  It should be noted, however, that Weyl’s defence of his theory  implicitly addresses an important methodological consideration  concerning the relation between theory and evidence. As Pauli puts it above, according to Weyl “the metric field must not be  defined by means of information taken from these measuring instruments [rigid rods and ideal clocks]”. That is,  Weyl rejects Einstein’s operational standpoint which gives  operational significance to the metric field in terms of the observable behaviour of ideal rigid rods and ideal   clocks.[51]  Unlike light propagation and freely falling (spherically symmetric,  neutral) particles, rigid rods and ideal clocks are relativistically  ill defined probative systems, and are thus unsuitable for the  determination of the inherent structures of spacetime postulated by  the theory of relativity. Weyl (1918a) clearly recognized this when  he said in response to Einstein’s critique “because of the  problematic behaviour of yardsticks and clocks I have in my book  Space-Time-Matter restricted myself for the specific  measurement of the \(g_{ik}\), exclusively to the  observation of the arrival of light signals.” It is interesting to note parenthetically that in the first edition of his book Weyl  thought that it was possible to have an intrinsic method of comparing the lengths of arbitrary spacetime intervals with an interval between two fiducial spacetime events, by using light signals only. It was  Lorentz who pointed out to Weyl that not only the world lines of  light rays but also the world lines of material bodies are required  for an intrinsic method of comparing lengths. Not only did Weyl  correct this mistake in subsequent editions, but already in 1921,  Weyl (1921c) discovered the causal-inertial method for determining  the spacetime metric (see §4.3) by proving an important theorem  that shows that the spacetime metric is already fully determined by  the inertial and causal structure of spacetime. Weyl (1949a, 103)  remarks “… therefore mensuration need not depend on  clocks and rigid bodies but … light signals and mass points  moving under the influence of inertia alone will suffice.” It  is clear that Weyl regarded the use of clocks and rigid rods as an  undesirable makeshift within the context of the special and general  theory. Since neither spatial nor temporal intervals are invariants  of spacetime, the invariant spacetime interval \(ds\) cannot be  directly ascertained by means of standard clocks and rigid rods. In  addition, the latter presuppose quantum theoretical principles for  their justification and therefore lie outside the relativistic  framework because the laws which govern their physical processes are  not   known.[52] 