Didn't originally see this comment. (That's what I get for leaving a tab open for hours before bothering to reply.)

No worries, I was offline anyway. Came back to find both your comments.

Your second paragraph differs from my understanding of Maitzen's core argument, but it's possible I misread him.

This is why my original, obscenely long comment contained quotes for every point.

Naturalists point to the many phenomena we used to attribute to supernatural agents but can now explain scientifically: the change of seasons, the course of a disease, the orbits of planets, and on and on. Their theistic opponents often admit that natural science has discovered not only good piecemeal explanations of the existence of particular phenomena but even good integrated explanations of the existence and operation of entire systems. In this sense, the opponents concede that natural science can answer not only mechanistic ‘‘how’’ questions but also existential ‘‘why’’ questions, such as ‘‘Why are there penguins?’’ or ‘‘Why is there cancer?’’ Yet they hasten to point out that natural science hasn’t explained why there exists anything at all: not specific things or kinds of things but anything in the first place, anything in general.

[...]

Properly put, then, the challenge to naturalism is that natural science may do a fine job accounting for particular contingent, concrete things and kinds of things, but it isn’t equipped or even meant to tell us why any such things exist at all.

Other philosophers regard the challenge as well-posed but sufficiently met if natural science can explain the existence of each given contingent, concrete thing. Their spokesman is Hume’s Cleanthes: ‘‘Did I show you the particular causes of each individual in a collection of twenty particles of matter, I should think it very unreasonable, should you afterwards ask me, what was the cause of the whole twenty. This is sufficiently explained in explaining the cause of the parts.’’

[...]

The nature of my complaint may become clearer if we imagine the following exchange:

A: Why is there anything?

B: What do you mean? Are you asking why numbers exist?

A: No. If numbers exist, they had to exist. Why is there anything that didn’t have to exist?

B: So you’re asking why there are any contingent things. Well, there are pens, which are contingent things, and here’s how pens come to exist—

A: —No! I’m not asking why there are any pens.

B: All right then. Penguins exist, and they’re contingent. Penguins evolved from—

A: —No! I’m not asking why there are penguins either. I’m asking why there are any contingent things at all.

B’s answers may seem deliberately obtuse, but they bring out the emptiness of A’s questions: A rejects each of B’s attempts to supply determinate content to the dummy sortal ‘contingent things’, but without such content there’s no determinate question being asked. Once ‘contingent things’ takes on content (e.g., in one of the ways B suggests), the resulting question becomes empirical and scientifically answerable. Or suppose I mention pens, plums, and penguins. You then ask me, ‘‘Why are there any of the things you just mentioned?’’ but tell me you don’t want explanations of the existence of pens, plums, or penguins in particular; instead, you want to know why there are any of the things I just mentioned (with table-pounding emphasis on ‘any’) rather than none at all. Clearly your attitude is perverse: ‘the things I just mentioned’ is only a covering term for pens, plums, and penguins; it doesn’t pick out a category of thing requiring an explanation beyond those I was already prepared to give and you didn’t want to hear. Likewise for ‘contingent things’ and the other dummy sortals I’ve discussed: there aren’t any contingent things whose explanations outstrip the explanations available for the individuals covered by the covering term ‘contingent things’. For the same reason, we can see that the question ‘‘Why does the Universe exist?’’ taken in the way that objectors to naturalism must intend it, also poses no unanswerable challenge to naturalism, for it amounts to asking (again) ‘‘Why are there any contingent, concrete things at all?’’ or (again) ‘‘Why are there these contingent, concrete things rather than none at all?’’ or perhaps ‘‘Why are there these contingent, concrete things rather than other such things?’’ Once we substitute true sortals (‘pens’, ‘plums’, ‘penguins’, etc.) so that those latter questions have more sense than the question ‘‘Exactly how many contingent, concrete things are you holding in your hand?’’ they seem to admit of naturalistic answers. If, moreover, the explanatory challenge to naturalism should consist of a long disjunctive question—‘‘Why are there pens, or plums, or penguins, or…?’’—then of course naturalism can offer a long disjunctive answer.

The closest he comes to answering the actual question is this...

At this point, defenders of supernaturalism might counter that naturalistic explanations must ultimately bottom out at brute, unexplained posits. But I see no reason naturalistic explanations can’t go forever deeper. One bad reason for concluding that they can’t is the notion that x can’t explain y unless x itself is self explanatory. I don’t see that notion as at all implied by our ordinary concept of explanation, which allows that x can explain y even if something else altogether explains x. Moreover, there are grounds for thinking that naturalistic explanations not only could but must go forever deeper. A common attitude among scientists is that the more they discover, the more there is yet to discover—the more they know, the more they realize they don’t know—a pattern there’s no reason to think won’t continue indefinitely. Indeed, scientific discoveries routinely raise at least as many questions as they answer. Biologists have described some 80,000 species of roundworm, for example, but suspect there might be a million species. More generally, having discovered organisms in places they didn’t think could support life, biologists now worry that they lack even a rough idea of the total number of species; knowing more shows us we know less than we thought we knew. Furthermore, history teaches, just when some scientists begin to think the explanatory end is in sight, a revolution comes along to open domains of further inquiry. Maxwell gives way to Planck and Einstein, and Hilbert gives way to Go¨del. Jonathan Schaffer usefully catalogues several other examples of this kind.

... which, naturally, misses the point. Yes, we can imagine something infinitely old and fractally complex existing - although there may be some technical reason why it's impossible, I don't know of any - but we can also, counterfactually, imagine it not existing, and declaring it's turtles all the way down does not explain why this counterfactual is not true (in fact, I think it probably is true, because blah blah complexity bah blah Occam's Razor.)

People automatically attach an implicitly understood meaning to the word "things" (or "anything") when it shows up in everyday sentences, and that often works OK. But given the atypical question "Why is there anything?", it's not obvious which meaning they should substitute in, and their brains start flailing. Or so Maitzen speculates.

Except that you don't need to substitute in something specific for "anything", since it's just the set of all things - including all those possible things he lists. This might be clearer if we said "everything" or "something"?