Special and General Principle of Relativity
It was at all times clear that, from the point of view of the idea it conveys to us, every motion must be considered only as a relative motion. Returning to the illustration we have frequently used of the embankment and the railway carriage, we can express the fact of the motion here taking place in the following two forms, both of which are equally justiﬁable:
(a) The carriage is in motion relative to the embankment. (b) The embankment is in motion relative to the carriage.