Abstract

At the first stage of setting up Kinematical Relativity, Milne modified Einstein\'s Principle of Relativity and assumed that the Universe had to appear the same to all observers. He called this an \"Extended Principle of Relativity\". In order to specify this postulate, Milne defined the notion of the \"equivalence of observers,\" and then formulated a new definition of the Principle of Relativity: all descriptions of the whole system made by equivalent observers must be identical. Under Freundlich\'s influence he called it the \"Cosmological Principle\" (CP). The content of Milne\'s CP was different from both Einstein\'s Principles of Relativity and the uniformity postulate of Relativistic Cosmology. Notwithstanding this, the adherents of the latter adopted this name for the isotropy and homogeneity postulate. Initially, Milne treated CP as a hypothesis about matter distribution. But when he separated model constructing from verifying to what extent it corresponded with the actual Universe, he began to emphasize that CP is not a law of nature, but a definition of the research domain