  McCrea’s effort operationalized the concept of “distance”, principally and originally by comparison of certain elements of Newtonian cosmology and de Sitter’s axiomatized version of relativistic cosmology (McCrea 1935). Walker’s paper specifically eschewed use of “any indefinable concepts”, in particular, he did not assume that the “associated metric [of relativistic cosmology] has any a priori physical significance” (Walker 1935). Robertson’s article, the first of three, is the most important, both in its content, and in the signal it sends, namely, that one of the original mainstream relativistic cosmology proponents has adopted a major element of Milne’s new philosophy for cosmology (Robertson 1935). Robertson’s conclusion exhibits this point clearly and explicitly: