Newton's first law is part definition and part experimental. Isolated bodies move uniformly in inertial systems by virtue of the definition of an inertial system. In contrast, the assertion that inertial systems exist is a statement about the physical world.
According to me, the assertion in the last lines follows from the following statement.
It is always possible to find a coordinate system in which isolated bodies move uniformly.
Am I right to think that the assertion follows from the above statement?
Next, an isolated body is considered to be free of forces. A body, not isolated, but experiencing a net zero force also moves uniformly in inertial systems. I state the following:
It is always possible to find a coordinate system in which bodies experiencing a net zero force move uniformly.
While I think this makes better sense to me, I am still leaving some room for doubt. Could I be wrong in making the above statement?