  Can it be argued that Ehlers, Pirani and Schild’s generalization of  Weyl’s causal-inertial method for determining the spacetime metric  constitutes a convention-free, and – in relevant respects  – theory-independent body of evidence that can adjudicate  between spacetime geometries, and hence between spacetime theories  that postulate them? As Weyl showed, we can empirically determine the metric field, provided certain epistemic conditions are satisfied,  that is, provided we can measure the conformal-causal structure, and  provided “we are able to recognize and observe as such the  motion of free mass points which follow the guiding field.”  Criticisms of Ehlers, Pirani and Schild’s constructive axiomatics  suggest that the causal-inertial method is not convention-free and  that it is ineffective epistemologically in providing a possible  solution to the controversy between geometrical realism and  conventionalism in favor of realism. Basically, all of the charges  laid against Ehlers, Pirani and Schild’s constructive axiomatics  concentrate on the roles which massive particles play in their  construction. One of the constructive axioms employed by Ehlers,  Pirani and Schild, the projective axiom, is a statement of the  infinitesimal version of the Law of Inertia, the law of free (fall)  motion which contains Newton’s first law of motion as a special case  in the absence of gravitation. Since Ehlers, Pirani and Schild do not provide an independent, non-circular criterion by which to  characterize free (fall) motion, their approach has been charged with circularity by philosophers such as Grünbaum (1973), Salmon  (1977), Sklar (1977) and Winnie (1977). 