wasted effort. But we can now demonstrate experimentally that virtue may go into even the adept forgetter; power once developed remains even though upon the moment's examination the mind seems to have lost all its contents. Thomas Hanna, after a brain injury by a blow upon his head, lost all the detailed knowledge from both life and schooling; his "education" had in a certain sense been knocked out of him. And yet it remained, since he rapidly relearned what he had lost. So, too, the normal person, after disuse of the typewriter for years, so that nearly all the original skill seems gone, needs but a small part of the original practice to restore the whole. And the same seems to be true of poetry once learned and apparently quite forgotten. This is evidence that education goes deeper than memory and gives power that cannot be lost. The measure of accomplishment is now known to lie, not solely in what the child can recollect, but also in an imparted ability, temporarily become latent, but ready with little effort to be brought to full expression. This is a fact of cheer to weary workers.