We shall reply, in the first place, by asking whether a mistake has really been committed. This cannot be taken for granted, since the proportion of defectives varies enormously from one school to another. But let us admit a mistake, and ask who is responsible. The master of the school understood badly what the circular had explained more badly still. In these circulars we actually read the following definition of defectives: "Subjects who are in a condition of mental debility, possessing only a limited intelligence and a limited responsibility, which do not admit of their acquiring, at the ordinary school and by the usual methods of education, the average elementary instruction which the other pupils receive." If one interprets this badly constructed formula literally, it is evident that half the children of France must be defective, being of necessity below the average. If the teacher is to work intelligently, he must have more precise directions. After having explained to him that a defective child is one who does not adapt himself, or who adapts himself badly, to school life, one will tell him that the degrees of non-adaptation vary indefinitely; for it is quite exceptional for even a defective child not to adapt himself at all, and to learn absolutely nothing at the ordinary school. It remains, therefore, to decide what degree of retardation or of non-adaptation is to be recognised as determining a defective. According to a convention accepted in Belgium, which we modify slightly, _the retardation which determines a child as a defective is two years when the child is under nine, and three years when he is past his ninth birthday_. Here we have a very precise rule, easy to apply to all children, with the corrections already indicated relating to school attendance. The rule is, perhaps, a little rigid, we admit, but it will always be possible to make allowances when examining closely the individual cases to which it will have to be applied. Thus, the method which we have just indicated permits the making of a first selection.