  The use of clocks and rigid rods is, within the context of either  theory, an undesirable makeshift for two reasons. First, since  neither spatial nor temporal intervals are invariants of the  four-dimensional spacetime of the special theory of relativity and  the general theory of relativity, the invariant spacetime interval  \(ds\) cannot be directly ascertained by means of standard clocks  and rigid rods. Second, the concepts of a rigid body and a periodic  system (such as pendulums or atomic clocks) are not fundamental or  theoretically self-sufficient, but involve assumptions that  presuppose quantum theoretical principles for their justification and thus lie outside the present conceptual relativistic framework.  Therefore, methodological and ontological considerations decidedly  favor Weyl’s causal-inertial method for determining the spacetime  metric. 