,
,
,
; this what is done by the calculus of absolute differentials. The results of the preceding paragraphs, the far-reaching consequences of which can be fully recognized only by a detailed study of the mathematical developments involved, may be summarized as follows:
A mechanics of the relative motions of bodies, which is in harmony with the two fundamental postulates of continuity and relativity, can be built up only on a fundamental law of motion that preserves its form independently of the kind of motion the system is undergoing. An available law of this kind is given if we raise the law of motion along a geodetic line, which, in the special theory of relativity, holds only for a body moving under no forces, to the rank of a general differential law of the motion in the gravitational field, too. In this general law, we must, it is true, give the line-element of the orbit of the moving body the general form:
