The teacher must guide the child's mind on these lines in every kind of teaching; he must, however, never substitute his own intelligence for that of the child, but rather make the child himself think, and induce him to exercise his own activity. For instance, in the association period, the master must not say: "Look at such and such an object, and at such and such another; see how much alike they are, etc...." He should ask the pupil: "What do you see when you look around? Is there not something which is like, etc.?" Again, in the definition period, the master should not say: "A bird is a vertebrate animal covered with feathers; it has two limbs which have been transformed into wings," but by rapid questions, corrections, and analogies, he should induce the child to find the precise definition for himself. If the mental process of Herbart's four periods is to come naturally, it would be essential that great interest in the object should exist; it is interest which would keep the mind amused, or, as the famous pedagogist would say, plunged in the idea, and would maintain it in a system nevertheless embracing multilateral ideas; and hence it is necessary that "interest" should be awakened and should persist in all instruction. It is well known that a pupil of Herbart's must, to this end, supplement Herbart's four periods by a prior period, that of interest; linking all new knowledge to the old, "going from the known to the unknown," because what is absolutely new can awake no interest. "To make oneself interesting artificially," that is, interesting to those who have no interest in us, is indeed a very difficult task; and to arrest the attention hour after hour, and year after year, not of one, but of a multitude of persons who have nothing in common with us, not even years, is indeed a superhuman undertaking. Yet this is the task of the teacher, or, as he would say, his "art": to make this assembly of children whom he has reduced to immobility by discipline follow him with their minds, understand what he says, and learn; an internal action, which he cannot govern, as he governs the position of their bodies, but which he must win by making himself interesting, and by maintaining this interest. "The art of tuition," says Ardigo, "consists mainly of this: to know up to what point and in what manner one can maintain the interest of pupils. The most skilful teachers are those who never fatigue one fraction of the pupil's brain, but act in such a manner that his attention, turning now here, now there, may rest itself and, gaining strength, return to the principal argument of the discourse with renewed vigor."