  Unlike de Sitter, Robertson accepts the cosmological principle, replete with its observers on far-separated particles. Moreover, as this statement shows, Robertson is intimately familiar with Milne’s latest operationalist reduction: space is to be reduced to time measurements given by clock readings on signals exchanged between observers. Robertson gets this idea from Milne’s book, which he had earlier reviewed (but which was only subsequently published) for Astrophysical Journal (Robertson 1936). 