  In classical mechanics, it was decided by convention (see above) that a rigid rod was the same length throughout space and this convention was modified in Special Relativity. But in General Relativity, the length of a rigid rod may also depend on its place and position in such a way that consistency with Special Relativity is maintained. Thus, to maintain the general postulate of relativity, it is necessary to reduce the objective spatial structure of the earlier physics to a non-intuitive topology. This is a radical departure from the objectivity of the spatial structure of these earlier systems, which was entirely an artifact of their fixed metrical structure. But in Relativity Theory, the resulting conceptual construction admits of distinct metrical structures in different regions depending on the gravitational field in that region. As Schlick reflects in the closing pages of Space and Time …, the very possibility of the objectivity of this conceptual construction depends entirely on the method of point-coincidences. Any features of the world-picture which do not contribute to the systematization of point-coincidences are not physically objective. And all world-pictures which contain laws governing point-coincidences are thoroughly equivalent.  Furthermore, since any functional, single-valued deformation of the world-picture leaves all point-coincidences undisturbed, the equations of physics retain their form under such transformations, implying that they are covariant under all substitutions. These substitutions also leave the form of physical equations unchanged for coordinate-systems in motion, allowing for the relativity of space within such coordinate-systems, thus depriving space and time of the “last vestige of physical objectivity” (Schlick 2006a, Sec. VII). 