 So where do these postulates come from? Well the relativity principle seems reasonable enough, right? If I get in a car and go for a ride, I don't expect the laws of physics to suddenly change. But what about the second postulate? Well the laws of electromagnetism actually predict that light should travel at a particular speed. Scientists used to believe that all of space was filled with an invisible fluid called the aether, and light travelled as a wave through this. Now if this was true, the aether as a fluid would flow, and if it was flowing in this direction, as any light travelling in this direction would be carried along by the fluid and travel faster, as opposed to if it was travelling in that direction. In 1887 however, a famous experiment by Michelsen and Moorley measured the speed of light in orthogonal directions, in this direction and in that direction. To their surprise, they found that it was the same. Speed of light didn't matter in which direction you shined. Now this spelled the end for aether-based theories, as this and future experiments showed, that even if the aether did exist, it would have no measurable effect on the physical world around us, so there would be no point talking about it. Ultimately, acceptance of the postulates comes down to experiment. However since the postulates and special relativity were first put forward, not a single reputable experiment has ever contradicted either of them. And because of this, it seems reasonable to build a theory using them as the basis, and we can be happy with this until such a point in the future when an experiment is performed which contradicts either of the postulates.