 This is done as follows. The empirical determination of the spacetime metric by measurement requires choice of some “metrical indicators”: this can only be done by laying down a coordinative definition stipulating, e.g., that the metrical notion of length is coordinated to some physical object or process. A standard choice coordinates lengths with “infinitesimal measuring rods” supposed rigid (e.g., Einstein’s “practically rigid body”). This however is only a convention, and other physical objects or processes might be chosen. (In Schlick’s fanciful example, the Dali Lama’s heartbeat could be chosen as the physical process to which units of time are coordinated.) Of course, the chosen metrical indicators must be corrected for certain distorting effects (temperature, magnetism, etc.) due to the presence of physical forces. Such forces are termed “differential forces” to indicate that they affect various materials differently. However, Reichenbach argued, the choice of a rigid rod as standard of length is tantamount to the claim that there are no non-differential—“universal”—distorting forces that affect all bodies in the same way and cannot be screened off. In the absence of “universal forces” the coordinative definition regarding rigid rods can be implemented and the nature of the spacetime metric empirically determined, for example, finding that paths of light rays through solar gravitational field are not Euclidean straight lines. Thus, the theory of general relativity, on adoption of the coordinative definition of rigid rods (“universal forces = 0”), affirms that the geometry of spacetime in a given region is of a non-euclidean kind. The point, however, is that this conclusion rests on the convention governing measuring rods. One could, alternately, maintain that the geometry of spacetime was Euclidean by adopting a different coordinative definition, for example, holding that measuring rods expanded or contracted depending on their location in spacetime, a choice tantamount to the supposition of “universal forces”. Then, consistent with all empirical phenomena, it could be maintained that Euclidean geometry was compatible with Einstein’s theory if only one allowed the existence of such forces. Thus whether general relativity affirms a Euclidean or a non-euclidean metric in the solar gravitational field rests upon a conventional choice regarding the existence of non-zero universal forces. Either hypothesis may be adopted since they are empirically equivalent descriptions; their joint possibility is referred to as “the relativity of geometry”. Just as with the choice of standard synchrony in Reichenbach’s analysis of the conventionality of simultaneity, a choice also held to be “logically arbitrary”, Reichenbach recommends the “descriptively simpler” alternative in which universal forces do not exist. To be sure, “ descriptive simplicity has nothing to do with truth”, i.e., has no bearing on the question of whether the spacetime metric really has a non-Euclidean structure (1928: 47 [1958: 35]).