 Finally, it is important to recognize that GTR is generally covariant in a very special sense: unlike all other prior theories (and unlike many subsequent quantum theories), it postulates no fixed ‘prior’ or ‘background’ spacetime structure. As mathematicians and physicists realized early on, other theories, e.g., Newtonian mechanics and STR, can be put into a generally covariant form. But when this is done, there are inevitably mathematical objects postulated as part of the formalism, whose role is to represent absolute elements of spacetime structure (see Friedman 1983, Pooley 2017). What is unique about GTR is that it was the first, and is still the only ‘core’ physical theory, to have no such absolute elements in its covariant equations. (Whether these claims are exactly correct is a matter of ongoing debate, relating to the question of the ‘background independence’ of GTR: for discussion, see e.g. Belot (2011), Pitts (2006), Read (2016), and Pooley (2017).) The spacetime structure in GTR, represented by the metric field, is at least partly ‘shaped’ by the distribution of matter and energy. And in certain models of the theory, such as the Big Bang cosmological models, some authors have claimed that the local standards of inertial motion – the local ‘gravitational field’ of Einstein’s equivalence principle – are entirely fixed by the matter distribution throughout space and time, just as Mach-heavy requires (see, for example, Wheeler and Cuifollini 1995).