Although this book is specially intended as a guide to the admission of mentally defective children to special schools or classes, we cannot commence by an exposition of the methods of recruiting such children. We must first describe the children and indicate their principal characteristics, mental and moral. We must also discuss the question what a mentally defective child really is--a very important question, upon which depends everything else, the organisation of the schools and the special methods of education. Every educational method depends upon a theory, formulated or implicit, which is at once its point of departure and its justification. One would run the risk of falling into a blind empiricism if one were content to apply an educational method independently of the theory which is its soul. There are two conceptions of a totally different nature, either of which may inspire the training of defective children. Let us examine each of these in turn, and find out which is the more reasonable. According to the first, the defective child is practically the same as a normal child several years younger; or, in other words, he is a child who has been retarded in his development. A backward child of twelve years of age, who has not yet been able to learn to read, would thus be comparable to an ordinary child of six, who is just beginning to spell. It is evident that such a comparison must not be pushed too far. Many reservations must be made. On the one hand, the defective has not so much time in front of him for development as a normal and younger child. He is then not strictly comparable to the latter. On the other hand, the very fact of his age has given to the defective of twelve a bodily and even a mental development never attained at six. For example, he is nearer puberty; his vocabulary is more extensive; and he possesses greater general knowledge. But these reservations once made, the theory that the defective is the victim of a retardation of development has seemed reasonable to many competent people. As a rule one just accepts it without taking the trouble to formulate it in precise terms. Perhaps it is for this very reason that one accepts it so easily; it is the classic theory. To the cursory reader it may seem that we adopt this theory ourselves, for we shall frequently use such phrases as "defective of eleven who is at the level of a child of nine." But the sense in which we use such an expression must not be misunderstood, because it is only intended to imply that a certain standard has been attained. It has no bearing on the cause of the retardation, nor upon its particular nature, nor upon the means of rectifying it.