On the most usual assumption, the universe is homogeneous on the large scale, i. e. down to regions containing each an appreciable number of nebulae. The homogeneity assumption may then be put in the form: An observer situated in a nebula and moving with the nebula will observe the same properties of the universe as any other similarly situated observer at any time.

Sir Hermann Bondi, "Review of Cosmology," Monthly Notices of the Royal Astronomical Society 1948, p. 107-8, as cited in: Hermann Friedmann. Wissenschaft und Symbol, Biederstein, 1949, p. 472