The speed of light is constant but with reference to what? Distances are not absolute and neither is time. Is it that all inertial observers agree upon the ratio of the distance to the time that it takes for light to go between the two ends of some object even though they don’t necessarily agree upon the length of the object or the time it takes light to traverse that object? One of the fundamental postulates of Einstein's special theory of relativity is that all inertial observers will measure the same speed of light in vacuum regardless of their relative motion with respect to each other or the source. I am sure that this statement is definitive but I find it confusing. I find it confusing because defining speed means defining it with reference to a distance as well as a ‘time span’ and saying that the distance divided by the ‘time span’ is the same for all inertial observers. But distances in SR are not absolute. Different inertial observers will measure different distances for a given object.