218. Our chief concern during the age of boyhood must be to prevent the premature fixation of the circle of ideas. It is for instruction to undertake the task of doing so. True, by far the greatest part of the process of learning, however manifold, is performed through the interpretation of words, the pupil supplying the meaning out of the mental store collected previously. But this very fact obviously implies that quantitively the pupil’s stock of ideas is for the most part complete; instruction merely works it up into new forms. Accordingly, such shaping must take place while the material is still in a plastic state; for with increasing years it gradually assumes a more solid character. 219. Boys differ from girls, individuals differ from one another; and the subjects taught, together with the methods of teaching them, should be differentiated accordingly. But here the family interposes the interests of rank or station, and claims the right to determine by these how much or how little instruction a boy needs. Looked at pedagogically, each study calls for a corresponding mental activity to be suited to the general condition of the individual. Its success must not involve exhaustion of the pupil’s powers, nor make demands upon them at the wrong time. But it would be an error to argue that one who is being initiated into one subject ought to combine with that subject a second, third, or fourth, on the ground that subjects one, two, three, and four are essentially interrelated. This conclusion holds for scholars, who, so far as they are personally concerned, have long passed beyond preliminary pedagogical considerations, and even in their case it applies only to those branches which are intimately connected with their specialties; it has nothing to do with the psychological conditions by which the course of education must be governed. Only too frequently do masses of ideas remain isolated despite the fact that the objects corresponding to them are most intimately and necessarily interconnected; and such isolation could not have been prevented by merely starting work in a large web of erudition in a number of places. The case is different where certain studies constitute the necessary preparation for thorough knowledge of one kind or another. Here we are right in concluding that one who cannot master the former is equally unable to get hold of the latter.[24] [24] These remarks upon correlation are instructive in view of later developments of the Herbartian school in Germany. The reader is referred to discussions in the First and Second Year-Books of the National Herbart Society.