The subject matter or content of painting is determined by its language, color, and line. These, as we have seen, by an imitation more or less exact, represent nature, the world of concrete things as directly presented to us in vision, colored and shapely. The inner world is expressed only so far as it is revealed in the gestures and attitudes of the bodies of men or so far as it is a mood attached to things and their colors and shapes. Now space is the universal container in which all elements of the visible world are disposed. Every painting, therefore, should include a representation of space; it should never represent things as if they stood alone without environment or relation. Even in the portrait of a single individual some relation to space should be indicated; this is accomplished by the background, in which the figures should be made to lie, and to which they should seem to belong. In front, the space of a picture is limited by the plane of the surface on which it is painted; everything should appear to belong in the space back of this; nothing should seem to come forward out towards the spectator. But behind this, backwards, the space represented is unlimited, and its infinite depths may well be indicated by the convergence of perspective lines and the gradual fading of the outlines and colors of objects. The represented space of the picture is not, of course, the real space of the canvas or of the room in which the picture hangs. The former is infinite, while the latter is only so many square feet in area. The frame serves the purpose of cutting off the represented space from all relation to the real space, of which the frame itself is a part. A confusion of these two spaces is sometimes found in crude work and in the comments of people upon genuine works of art. I have, for example, seen a picture of a lion with iron bars riveted to the frame and extending over it,--a represented lion in a real cage! And I once heard a man criticise one of Degas' paintings on the ground that "if the dancing girl were to straighten her bent body she would bump her head on the frame!" The rule that the color of the frame should harmonize with the main tones of the picture is no proof that they belong together; its purpose is merely to protect the colors of the painting from being changed through their neighborhood with those of the frame.