 The second postulate is a bit strange. It says that the speed of light is the same in all inertial reference frames. What's so strange about this? Well, let's suppose there's a ball moving forwards at 30 meters per second and Bob is chasing after it at 10 meters per second. How fast will Bob see the ball moving away from him? Well, the difference in their speeds is 30 minus 10 equals 20 meters per second. Or equivalently, in Bob's reference frame, where Bob is stationary at the origin, the ball will be moving forwards away from Bob at 20 meters per second. This is what we expect from Newtonian mechanics. However, let's see what the second postulate says. Suppose there's a photon, a particle of light, and it's moving forwards at a speed c. c just means the speed of light. Now Bob is chasing after this photon at 0.25c, a quarter the speed of light. Now, I measure the difference between their velocities to be c minus 0.25c is 0.75c, three quarters the speed of light. However, the second postulate says that in Bob's reference frame, the photon will still be moving away from him at the speed of light. Despite the fact that I see Bob running towards it at a quarter the speed of light, he will still measure that photon as moving away from him at c, the full speed of light.