may be called the kinetic energy of the material point. Since _dt_ is always greater than _d_τ we may call the quotient (_dt_ - _d_τ)/_d_τ as the “Gain” (vorgehen) of the time over the proper-time of the material point and the law can then be thus expressed;—The kinetic energy of a material point is the product of its mass into the gain of the time over its proper-time. The set of four equations (22) again shows the symmetry in (_x_, _y_, _z_, _t_), which is demanded by the relativity postulate; to the fourth equation however, a higher physical significance is to be attached, as we have already seen in the analogous case in electrodynamics. On the ground of this demand for symmetry, the triplet consisting of the first three equations are to be constructed after the model of the fourth; remembering this circumstance, we are justified in saying,— “If the relativity-postulate be placed at the head of mechanics, then the whole set of laws of motion follows from the law of energy.” I cannot refrain from showing that no contradiction to the assumption on the relativity-postulate can be expected from the phenomena of gravitation. If B*(_x_*, _y_*, _z_*, _t_*) be a solid (fester) space-time point, then the region of all those space-time points B (_x_, _y_, _z_, _t_), for which (23) (_x_ - _x_*)² + (_y_ - _y_*)² + (_z_ - _z_*)² = (_t_ - _t_*)² _t_ - _t_* >= 0 may be called a “Ray-figure” (Strahl-gebilde) of the space time point B*. A space-time line taken in any manner can be cut by this figure only at one particular point; this easily follows from the convexity of the figure on the one hand, and on the other hand from the fact that all directions of the space-time lines are only directions from B* towards to the concave side of the figure. Then B* may be called the light-point of B. If in (23), the point (_x_ _y_ _z_ _t_) be supposed to be fixed, the point (_x_* _y_* _z_* _t_*) be supposed to be variable, then the relation (23) would represent the locus of all the space-time points B*, which are light-points of B. Let us conceive that a material point F of mass _m_ may, owing to the presence of another material point F*, experience a moving force according to the following law. Let us picture to ourselves the space-time filaments of F and F* along with the principal lines of the filaments. Let BC be an infinitely small element of the principal line of F; further let B* be the light point of B, C* be the light point of C on the principal line of F*; so that OA′ is the radius vector of the hyperboloidal fundamental figure (23) parallel to B*C*, finally D* is the point of intersection of line B*C* with the space normal to itself and passing through B. The moving force of the mass-point F in the space-time point B is now the space-time vector of the first kind which is normal to BC, and which is composed of the vectors (24) _mm_*(OA′/B*D*)³ BD* in the direction of BD*, and another vector of suitable value in direction of B*C*.