I. A Common Apologetic Strategy
Many apologetic critiques of naturalism share a common basic strategy: point to a piece of data (e.g., abstract objects, morality, consciousness, the apparent fine-tuning of the universe, the apparent contingency of the universe, etc.), say that it doesn't fit in the naturalist's sparse ontology, and then argue that the data is better explained on the hypothesis of theism. Call this form of argumentation The Common Apologetic Strategy.
What to make of arguments that instantiate The Common Apologetic Strategy? Instead of evaluating particular instances of this strategy -- i.e., evaluating this or that theistic argument from morality, or consciousness, or cosmic fine-tuning, etc. -- I'd like to raise a worry about the general line of reasoning such arguments take, as outlined above. In order to do so, I'll need to spend some time making some basic distinctions. This in turn will provide a framework that (I hope) will help in evaluating such arguments.
II. First Preliminary: Varieties of Naturalism
There are several versions of naturalism. Naturalists share in common the view that the natural world is all there is -- there is no supernatural realm of spiritual beings. However, naturalists differ in how they define 'the natural world'. Now there are at least three broad ways of characterizing "the natural world", and so there are at least three kinds of naturalists -- let's call them 'Conservatives', 'Moderates', and 'Liberals'.
Conservative naturalists are straight physicalists -- nothing exists but the physical, and the physical is characterized by all and only the properties listed in physics and chemistry textbooks. Recent proponents include Andrew Melnyk, Paul and Patricia Churchland, and some of the members of the so-called New Atheist movement.
Moderate naturalists differ from Conservative naturalists, in that they expand their conception of the natural world so as to include abstract objects (e.g., propositions, properties, possible worlds, etc.). Recent proponents include Tyler Burge, Jeff King, W.V.O. Quine, and Kit Fine.
Finally, Liberal naturalists differ from Moderates and Conservatives, in that they admit into their ontology of the natural world the abstracta of the Moderates, but they also allow for a conception of concreta according to which they have more properties and powers than the Conservatives and Moderates allow. Thus, perhaps they're straight Spinozists, or type-F monists, or panprotopsychists, etc. Famous past Liberal naturalists include people like Spinoza; more recent Liberal Naturalists include Donald Davidson, Thomas Nagel, David Chalmers, Daniel Stoljar, and Derk Pereboom. Since liberal forms of naturalism are no doubt the least familiar, perhaps it'll help to sketch one such account. David Chalmers' version of contemporary Liberal Naturalism is representative, so I'll sketch his version (call it 'CLN'):
(CLN) the world of concrete objects is composed of just one kind of substance, and its essence has both physical and phenomenological or proto-phenomenal (or at least representational or proto-representational) attributes (an alternative version of LN: the one kind of substance is neither physical nor mental, but but the physical and mental are composed of it).
III. Second Preliminary: Two Basic Explanatory Approaches Available to Naturalists
There are two basic approaches a naturalist can take to explaining the relevant data. According to one, one keeps one's ontology sparse by adopting (say) Conservative Naturalism, and then tries to explain all the data in terms of of the entities in that limited ontology. Furthermore, if something doesn't "fit", then one eliminates it -- i.e., one says that such entities aren't real. Call this approach, the Shoehorning Approach.
Now if naturalism were limited to Conservative Naturalism, The Shoehorning Approach would be the naturalist's only option. But as we have seen above, naturalists are not so limited -- Moderate and Liberal forms of naturalism are live possibilities as well, and there is no a priori or a posteriori basis for ruling them out. This fact about naturalism leaves room for a second approach to explanation, which I will call The Base-Expanding Approach. The Base-Expanding Approach starts out like the Shoehorning Approach: start with a sparser ontology, and then try to explain all the data in terms of it. However, the Base-Expanding Approach diverges from the Shoehorn Approach when it comes to entities that don't fit: if the data to be explained cannot be reduced to the sparser ontology, but the data really seem to be recalcitrant, then one does not eliminate them. Rather, one expands one's ontology. So, for example, a naturalist might start out tentatively adopting Conservative Naturalism as a working hypothesis, and then find that he can't reduce abstract objects to such a sparse ontology; nor can he plausibly eliminate them. He may then broaden his ontology by allowing for abstract objects, thereby moving from Conservative to Moderate Naturalism.
In light of the preceding, we can now provide a broad characterization of the ontologies and explanatory strategies available to naturalists. Thus, naturalists have available to them at least three basic ontologies: Conservative, Moderate, and Liberal. Conservatives are straight physicalists; Moderates go further by adding abstract objects to the ontology of Conservative Naturalism; and Liberals go further than both Conservatives and Moderates by positing a richer nature to concrete objects by allowing representational or proto-representational properties to be part of the essence of matter.
Furthermore, given that naturalists aren't limited to Conservative Naturalism, they have two basic explanatory approaches available to them: the Shoehorning Approach and the Base-Expanding Approach. Shoehorners reduce all phenomena they can to the fundamental elements of the naturalistic ontology they've adopted, and eliminate all else that they cannot so reduce. And Base-Expanders, by contrast, add more elements to their ontology when they cannot reduce or plausibly eliminate a given phenomenon to their fundamental ontology.
IV. The Framework Applied: Two Examples
In light of the preceding preliminaries, we are in a better position to evaluate The Common Apologetic Strategy outlined at the beginning of our discussion. First of all, we see that apologists who adopt it fail to appreciate that naturalists need not adopt Conservative Naturalism. And because of this, they fail to appreciate that naturalists need not adopt the Shoehorning Approach to explanation. Therefore, pending good arguments against other versions of naturalism and against the Base-Expanding Approach, arguments that adopt The Common Apologetic Strategy are bound to fail.
At least two contemporary naturalists --David Chalmers and Erik Wielenberg -- exemplify the success of the Base-Expanding Approach in their work. In so doing, they expose the inadequacy of The Common Apologetic Strategy.
Case 1: Erik Wielenberg, Moderate Naturalism, and the theistic argument from morality
Erik Wielenberg's version of Moderate Naturalism exposes the inadequacy of a contemporary apologetic argument from morality to theism, the latter of which exemplifies the Common Apologetic Strategy. The argument states that the naturalist is limited to the ontology of Conservative Naturalism, and that the data of moral facts cannot be shoehorned into such a sparse ontology; nor can it be plausibly eliminated. By contrast, theism can, with God as the ground of moral facts. Therefore, the data of moral facts is best explained in terms of theism and not naturalism.
Wielenberg agrees that moral facts cannot be adequately reduced to the ontology of Conservative Naturalism; he also agrees that they cannot be plausibly eliminated. However, he also thinks there are good reasons to reject theism. He therefore feels pressure to expand the base of his ontology and move from Conservative to Moderate Naturalism. Thus, he posits abstract objects such as properties and states of affairs. These abstract objects ground the necessity of basic moral truths, such as that it's wrong to cause a person or animal significant pain without a sufficient reason for doing so. And given that this view is epistemically possible, the theistic argument from morality is undercut.
Case 2: David Chalmers, Liberal Naturalism, and the theistic argument from consciousness
David Chalmers' version of Liberal Naturalism exposes the inadequacy of a contemporary apologetic argument from consciousness to theism, the latter of which also exemplifies the Common Apologetic Strategy. The argument states that the naturalist is limited to the sparse ontology of Conservative Naturalism, and that the data of phenomenal consciousness cannot be shoehorned into such a sparse ontology; nor can it be plausibly eliminated. By contrast, theism can, with its substance dualist ontology. Therefore, the data of phenomenal consciousness is best explained in terms of theism and not naturalism.
Chalmers agrees that the data of phenomenal consciousness cannot be adequately reduced by means of the sparse ontology of Conservative Naturalism. He also rejects the strategy of eliminating the data, as the Churchlands do. However, he also finds theism implausible. He therefore opts for the Base-Expanding approach, thus moving from Conservative to Liberal Naturalism. Thus, he grants that consciousness can't be squeezed out of the properties of objects listed in physics and chemistry textbooks, and thus posits that they must therefore have more properties as a part of their essence -- proto-phenomenal or proto-represenational properties. This account allows that simpler concrete objects aren't conscious, but it also entails that when a complex collection of such objects exists and is suitably arranged, it necessarily exemplifies consciousness. But if this account of the origin of phenomenal consciousness is epistemically possible, then the theistic argument from consciousness to theism is undercut.
What went wrong with the theistic arguments above? I submit that my framework sketched above provides the means for an illuminating diagnosis: In both cases, the flaw was not with something particular to either argument. Rather, the problem was with the Common Apologetic Strategy exemplified by both arguments. Thus, both arguments assumed that the naturalist was limited to Conservative Naturalism, and thus that the naturalist was stuck with the Shoehorning Approach to explaining the relevant data (moral facts in the first case, phenomenal consciousness in the second). But we saw that both assumptions were false: Moderate and Liberal versions of naturalism are prima facie epistemically possible, in which case Wielenberg and Chalmers were free to opt for the Base-Expanding approach to explanation, which they did: Wielenberg broadened his ontology to adopt Moderate Naturalism, thereby allowing him to account for necessary truths about morality within a naturalist framework; and Chalmers broadened his ontology to adopt Liberal Naturalism, thereby allowing him to account for phenomenal consciousness within a naturalist framework.