So, basically, I think Aquinas can prove effectively that there should be a first mover. Here, I'm refering to the First Way, more specifically, for I was reading Feser's book, from which the question I'm going to ask came, but I do think this argument works for any of Aquinas' Ways.
The thing is, I have the following question: how can he prove that a first mover should be necessarily not only unmoved, but also unmovable? For if it's movable, then it has potentiality, and if it does, then it's not pure act. I mean, I do assume Aquinas was right in saying that nothing made the first mover come to exist; in fact, it doesn't have a mover himself, but only before it created other things. This does lead us to the conclusion that there wasn't potentiality in Aquinas' first mover's existence, for it was actually act (precisely because he's the first in the causal order). But even though this proves – let's assume it does, at least for now – there should be a first mover whose existence wasn't a potentiality made act, but was already act, it doesn't prove it (the mover) doesn't have any potential, nor does it prove that after creating things, it couldn't be putting towards movement by them.
Consider a father. Obviously, his son just exists because his father was his efficient cause. But this doesn't mean that, because the father was such a cause here, the product of this act can't actualize his potentials, for even though for exist, the son pressupposed the father, as soon as the son was created, turns out it could actualize the father by himself, since he already exists. I'd think the same should apply to the second cause, for if it exists, then it should have the logical right of possibly actualizing what caused it (the first mover). This, obviously, would mean the mover isn't pure act at all. An external act would be actualizing its potentials.
How does Aquinas answer to this? How can he prove that, as soon as a second cause exists (due to the first mover, it's true), this second cause can't actualize any potential in the first? He should prove it because the opposite would mean this first cause isn't pure act at all, therefore, not God. Thanks in advance.