  Milne’s metaphysical views were based in positivism, most especially in operationalism: only those objects whose properties could be directly revealed by some observational procedure, or operation, were to be counted among the real. Thus, for Milne, reality did not contain the usual relativistic cosmology referents “curved space”, “expanding space” or even “four-dimensional spacetime”, simply because none of these entities were operationalizable. Milne held that ‘what you see is what you get’; since “curved space” of various geometries couldn’t be observed, space was just as it looked—Euclidean. In the same way, the expanding of the Universe was a genuine expanding—each galaxy was retreating from each other—not just a change in the geometry of the curvature, as it was for the relativists. Moreover, in place of relativistic cosmology’s inductive empiricism, Milne opted for a hypothetico-deductive rationalism. Cosmologists, Milne believed, were bound to dream up any and all possible models of a universe, and then deduce what, if any, observable consequences followed from their hypotheses. Hypothesizing could be based upon just about anything, although Milne believed that certain very general rational principles derived from aesthetic beliefs about universal order and regularity would be the most fruitful. His ‘cosmological principle’, namely, that every cosmologist in the universe should look out upon the same cosmos, was the prime example of such thinking. Later, Bondi would found Steady-state Cosmology on an even more general version of Milne’s principle, which he called the ‘perfect cosmological principle.’