  Incidentally, without a world structure the concept of relative  motion of several bodies has, as the postulate of general relativity  shows, no more foundation than the concept of absolute motion of a  single body. Let us imagine the four-dimensional world as a mass of  plasticine traversed by individual fibers, the world lines of the  material particles. Except for the condition that no two world lines  intersect, their pattern may be arbitrarily given. The plasticine can then be continuously deformed so that not only one but all fibers  become vertical straight lines. Thus no solution of the problem is  possible as long as in adherence to the tendencies of Huyghens and  Mach one disregards the structure of the world. But once the inertial structure of the world is accepted as the cause for the dynamical  inequivalence of motions, we recognize clearly why the situation  appeared so unsatisfactory. … Hence the solution is attained  as soon as we dare to acknowledge the inertial structure as a  real thing that not only exerts effects upon matter but in turn suffers such effects.