In Dr. Feser's second response, he addresses the issue of local motion. It seems I need to read his other book, as well as a paper of his that hasn't even been published yet, to get the full story on local motion. Hmm. Since the one requires effort and the other requires violation of causality (physicist's version), it will be a while before I get to that.
In the book (p.102), Feser says local motion is irrelevant because Aquinas means change in general, and so "there would still be other kinds of motion so which Aquinas's argument would apply." As we saw earlier, though, this renders the First Way argument invalid. In the blog post, Feser seems to have abandoned that line. Maybe an implicit concession that it doesn't work?
Instead, he claims that inertial motion is not real change - "it involves no
actualization of potential." (He's actually cagey in how he phrases this, writing, "... if such motion really is a state...." Of the five points he makes, though, this is the only one that explains how Aquinas's argument can be preserved, so I will take it as his (now) preferred solution.)
Now, picture two objects, one in inertial motion toward the other. They get closer ... and closer ... but (according to Feser) there is no change occurring. This is a perfect example of the kind of absurdity you get when you don't allow physics to inform your metaphysics. It is obvious that at least one thing - the distance between the two objects - is changing. It is only by insisting on a rather bizarre definition of "change" that you can call this a case of no change.
A better way to go is to recognize that there are different kinds of change. Then we can decide what kinds of change need an explanation and what kinds do not.
Newtonian physics has a simple and elegant solution. We distinguish between velocity (the rate of change of position) and acceleration (the rate of change of velocity). Then we declare that velocity doesn't need an explanation (it is something that is just in the nature of massive objects), but acceleration does. (The cause of an acceleration is what physicists call "force.") This is not just an advance in physics, it is also an advance in metaphysics. By making finer distinctions among the types of change that occur, we come to a better understanding of the meaning of "cause." Unfortunately for Feser and the First Way, with these new definitions it is no longer true that "Whatever changes is changed by something else."
As I said earlier, of course you can find definitions of "actuality", "potentiality," and "change" so that every change always has an external source. But if you find yourself insisting that even though those two objects are getting closer together, no change is occurring, then you ought to suspect that something is wrong with your definitions.
ETA: I apologize that the middle part of this post is so poorly worded. In his second point under "Newton and local motion", Feser does say that there are two kinds of "change" (what he calls "motion"): the Newtonian kind and the AT actualization-of-potentials kind. Let's call the first n-change and the second at-change. Now he can proceed with the First Way:
1'. Whatever at-changes must be at-changed by something else.
2. The causal series can't go on to infinity.
3. Therefore, there must be a first at-changer.
(Note that the "first mover", which here becomes "first at-changer", is not yet the Unmoved Mover that Aquinas eventually reaches in his argument. All we can get from (1'.) and (2.) is that there is some first element in this particular causal series. There is no reason, as yet, to think that this must be "pure act".)
At this point, though, we have to allow that the whole series of at-changes might have been set off by an n-change. For instance, suppose our inertially moving object (A) and our stationary object (B) have a short-range force that acts between them, such that there is no force when the objects are far away, but a force begins to act when A gets close enough to B. When B feels this force, a series of at-changes begins: B breaks a laser beam that sets off a bomb, say.
So, even though the argument up to (3.) is logically sound, it doesn't get Feser what he needs: even though there is a first at-changer, the true initiating cause of the whole series is an n-change: something that doesn't count as a change at all in Feser's metaphysics!
Hope this makes things clearer....