The structure of works of art is even more complicated than would appear from the description given thus far. For there is not only the unity of the elements among themselves, but between the two aspects of each element and of the whole--the form and content. This--the unity between the sense medium and whatever of thought and feeling is embodied in it--is the fundamental unity in all expression. It is the unity between a word and its meaning, a musical tone and its mood, a color and shape and what they represent. Since, however, it is indispensable to all expression, it is not peculiar to art. And to a large extent, even in the creative work of the artist, this unity is given, not made; the very materials of the artist consisting of elementary expressions--words, tones, colors, space-forms--in which the unity of form and content has already been achieved, either by an innate psycho-physical process, as is the case with tones and simple rhythms, or by association and habit, as is the case with the words of any natural language, or the object-meanings which we attach to colors and shapes. The poet does not work with sounds, but with words which already have their definite meanings; his creation consists of the larger whole into which he weaves them. Of course, even in the case of ordinary verbal expression, the thought often comes first before its clothing in words, when there is a certain process of choice and fitting; and in painting there is always the possibility of varying conventional forms; yet even so, in large measure, the elements of the arts are themselves expressions, in which a unity of form and content already exists. In art, however, there are subtler aspects to the relation between form and content, and these have a unique aesthetic significance. For there, as we know, the elements of the medium, colors and lines and sounds, and the patterns of these, their harmonies and structures and rhythms, are expressive, in a vague way, of feeling; hence, when the artist employs them as embodiments of his ideas, he has to select them, not only as carriers of meaning, but as communications of mood. Now, in order that his selection be appropriate, it is clearly necessary that the feeling tone of the form be identical with that of the content which he puts into it. The medium as such must reexpress and so enforce the values of the content. This is the "harmony," as distinguished from the mere unity, of form and content, the existence of which in art is one of its distinguishing properties. I have already called attention to this in our second chapter. It involves, as we observed, that in painting, for example, the feeling tone of the colors and lines should be identical with that of the objects to be represented; in poetry, that the emotional quality of meter and rhythm should be attuned to the incidents and sentiments expressed. Otherwise the effect is ugly or comical.