Inertial Motion and the First Way

The following is a summary of Edward Feser's answer to the inertia objection as seen in his book Aquinas: A Beginner's Guide.

It is sometimes suggested that the law of inertia provides a counter example to the premise of the argument from change that says "whatever is changing is being changed by something else". An object in motion tends to remain in motion, unless acted upon by an outside force. While the object is moving, nothing is pushing it, and so there is no need for an unmoved mover.

Several responses can be made:

• The word "motion": The word "motion" in the argument refers to all kinds of change, and not just movement from place to place. Changes in quality and changes in quantity still must be changed by something else, per the argument. However, there is no reason to exclude movement from place to place from the argument, as seen in the next points. It is often said that the law of inertia is a "law of physics", and objects just behave that way. But what is it to be a law of physics in the first place? There are different ways of understanding this:
• Instrumentalist view of inertia: One could say that while the law of inertia is a convenient and useful mathematical model of the motion of objects, it does not reflect anything in reality (in the same way that the geocentric model of the solar system still manages to predict the motions of the planets, even though it is not real). One motivation for saying this is that it leads to the absurd conclusion that a finite cause can have an infinite effect.
• Impetus: If the instrumentalist view is correct, and the tendency of objects to remain in motion without any cause is not real, then they must have some cause after all. Some kind of force or impetus keeps them moving at all times,and so inertial movement does not serve as a counter example to the premise that "whatever is changing is being changed by something else."
• Realist view of inertia: Alternatively, one could take the realist view and say that objects in motion do tend to remain in motion without any continual cause of that motion. Objects just have that property. But then something else would have to actualize the potential of such objects to exist, and the argument is unaffected. 
• Inertial motion is a state, not change: Inertial motion is often described (by physics) as a "state" rather than an example of true change. If so, then for that state to alter requires something else, just like the law of inertia says. And the argument from change is unaffected.