According to Einstein:
where "determination of space-time coincidences" is thought, at least in principle, to be unambiguously, definitively and consistently obtained by each individual participant.
The statement that
the laws of physics are invariant (i.e., identical) in all inertial systems (non-accelerating frames of reference)
can be understood as a less precise (possibly circular) and more restrictive formulation of Einstein's maxime quoted above. (It is based on Einstein's earliest, preliminary attempts at trying to express his maxime.)
(I obviously know what an inertial frame is)
Really?!? (cmp. "What determines which frames are inertial frames?", PSE/q/3193)Really?!? (cmp. "What determines which frames are inertial frames?", PSE/q/3193)