In structure the expression of life is gained by so arranging the lines and masses as to give an impression that power is at work—that something is being done—done triumphantly yet not without strain and effort. Every object of utility does something—art shows it to us in the act. An example may help to make clear what I mean, and may show how the principle can be applied to any kind of object which may be the subject of artistic treatment. A Greek temple in its simplest external aspect consists of a quadrilateral group of columns surrounding a walled shrine and supporting a low-pitched roof. Nothing could well be simpler than the structural conditions thus expressed. But the artistic expression of them is not so simple. This depends in the main upon the proportion observed between the pillars and the weight, or apparent weight, above them. If the pillars are too massive or too numerous there will be no sense of strain, and if they are too slender or too few there will be no sense of security. In either case the expression of vital energy in the structure will be imperfect, and beauty, which waits on the golden moment of the perfect adaptation of means to ends, will not dwell in that structure. There is nothing more inartistic than superfluity; and there is no lesson more emphatically taught by nature than this. The avoidance of insufficiency is generally enforced in practice on utilitarian grounds, but its artistic justification is equally evident. The golden mean is what we call Just Proportion. The kind of vitality expressed in Greek architecture is quite different from that expressed in Gothic, but the æsthetic basis of both styles is the same; the principle we have in view will justify any art in which there is the spirit of life. A Greek temple shows us power, braced and conscious, but in repose. There is nothing daring or sensational in its construction. Stress and thrust answer each other directly, simply, massively. The stately calm of such a structure might easily become dull and monotonous were it not for the delicate sense of proportion governing the relations of the parts, for the introduction of slight deviations from strict rectangularity and symmetry,[181] and for the beautiful decoration in form and colour on frieze, pediment, and capital.