So here’s a thought.
We imagine the Leibnizian saying that if there were an infinitely regressive series of facts, all of which were contingent, one would still require an explanation over and beyond the set of such facts. The Humean, we can imagine, responds by noting that if every fact in an infinite series is explained by its immediate antecedent, then any fact you pick out is explained. If all the facts in the set are explained, then there seems to be nothing left-over, nothing left-out. The Leibnizian then responds that this explanation has the logical form of a viciously circular argument; she points out that since the set of every second fact (P2n) is what explains the set of all facts which immediately follow (P2n-1), and since that latter set (P2n-1) explains the former (P2n), the Humean is running an explanation in a circle. Suppose then that the Humean replies as follows:
For any fact (Pn) one picks out of the set of infinitely many facts, that fact is itself explained by the set of facts Px where x>n.
Obviously if any fact is explained by the infinitely long conjunction of antecedent facts, most of which are mediately antecedent instead of immediately antecedent, then one may be able to wiggle out of the objection that the reasoning is circular. The Pruss-ian parody cannot be run.
I’m not sure this is right, since the Humean generally wants us to believe that if a chicken appeared ex nihilo, and laid an egg, then the egg will clearly have a satisfactory explanation regardless of whether the chicken has one. Moreover, the Humean generally wants to say that it is logically possible that a chicken (or whatever) exist ex nihilo. However, the counter-apologist could always just adopt the Humean move and then drop the rest of the Humean philosophy, and thus claim that the way in which they intend to explain any given fact in an infinite series of facts is not by merely citing it’s immediate antecedent.