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There is an explicit statement of a restricted form. of what Einstein later called the Principle of Relativity in Sir Isaac Newton's Principia. It may be translated:

"The motions of bodies enclosed in a given space are the same relatively to each other whether that space is at rest or moving uniformly in direction [i.e. "moving with a constant velocity in a straight line] without circular motion." ...

A change in the outlook of physicists and in the associated physical theory began, rather gradually, as the end of last century approached. This had its origin in the unsuccessful efforts of the natural philosophers of that time to account satisfactorily for certain well-authenticated phenomena in terms of the accepted physical theory, which of course was based on Sir Isaac Newton's mechanical principles. One formidable problem emerged from. the fact that the interpretation of stellar aberration, on the one hand, and of the result of the experiments of Michelson and Morley (1887) on the other, led to conflicting conclusions about the motion of the luminiferous medium. relatively to the earth. Another, different, problem was that presented by the experimentally determined distribution of energy in the normal spectrum. (spectrum of black body radiation).