Having decided to try to understand the mathematics underlying Special Relativity, I have come up against an initial problem. I read that one of its postulates is that measurements of the speed of light, in any inertial frame of reference, return the same value, the value c. What I have so far failed to find is the basis for this postulate. To me, the term postulate suggests that the notion of the constancy of the velocity of light was not derived by experiment or theory. Is that, in fact, the case? If so, was it plucked out of thin air, so to speak (and by whom?), and If not, then what is its basis?