128. With regard to the sequence of studies we need to distinguish first of all between preparatory knowledge and ability to do. As is well known, the latter, even when it has been fully attained, can be secured against loss only by long-continued practice. Hence the practice of the pupil’s skill must go on constantly from the time when he first learns to apply what he knows. But merely preliminary knowledge, which produced fatigue before it was mastered, may be allowed to drop out of the memory. Enough remains to make it easier to resume the subject at a later time (92, 103). Accordingly, not the preliminary knowledge just referred to, but the pupil’s facility in doing, supplies the principle determining sequence. In the case of all essential elementary information--knowledge of rudiments of grammar, arithmetic, and geometry--it will be found expedient to begin with the simplest elements long before any practical application is made. In such first lessons individual facts only are presented. These are made clear to the pupils (68, 69); here and there they are associated. Fatigue is avoided if possible. Even if the earliest attempts at memorizing should prove successful, it will be safer, instead of relying on this fact, to postpone the whole matter for a time. At a later period the same subject is resumed from the beginning without any demand on the teacher’s part that some things should have been retained. This time, however, it will be possible to introduce a somewhat larger quantity of the instruction-material, and it will not be too early to make pupils perceive the connection between individual facts. If pupils experience difficulty in comprehending, we should be careful not to advance too rapidly; the greater the difficulty, the greater the need for caution. When the time comes for practical application, an earnest, diligent effort must be insisted on, but only for tasks of moderate length, and without exacting too much by harsh means. Not every pupil can do everything. Sometimes a pupil will at a later period acquire the power he does not possess now, if only his chances for success have not been spoiled by earlier blindness on the part of his teacher. 129. Again, corresponding to each stage of instruction, there is a certain capacity for apperceiving attention (77) which deserves careful consideration. For we ought to avail ourselves of the comparatively easy in order to facilitate indirectly what would otherwise prove difficult and time-consuming.