Taking the metaphysical question of the ontology of objects as his test case, Kriegel argues that metaphysical disputes often fail to admit of resolution. They fail to admit of resolution, Kriegel argues, not because such disputes lack substance or are merely terminological, but for the more interestingly skeptical reason that although there may be a fact of the matter which metaphysical position is correct, we have no good means of discovering that fact. I have argued for a similarly skeptical position about the "mind-body problem", that is, the question of the relationship between mind and matter. (Hence my pro-Kriegel bias.) But Kriegel develops his argument in some respects more systematically.
Consider a set of four fundamental particles joined together in a tetrahedron. How many objects are there really? A conservative ontological view might say that really there are just four objects and no more: the four fundamental particles "arranged tetrahedronwise". A liberal ontological view might say that really there are fifteen objects: each of the four particles, plus each of the six possible pairings of the particles, plus each of the four possible triplets, plus the whole tetrahedron. An intermediate ("common sense"?) view might hold that the individual particles are each real objects, and so is the tetrahedron, but not the pairs and triplets, for a total of five objects.
Now who is right? Kriegel envisions three possible approaches to determining where the truth lies: empirical testing, appeal to "intuition", and appeal to theoretical virtues like simplicity and parsimony. However, none of these approaches seems promising.
Contra empirical testing: There is, it seems, no empirical fact on which the conservative and liberal would disagree. It's not like we could bombard the tetrahedron with radiation of some sort and the conservative would predict one thing, the liberal another.
Contra appeal to intuition: "Intuition" is a problematic concept in metaphilosophy. But maybe it means something like common sense or coherence with pre-theoretical opinion. Intuition in this sense might favor the five-object answer (the four particles plus the whole), but that's not entirely clear. However, Kriegel argues, hewing to intuition means doing only what P.F. Strawson calls "descriptive metaphysics" -- metaphysics that aims merely to reveal the structure of reality implicit in people's (possibly misguided) conceptual schemes. If we're aiming to discover real metaphysical truths, and not merely what's already implicit in ordinary opinion, we are doing instead what Kriegel and Strawson call "revisionary metaphysics"; and although descriptive metaphysics is beholden to intuition, revisionary metaphysics is not.
Contra appeal to theoretical virtue: Theoretical virtues like simplicity and parsimony might be pragmatic or aesthetic virtues but, Kriegel argues, there seems to be no reason to regard them as truth-conducive in metaphysics. Is there reason to think that the world is simple, and thus that a simple metaphysical theory is more likely to be true than a complex one? Is there reason to think the world contains few entities, and thus that a parsimonious metaphysical theory that posits fewer entities than its rivals is more likely to be true? Kriegel suggests not.
As I said, I love this paper and I'm sympathetic with its conclusion. But I'm a philosopher, so I can't possibly agree entirely with another philosopher working in the same area. That's just not in our nature! So let me issue two complaints.
First: I'm not sure object ontology is the best test case for Kriegel's view. Maybe there's a real fact of the matter whether there are four, five, or fifteen objects in our tetrahedron, but it's not obviously so. It seems like a good case for reinterpretation as a terminological dispute, if any case is. If Kriegel wants to make a general case for metaphysical skepticism, he might do better to choose a dispute that's less tempting to dismiss as terminological, such as the dispute about whether there are immaterial substances or properties. (In fairness, I happen to know he is working on this now.)
Second: It seems to me that Kriegel commits to more strongly negative opinions about the epistemic value of intuition and theoretical virtue than is necessary or plausible. It sounds, in places, like Kriegel is committing to saying that there's no epistemic value, for metaphysics, in harmony with pre-theoretical intuition or in theoretical virtues like simplicity. These are strong claims! We can admit, more plausibly I think, that intuitiveness, simplicity, explanatory power, etc., have some epistemic value while still holding that the kinds of positions that philosophers regard as real metaphysical contenders tend not to admit of decisive resolution by appeal to such factors. Real metaphysical contenders will conflict with some intuitions and harmonize with others, and they will tend to have different competing sets of theoretical virtues and vices, engendering debates it's difficult to see any hope of resolving with our current tools and capacities.
Consider this: It seems very unlikely that the metaphysical truth is that there are exactly fourteen objects in our tetrahedron, to wit, all of the combinations admitted by the liberal view except for one of the four triplet combinations. Such a view seems arbitrary, asymmetric, unsimple, and intuitively bizarre, compared to the more standard options. If you agree, then you should accept, contra a strong reading of Kriegel's argument, that those sorts of theoretical considerations can take us some distance, epistemically. It's just that they aren't likely to take us all the way.