Hume asks the question of how a finite effect can have an infinite cause.  Kant says that you can base it on the moral argument. If you are explaining the subsets, you have the explanation of the infinite set (according to Hume).  However, in explaining the first member of a finite set, one must go out of that set.  Focus on a series in which a member’s existence is explained by the preceding cause.  In explaining a set you go outside of the set.  The point is not if it’s an infinite set but if it gets outside of the set (relies on PSRb in a way–that there must be some sufficient reasons for any positive fact and denies brute facts.)

Hume also argues against the CA on a priori grounds.  He formulates the CA in a way that combines causal (Thomistic or kalam) and reason (Leibnizian).  He considers the causal closure of the universe.  Under his idea of how the universe is he considers it to be a causally closed system, which, by definition, rules out any external causation.  Any a priori argument is absurd to Hume—it’s impossible to deduce the existence of God from fixed necessary premises or Kantian/Newtonian spacetime intuitions.