「慣性系」が初めて定義されたのは、ニュートン死後150年経ってから。直線の定義と不可分よ。

https://stanford.library.sydney.edu.au/archives/fall2016/entries/spacetime-iframes/
https://stanford.library.sydney.edu.au/archives/fall2016/entries/spacetime-iframes/figure3.gif
Figure 3: Lange's Definition of ‘inertial system’ (1885):
An inertial system is a coordinate system with respect to which three free particles, projected from a single point and moving in non-coplanar directions, move in straight lines and travel mutually-proportional distances. The law of inertia then states that relative to any inertial system, any fourth free particle will move uniformly.