I know this is a physical principle and cannot be derived at from other physical laws. But how can it be arrived at? What observations or what train of thoughts leads to this conclusion?
e.g. the Newton's second law of motion is also a physical principle and cannot be derived. But from observations like 'a body moving at constant velocity on a frictionless surface doesn't tend to stop' and many others, it is understandable to arrive at the Newton's laws. What observations or thought process lead Einstein to this postulate?
