 Since the Maxwell-Lorentz equations for electromagnetics are not Galileo-covariant, the classical principle of relativity according to which measurement in one inertial reference frame can be converted to another by Galilean transformation is called into question. Using in his famous St. Louis lecture of 1904 the designation (physical) “principle of relativity” (Poincaré 1905a: 607), which does not apply “to finite equations that are directly observed, but to differential equations” (Poincaré 1913a: 103; 1963: 19), Poincaré reports that Lorentz introduces the conjectures (i.e., the ad hoc hypotheses) of “local time” and of “uniform contraction in the direction of motion” in an attempt to save the principle in its application to the electromagnetic domain (see Poincaré 1905b: 132 ff.; 1913b: 305 ff.). It was indeed Poincaré (1906) who made Lorentz’s theory fully compatible with the relativity principle. We are thus naturally inclined, remarks Poincaré, to admit the postulate of relativity in every domain. In postulating the principle as a “general law of Nature” (Poincaré 1906: 495), its extended form finds its origin in the Galilean principle, and its motivation in its compatibility with a theory explaining why no experiment is able to inform us of the earth’s motion relative to the ether. So, the principle is an element of a work in progress containing several conjectures and there is the possibility that it will lose its soundness (Poincaré 1905b: 146; 1913b: 319).