  … Only such relations will have objective meaning as are  independent of the mapping chosen and therefore remain invariant  under deformations of the map. Such a relation is, for instance, the  intersection of two world lines. If we wish to characterize a special mapping or a special class of mappings, we must do so in terms of the real physical events and of the structure revealed in them. That is  the content of the postulate of general relativity.  According to the special theory of relativity, it  is possible in particular to construct a map of the world such that  (1) the world line of each mass point which is subject to no external forces appears as a straight line, and (2) the light cone issuing  from an arbitrary world point is represented by a circular cone with  vertical axis and a vertex angle of 90°. In this  theory the inertial and causal structure and hence also the metrical  structure of the world have the character of rigidity, they are  absolutely fixed once and for all. It is impossible objectively,  without resorting to individual exhibition, to make a narrower  selection from among the ‘normal mappings’ satisfying the above conditions (1) and (2).