I'm currently reading through Hume's An Inquiry Concerning Human Understanding, and I'm having trouble understanding one of his big arguments in the section entitled "Skeptical Doubts Concerning the Operation of the Understanding: Part 1". After arguing that so-called "Matters of Fact" can only be known via experience, and that such understanding rests on cause and effect reasoning, Hume goes on to argue the following:
Hence we may discover the reason why no philosopher, who is rational and modest, has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power, which produces any single effect in the universe. It is confessed, that the utmost effort of human reason is to reduce the principles, productive of natural phenomena, to a greater simplicity, and to resolve the many particular effects into a few general causes, by means of reasonings from analogy, experience, and observation. But as to the causes of these general causes, we should in vain attempt their discovery; nor shall we ever be able to satisfy ourselves, by any particular explication of them. These ultimate springs and principles are totally shut up from human curiosity and enquiry
My questions are multiple. Firstly, what (if anything) is the difference for Hume between an "ultimate" and a "general" cause? Are they interchangeable, or does "ultimate" cause mean something more akin to an uncaused cause?
Secondly (this is my central confusion), why exactly is it the case according to Hume that ultimate causes cannot be ascertained, given what he's argued so far in the essay? Hume has so far said that causes and effects can't be known simply through a priori reason alone, an argument I follow. However, is it not possible that causes and effects, and certain general "laws" that follow from them (e.g. laws of physics) which have been learned from experience, can be applied to situations where a cause and effect relationship is not yet known, i.e. in the investigation of some "ultimate" (or "general", depending on the answer to the first question) cause? For instance (and my lack of knowledge of physics and cosmology shows here), is the theory of the Big Bang not such a deduction? Have scientists not extrapolated from existing theories (themselves learned from experience) and deduced the necessity of the Big Bang? Is the Big Bang the kind of "ultimate" cause Hume is referring to? Obviously the theory wasn't known to Hume, but is it the kind of cause he was referring to?
Finally, when Hume says that we will never "be able to satisfy ourselves" with justifications of matters of fact, is this because such justifications must always either, 1) end with an uncaused cause (which may be unsatisfying) or, 2) must involve an infinite regress (which may also be unsatisfying)? Thanks in advance for any insights into these questions!