I’ve struggled with the concept of inertia as derived from Newton’s first law of motion for the following reason: IF, as postulated by Einstein, gravitational acceleration is not caused by the application of a force to an object but is caused by the curvature of space-time… AND IF space-time is curved at all locations in the universe to some extent, however small… THEN all free objects at all locations everywhere will accelerate without the application of any force. …which directly contradicts the definition of inertia as the property of objects to resist acceleration unless acted upon by an external force. In fact the opposite of this definition of inertia true. Let’s call the acceleration at all locations in the universe due to space-time curvature the “ambient” acceleration for emphasis. Then we can say that all free objects will accelerate without any force acting on them meaning that inertia is not that property of an object that causes it to resist acceleration, rather inertia is that property of an object that causes it to resist deviating from the ambient acceleration. A (real = measure-able) force is required to cause an object to deviate from the ambient acceleration. This definition of inertia makes sense to me and has the added benefit of by-passing much of the confusion surrounding attempts to define inertial and non-inertial reference frames.