 Einstein’s special theory of relativity (STR) is constructed on the basis of two fundamental postulates. One is the light postulate (that the speed of light, in the “rest frame”, is independent of the speed of the source), and the other is the principle of relativity. The latter was adopted by Einstein explicitly as a means of restricting the form of the laws, whatever their detailed structure might turn out to be. Thus, we have the difference between a “constructive” and a “principle” theory: in the former case we build our theory based on known facts about the constitution and behaviour of material bodies; in the latter case we start by restricting the possible form of such a theory by adopting certain  principles.[9]