But these generalizations remain rather insignificant so long as they are not connected with one definite image which stands as a symbol for the whole class of things. Nature scarcely presents to us any images which could be used as symbols of this kind. What are we invariably conscious of when thinking of books, or of trees, or of houses--something that is not only invariable, but also readily separable in our imagination? It is difficult to name anything which fulfills these conditions. But man created what he did not find in nature, symbols which can be used as meaning whole classes of objects and relations of objects. The totality of these symbols is human language. These symbols are normally divided into four classes of imagery, four languages, so to speak, in such a manner that each class of objects has a symbol in each of the four languages. The first of these languages acquired by the child is the auditory language, made up of the sounds of the words spoken by others. Soon after having begun to understand spoken words, the child begins to speak himself. Thus he acquires a second language, made up of kinesthetic imagery of his vocal organs. These languages are the only ones possessed by illiterates. In school the child learns to read, that is, he acquires a third class of symbols, consisting of visual images of written and printed words. One might of course speak of these as two visual languages, since the sight of written words differs somewhat from the sight of printed words. Finally the child learns to write, and thus acquires a fourth language, made up of kinesthetic images of the writing hand. These are, of course, not the only languages possible. The blind-born, unable to acquire visual imagery, substitute tactual word imagery by learning to read raised letters or the raised point script generally taught in institutions for the blind. But a seeing person, too, may acquire this tactual language in addition to the other four. The deaf-born acquire a visual language made up of the images of the hand and the fingers representing symbolically letters and words. But it is hardly worth while to enumerate all these minor languages. The most important ones practically are these four: the auditory, the visual (written and printed), the kinesthetic of the vocal organs, and the kinesthetic of the writing hand.