How I studied relativity, we postulated that a particle travelling at c in one inertial frame travels at c in all inertial frame. But now looking through a book, I see that they just postulate that all laws of physics are same in all inertial frames, and that there is a speed limit (c). However then I don't quite get the reasoning on why if an object travels at c in one inertial frame, it follows that it must travel at c in all inertial frames...Is there a contradiction I'm not seeing that if you measure a particle travelling at c, someone moving much faster than you doesn't necessarily have to? This wouldn't change the speed limit in their frame. (I know this is wrong, I'm just trying to deduce it from how the book did it).