Galileo first developed the principle of relativity, which was the postulate that claimed that the laws of physics be the same for all observers, and advocated a classical view that time was a universal constant.
Einstein'stheory of relativity consists of special relativity and general relativity, which reshaped the classical view, arguing instead that observed time is not constant, but relative to the observer, and therefore the "inertial frame of reference" is only meaningful if qualified by theories of relativity that defines the general context for the frame.
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