The laws by which the states of physical systems undergo changes are independent of whether these changes of state are referred to one or the other of two coordinate systems in uniform relative translational motion. All logical consequences of the laws by which the states of physical systems undergo changes are independent of whether these changes of state are referred to one or the other of two coordinate systems in uniform relative translational motion. All the laws of physics are the same in every inertial system. The laws of physics cannot provide a way to distinguish one inertial frame from another. I.e., not only the laws of mechanics but also the laws of electromagnetism and all other laws of physics hold true as well in one inertial reference frame as in any other inertial reference frame.
All inertial observers are equivalent.
It is impossible to distinguish one inertial observer from any other by internal experiment. There are no privileged inertial observers. The laws of physics are the same for all non-accelerating observers. The laws by which the states of physical systems undergo changes are independent of the inertial fram to which they are referred. The laws of physical change are independent of the inertial frame to which they are referred.