I was reading on a conceptual situation, in which the propagation of light was analyzed from two different reference frames. One was from a stationary reference point, whereas the other was from a reference point that was moving with a velocity v in the direction of the propagation of light (v<<c). The velocity of light with respect to the stationary reference body would be c, whereas the velocity of light with respect to the moving reference body would be c-v. This presented a contradiction. Now the contradiction that was being referred to was not a contradiction of the principle of relativity in itself, but a contradiction between the principle of relativity and the constancy of light, right? If so, then what made physicists so sure that light had to retain a constant velocity c in all reference frames?