Having now considered these three cases, what shall we say about the plausibility of specific naturalistic constraints on metaphysics? First, I believe that the cases illustrate (what seems independently plausible) that not every apparent demand that issues from science should be accorded equal weight. Not all scientific claims are on an epistemic par; nor are all metaphysical claims. We cannot, then, assume that entire disciplines can be neatly stacked in terms of their epistemic superiority. Perhaps some claims that appear to be metaphysical in nature—assuming that this status can be defined independently of their evidential grounds—are in a position of trumping claims extending from the natural sciences. At the level of conflict between particular claims, science might well admit of epistemological rivals even if it is unrivaled as an intellectual/epistemic corpus. We cannot reasonably prioritize the scientific over the metaphysical any more than we may reasonably prioritize beliefs formed by, say, vision over those formed via testimony as justificatorily superior.
Second, in evaluating and resolving apparent conflicts, we need to take into account both the relative epistemic standing of the competing claims and the role that potentially silent auxiliary metaphysical theses might play in generating that competition. Third, and relatedly, we should be cognizant of a problem of unconceived alternatives for scientific metaphysics and draw inferences about the correct metaphysics of species (or what have you) cautiously.
As these conclusions function as partial checks on naturalistic constraint, the story so far is largely negative. Is there anything we can say at this point on the positive side about how we ought to naturalize metaphysics? Are there principles for resolving the sorts of apparent conflicts we have examined between scientific and metaphysical claims? How, in general, can we give specific, defensible, and nonvacuous content to the concept of naturalized metaphysics ? I am not confident that we can. This is not to deny that there are many cases of metaphysical overreach where naturalistic ideals are flagrantly ignored. But we can often diagnose such cases—as illustrated by essentialism about species—as metaphysics failing to take into account well-established facts that bear on the theses in question. I take it, however, that such input can come from any source. Science is generally good at producing claims that deserve to be taken seriously. Many of these obviously bear on philosophical claims. Thus they should be sought out and acknowledged by philosophers and taken into account as appropriate. It is this last imperative that seems to me difficult to assign precise content. My suggestion that we distinguish between scientific practice, norms of classification (or other aspects of scientific practice), and empirical findings, theories, proposals, and working hypotheses (and distinguish further within these categories according to their epistemic standing) may be part of the story, but I do not assume that we can map very precise methodological constraints onto such divisions.
This story might change somewhat depending on how we understand the project of scientific metaphysics (or metaphysics generally). I argued in the previous section that naturalistic constraint should be at best hesitant in contexts in which pluralism may be expected to hold sway (say, in cases where methodological norms or classificatory practice was largely contingent). Even if a particular piece of scientific practice has developed in a uniform way, if it might have legitimately developed differently (with different metaphysical consequences), then the naturalist cannot reasonably insist on grounds of scientific consensus that the corresponding metaphysics must be this or that other way. But suppose we adopted an approach to metaphysics like the one Peter Godfrey-Smith describes, according to which “the theoretical constructs developed in systematic metaphysics are best seen as models. Metaphysical system-building is model building” (2006, 6). On this view, concerns about pluralism and contingency fall away. A consensus classificatory practice might be as contingent as you like; we can still model it and discuss its ontology in this deflationary sense without any pretense that we are mirroring the fundamental structure of nature.
Your mileage may vary; maybe this does not seem like doing metaphysics. Fair enough. Yet perhaps we can maintain something of the spirit of Godfrey-Smith’s conception while moving in the direction of (traditional) metaphysics naturalized. Here I am reminded of some remarks of Mark Johnston’s at the outset of an important essay on how we might formulate a defensible, non-verificationist Pragmatism; he writes,
Let us say that metaphysics in the pejorative sense is a confused conception of what legitimates our practices; confused because metaphysics in this sense is a series of pictures of the world as containing various independent demands for our practices, when the only real legitimation of those practices consists in showing their worthiness to survive on the testing ground of everyday life. Then metaphysics is not just a technical discourse within philosophy to which, since Kant, a technical apparatus of philosophical criticism has been opposed.
It is endemic to our culture. So defined, metaphysics is the proper object of that practical criticism which asks whether the apparently legitimating stories which help sustain our practices really do legitimate, and whether the real explanations of our practices allow us to justify them. There then ought to be a critical philosophy which not only corrals the developed manifestations of metaphysics within philosophy but also serves the ends of practical criticism. Such a critical philosophy would be the content of anything that deserved the name of a progressive Pragmatism. (1993, 85)
Some versions of naturalism can be characterized as strong reactions against Johnston’s “metaphysics in the pejorative sense”; I have argued that such naturalists err in asserting too strong an asymmetry in the opposite direction, “corralling” metaphysics too aggressively. I do not claim that naturalized metaphysics should be seen as an outcropping of pragmatism. Rather I see a progressively naturalized metaphysics as a regulative ideal in which competing demands of practice, norms, features of disciplinary discourse (both scientific and metaphysical), and our best theories (both scientific and metaphysical) are weighed against one another in order to develop the best picture of the world that we can manage. This pragmatic stance seems superior to a discipline-centered approach in its ability to avoid the problems discussed above while making flexible use of our epistemic resources. If more can be said in this direction, we will just have to evaluate such proposals on their merits.
-  I read McLeod and Parsons as making a similar claim when, in the finale of their critical response to Maclaurinand Dyke’s previously discussed criterion for naturalistic acceptability (2012), they write that “it’s not possibleto determine which theories are [naturalistically acceptable] just by understanding what those theories say. Todetermine whether a theory is naturalistic, we have to do some philosophy (and possibly some science too)”(2013,177-78).