  The mathematical model of physical spacetime is the four-dimensional  pseudo-Riemannian manifold. Weyl (1921c) distinguished between two  primitive substructures of that model: the conformal and  projective structures and showed that the conformal  structure, modelling the causal field governing light  propagation, and the projective structure, modelling the inertial or  guiding field governing all free (fall) motions, uniquely  determine the metric. That is, Weyl (1921c) proved 