  We have known since Galileo and Newton, that the motion of a body  involves an inherent struggle between inertia and force. According to the old view, the inertial tendency of persistence, the  “guidance”, which gives a body its natural inertial  motion, is based on a formal geometric structure of the spacetime  (uniform motion in a straight line) which resides once and for all in spacetime independently of any natural processes. This assumption  Einstein rejects; because whatever exerts as powerful effects as  inertia—for example, in opposition to the molecular forces of  two colliding trains it rips apart their freight cars—must be  something real which itself suffers effect from matter. Moreover,  Einstein recognized that the guiding field’s variability and  dependence on matter is revealed in gravitational effects. Therefore, the dualism between guidance and force is maintained; but 