From Barry's post:
According to our putatively final metaphysics, the world is one big chunk of matter. I think this leaves us with two questions, which the naturalist lacks the resources to answer. (1) Why is the world made of this stuff, and not some other stuff? (2) Why is there this stuff and not nothing at all?
Our explanans must always be explanatorily broader than our explanandum. You cannot explain alpha with alpha; you must explain it with some beta that may include alpha. (The fact that the girl crossed the road does not explain the fact that the girl crossed the road, but it might explain the fact that the girl crossed some part of the road.) As the Aristotelians say, explanations must involve not just the 'that' but also some 'because.'
In Nozick's Philosophical Explanations, he says something like: "What did philosophers expect? Of course if they keep investigating things they're going to reach something that can't be explained, an intractable problem: why is there something rather than nothing?"
But the solution need not be some brute fact. Rather, I see three possibilities:
1. The series of explanations is infinite. Every explanandum has its explanans, forever.
2. There really is some starting point, some brute fact of the matter. The world just is, full stop (some sort of recursive explanation might fit here).
3. It's circular: A is explained by B, which is explained by C, which is explained... which is explained by A.
I find all of these profoundly unsatisfying, but I know of no other options. Infinity's unpalatable, a brute fact seems arbitrary, and circularity is widely considered erroneous.* This dilemma (trilemma) shows up in various areas.
Think of justification of beliefs: do we have to 1. justify every belief with some other belief, ad infinitum? Or 2. take some beliefs as brute facts? Or 3. are beliefs justified in a circular fashion?
Or think of the creation of the Universe. Was 1. God created by meta-God, who in turn was created by meta-meta-God, ad infinitum? Or is 2. God a primitive? Or 3. did God create us so we can later create him?
*I have heard that the viability of circular arguments is not logically impossible. I'm not sure how I feel about that. I do admit that I seem to be able to conceive of a circular argument proving something--whereas I certainly cannot conceive of a four-sided triangle, or the like. If so, there's nothing logically wrong with the circular argument--the problem is elsewhere. Alternatively, I simply haven't reflected enough on the problem, when, if I did so, it would become apparent that circular arguments really are necessarily invalid.