So, Oristar, do you feel comfortable with the idea of Galilean relativity?
One example of Galilean relativity is to imagine two spaceships.
Person A (on spaceship A) notices that each second Person B gets 20 meters closer. Person A assumes that he is stationary and concludes that Person B is moving toward him at 20 m/s.
Person B notices that each second person A gets 20 meters closer and assumes that she is stationary and that Person B is moving at 20 m/s.
The problem that each has is that there is no experiment possible that either person could perform to say that he is right.... and there is no experiment possible to show that either person is wrong.
Of course science can make the claim that either is moving within a specified frame of reference... once you choose an arbitrary frame of reference, there is any number of experiments, from measuring how distance changes directly to using sonar to using the doppler effect.
But as long as there is no experiment possible to prove or disprove either persons claim to be stationary in an absolute sense... the question of who is "truly" stationary is not a scientifically valid question.
If there was an experiment that could distinguish a truly stationary person from a not stationary person in an absolute sense, then this would be a valid question.
The claim of Galilean relativity is that laws of mechanics (the branch of physics dealing with velocity, acceleration and motion) work exactly the same, under any experimental conditions, in any frame of reference. If you could devise an experiment where a person who was "truly stationary" could prove the other person, who was not "truly stationary" wrong, then you would disprove Galilean Relativity (an experiment that worked differently for person A from person A's perspective then it did for person B from person B's perspective would do the trick... but there is no such experiment).
But since there is no way by experiment to distinguish between "truly stationary" and "not truly stationary"... the term "truly stationary" has no meaning in any scientific context.
Does this make sense?