  Ehlers et al. (1972) generalized Weyl’s causal-inertial method by  deriving the metric field directly from the conformal and  projective fields and derived a unique pseudo-Riemannian spacetime  metric solely as a consequence of a set of natural, physically  well-motivated, constructive, “geometry-free” axioms  concerning the incidence and differential properties of light  propagation and free (fall) motion. Ehlers, Pirani and Schild adopt  Reichenbach’s (1924) term, constructive axiomatics to  describe the nature of their approach. The  “geometry-free” axioms are propositions about a few  general qualitative assumptions concerning free (fall) motion and  light propagation that can be verified directly through experience in a way that does not presuppose the full blown edifice of the general  theory of relativity. From these axioms, the theoretical basis of the theory is reconstructed step by step. 