 Special relativity has two postulates. Now a postulate is something that you accept as fact and use it to derive the rest of the theory. So how do we know our postulates are correct? Experiments. Throughout the course of human history, since the dawn of special relativity, no reputable experiment has contradicted either of these postulates. And because of that, we feel it's reasonable to take this to be fact for the time being until they are contradicted by an experiment in the future. So the first postulate is the principle of relativity. And this states that the laws of physics are independent of your inertial reference frame. If I'm standing here and Alice is driving in that direction at 100 miles an hour, both of us observe the same laws of physics. To put it in another way, if I put you on a train and that train was traveling very smoothly, no bumping on the tracks, no acceleration, my were to black out all the windows and stop you from communicating in any way with the outside world, you would not be able to tell what speed the train was traveling at, if it was moving forwards, backwards, or not moving at all. Because in each of those cases, the laws of physics that you would observe would be exactly the same.