 Einstein’s Special Theory of Relativity (STR) is notionally based on a principle of relativity of motion; but that principle is ‘special’ – meaning, restricted. The relativity principle built into STR is in fact nothing other than the Galilean principle of relativity, which is built into Newtonian  physics.[3]  In other words, while there is no privileged standard of velocity, there is nevertheless a determinate fact of the matter about whether a body has accelerated or non-accelerated (i.e., inertial) motion. In this regard, the spacetime of STR is exactly like Galilean spacetime (discussed in the entry on  absolute and relational space and motion: classical theories,  section 5). In terms of the question of whether all motion can be considered purely relative, one could argue that there is nothing new brought to the table by the introduction of Einstein’s STR – at least, as far as mechanics is concerned. (See the entry on  space and time: inertial frames  for a more detailed discussion.)