I know a classical mechanics law points out the following (Newton's first law): material particles with constant velocity will continue to move uniformly in straight line. If material particles are in a state of rest they will continue to be at rest. This law is just valid for some kind of reference frames (RFs) (to be more accurate, it is just valid for RFs with certain singular states of motion).
The first question coming to my mind was:
Why are there valid and invalid reference-bodies in classical mechanics? I know Newton's laws are just valid at inertial reference frames, but the same question came to my mind: why?
Doing some research I read Einstein thought it was not possible to find the reason why bodies had different behaviours considered with respect to different reference systems (using classical mechanics). Also I read Newton tried to invalidate this preference but it was not possible.