The mere expression of mechanical forces in a building would not, however, be aesthetic by itself, no matter how obvious to the mind. We must not only know these forces to be there, we must also feel them as there; we must appreciate them in terms of our own experiences in supporting weights and overcoming resistances. We must transform the mechanical into the vital, the material into the human. Art is an expression of life, not of mathematics. And this translation is not the result of an unusual, artificial attitude assumed for the sake of aesthetic appreciation; it is the natural mode of apperceiving force and mass. We cannot see a column supporting an entablature without feeling that it stands firm to bear the weight, much as we should stand if we were in its place. If this is a "pathetic fallacy," it is one which we all inevitably commit. Even the skeptic, if he were to examine carefully into his own mind, would find that he commits it, whenever he gives to the column, not a casual or merely calculating regard, but a free and earnest attention. If he gives his mind to the column and lets the column take hold of his mind, allowing his psychological mechanism to work unhampered, he will commit it. The aesthetic intuition of force--the human way of appreciating it--is, in fact, primary; the purely mechanical and mathematical is an abstraction, superimposed for practical and scientific purposes. The interplay of humanized mechanical energies, of which architecture is the expression, may be conceived as the resultant of four chief forces, acting each in a definite direction: upward, downward, outward, and inward. The downward force is associated with the weight of the materials of which the building is constructed. To all physical objects we ascribe a tendency toward the earth. An unsupported weight will fall, and even when supported will exert a pressure downward. And this tendency is no mere directed force in the physical sense, but an impulse, in the personal sense. For when with hand or shoulder we support a weight, we inevitably interpret it in terms of our own voluntary muscular exertion in resisting it; even as we strive to resist it, so it seems to strive to fall. Although this force is exerted downward, it shows itself in the horizontal lines of a building, in string courses, parapets, cornices, friezes; for the horizontal is the line parallel to the earth, toward which the force is directed, and along which we lie when we rest.[Footnote: Compare the discussion of Lipps, _Aesthetik_, Bd. 1, Dritter Abschnitt, although I am far from accepting all of his analyses.]