There is an organic unity of line in sculpture, similar again to that in painting. And by line I mean not only surface lines, but the lines made by the planes in which the body lies, the lines of pose and attitude. The predominance of a single type of line, the union of many lines to form a single continuous line, balance and symmetry of line, proportion of length and parallelism, are all to be found in sculpture. Especially important is rhythm--the harmonious, balanced movement of lines. In the "Venus de Milo," for example, the plane of the lower limbs from the feet to the knees moves to the left; there is an opposite and balancing movement from the right knee to the waist; the first movement is repeated in the parallel line from the right hip to the top of the head; this, in turn, is balanced by a line in the opposite direction running from the left hip to the right shoulder, parallel to the second line; but the equilibrium of line is not a rigid one, for the body as a whole moves in an undulating line to the left, imparting grace and a total unity. The beauty of line in sculpture is, of course, no invention of the artist; for nature has created it in the body itself. The sculptor takes this beauty as the basis of his work, remodeling only by the elimination of details, through which purer effects of line are obtained, or by the selection and emphasis of pose, through which these effects are rendered more intensely expressive. All conventionalization is in the interest of increased beauty of line. But too great a sacrifice of the natural contours of the body, as in some of the work of the Cubists, results in a lifelessness that cannot be atoned for by any formal beauty. The unification of line in sculpture is a matter not only of lines within the whole and of single contours, but of the total visual form of the whole, of silhouette. Although three-dimensional, every statue casts a two-dimensional image on the retina. It makes as many of these plane pictures as there are points of view from which it can be seen. One can easily convince oneself of this by viewing a statue from a distance, when it will flatten out to a mere outline or silhouette. As such, it should be clear and simple and pleasing, capable of being grasped as a whole irrespective of detail. Michelangelo demanded that every statue be capable of being put inside of some simple geometrical figure, like a pyramid or a cube; that there be no wayward arms or legs, but close attachment to the body, so close that the statue might be rolled down hill without any part being broken off. This last is perhaps too rigorous a requirement, but the best work of all periods exhibits visual clarity and concentration.[Footnote: Compare Adolf Hildebrand, The Problem of Form in Painting and Sculpture.]