Letting 'CCB' abbreviate 'concrete contingent beings,' we may formulate the modified Leibniz question as follows: Why are there any CCBs at all? We have been discussing whether this question is a pseudo-question. To be precise, we have been discussing whether it is a pseudo-question on the assumption that it does not collapse into one or more naturalistically tractable questions: questions that can be answered by natural science.
My thesis is that the modified Leibniz question is a genuine question that does not collapse into one or more naturalistically tractable questions.
Consider a universe that consists of a beginningless actually infinite series of contingent beings. Let us assume that each CCB in this universe is (deterministically) caused by a preceding CCB. The beginninglessness of the series insures that every CCB has a cause. Since every CCB has a cause, each has a causal explanation in terms of an earlier one. And since each has a causal explanation, the whole lot of them does. (Some may smell the fallacy of composition in this last sentence, but let's assume arguendo that no fallacy has been committed.) Accordingly, the totality of CCBs, the universe, has an explanation in virtue of each CCB's having an explanation.
Some will say that on this scenario the modified Leibniz question has received a naturalistic answer. Why are there CCBs as as opposed to no CCBs? Because each CCB is causally explained by other CCBs, and because explaining each of them amounts to explaining the whole lot of them. And since the question has this naturalistic or universe-immanent answer, the specifically philiosophical form of the question, the question as Leibniz intended it, is a pseudo-question.
Others, like me, will insist that on the scenario sketched the question has not been answered. We will insist that a legitimate question remains: why is there this whole infinite system of contingent beings? After all, it is contingent, just as its parts are contingent whether taken distributively or collectively. There might not have been any concrete contingent beings at all, in which case there would not have been any CCBs to cause other CCBs. And nothing is changed by the fact that the series of CCBs is actually infinite in the past direction. The fact that the series always existed does not show that it could not have failed to exist. The temporal 'always' does not get the length of the modal 'necessarily.' If time is infinite in both directions, and the universe exists at every time, it does not follow that the universe necessarily exists. But if it contingently exists, then we are entitled to ask why it exists.
It is no answer to be told that each member of the universe, each CCB, is caused by others. I may cheerfully grant that but still sensibly ask: But what accounts for the whole causal system in the first place?
Please note that a possible answer here is: nothing does. The existence of the universe is a brute fact. Nothing I have said entails a theistic answer. My point is simply that the modified Leibniz question is a genuine question that cannot be answered by invoking causal relations within the universe.
There another line of attack open to me, one that focuses on the connection between causation and existence. It seems to me that the naturalist or 'immanentist' must assume that if x causes y, then x causes y to exist. The assumption, in other words, is that causation is existentially productive, that the cause brings the effect into existence. But on what theory of causation that the naturalist is likely to accept is causation productive?
This is a huge topic and I can only begin to explore it in this post. Suppose our naturalist, good empiricist that he is, subscribes to a Humean or regularity theory of causation along the following lines:
RT. x (directly) causes y =df (i) x and y are spatiotemporally contiguous; (ii) x occurs earlier than y; (iii) x and y are subsumed under event types X and Y that are related by the de facto empirical generalization that all events of type X are followed by events of type Y.
If this is what causation is, it is is not existentially productive: the cause does not produce, bring about, bring into existence the effect. On the contrary, the holding of the causal relation presupposes the existence of the cause-event and the effect-event. It follows that causation as understood on (RT) merely orders already existent events and cannot account for the very existence of these events.
Of course, the naturalist needn't be a Humean about causation. But then he ought to tell us what theory of causation he accepts and how it can be pressed into service to explain the very existence of CCBs.
For details and a much more rigorous development, see my article "The Hume-Edwards Objection to the Cosmological Argument," Journal of Philosophical Research, vol. XXII, 1997, pp. 425-443.