I raised a pretty dumb question in our discussion last week, but in keeping with well-known clichés still managed to learn something from it. Puzzling over Aristotle's concept of this highest science, I wondered whether he changes his concept of the science after Alpha, or merely changes the focus of his inquiry a bit. In Alpha he says it's all about aitia and archai but then later he seems more concerned with ousia.
My badly-put question was something like "Does Aristotle switch his attention from describing the causes, to describing what they act upon?" And of course that's not a sensible question: three of the causes sort of "act upon" the fourth. The efficient cause imparts motion on the matter, the material cause. The formal cause is what is imposed on the matter through the efficient cause. And on one reading of final cause, it is what draws this material-formal complex thing into being: it has a purpose, else it wouldn't have been made.
The question betrays that I'm still thinking of all four "causes" as efficient causes, just like a modern materialist scientist. Which I sort of am, I suppose, to the extent that I retain any residual effects of some scientific training that took place a long time ago.
A better way to put the question (and here's where I managed to learn something) would be this: "What is the being of which these four are the archai or aitia?" Metaphysics is a science of aitia, and the aitia are the "causes" of, what...? Well, of beings. Of ousia. So understanding causes is one side of the coin; the other side is the understanding of ousia. Hence the attention to both.