  In McCrea’s discussion of the theories he asserts that “the consituents [of the theories themselves] which are of physical significance are sets of mathematical relations, coupled with sets of rules of interpretation” which yield, “after observational test, descriptions rather than explanations of physical phenomena.” According to McCrea, one real advantage of his view is that it “leads to simple criteria for comparing the merits of different theories.” Finally, on the metaphysics of the original hypotheses, McCrea claims that “the initial hypotheses from which the mathematical relations are deduced do not ultimately have any direct physical significance.”