The natural vs. the supernatural
If metaphysical naturalism, simply put, is the thesis that nothing besides the natural world, or nature, exists, it would be helpful to have a definition of nature. One idea is that anything science describes and explains is a natural phenomenon. This means that if science discovered and incorporated in its theories something which, for good empirical and conceptual reasons, we decided to categorize as non-physical or mental, that too would be part of the natural world. Some naturalistic philosophers such as David Chalmers hypothesize that categorically mental phenomena might exist that relate to physical phenomena via psycho-physical laws. Such phenomena would be perfectly natural, just not physical. Although I don’t see any evidence as yet for this hypothesis, it seems logically possible. Nature might not be limited to what’s physical; so speaking philosophically, we shouldn’t equate naturalism with physicalism.
Because scientific theories tend to show the connections between natural phenomena by means of various laws and relationships, nature forms a more or less unified whole, from the sub-atomic to the multi-cosmic. But is it possible that nature itself doesn’t exhaust what exists? That is, is it possible that something exists which, for good empirical and conceptual reasons, we’d decide to categorize as supernatural or non-natural? This raises the question of what we have in mind by super- or non-natural (I’ll stick with supernatural in what follows). Commonsensically, what’s supernatural will likely contrast with, or be the opposite of, what’s natural in some important respects.
What if, as is conceivable, we discovered something that, despite our best efforts, we couldn’t describe or explain in terms of currently known phenomena, physical laws, psycho-physical laws, or in terms of any other sort of explanatory relationship, such as non-reductive emergence? Imagine your favorite prototypical supernatural phenomenon: a god, an angel, an ectoplasmic apparition, the proverbial 900 foot Jesus that materializes in Poughkeepsie on alternate Fridays. Would this obdurate anomaly, which we’re imagining in this thought experiment to be robustly and reliably observable by everyone, including the scientific community, count as something supernatural?
It seems to me the answer is at least a provisional yes, in that our ordinary concept of the supernatural includes the idea that it isn’t explicable in terms of natural laws and other empirically-derived explanatory relationships. Moreover, the supernatural is ordinarily thought of as being beyond our ken: we can’t understand or predict it; it has powers and capacities that aren’t reducible to, or explicable in terms of, known laws and mechanisms. So if we encountered something that defied natural laws in ways beyond our ken it would be fair to categorize it as supernatural. It would stand in stark contrast to the natural as we ordinarily conceive it.
[Note: Another characteristic of the supernatural as it's ordinarily conceived is that it's usually personified or agential, something with a mind and intentions, not impersonal. When we think of supernatural phenomena, we usually magine supernatural beings with agendas, not any of their constituent parts. This feature of the supernatural doesn't figure in what follows since of course there are natural agents and persons too. Being an agent doesn't distinguish the supernatural from the natural. TWC 7/5/15]
Naturalism as a defeasible hypothesis
Since we can’t rule out the possibility of such an encounter - of the 4th kind, let us say - the claim that the natural world is all there is can in principle be falsified. So although metaphysical naturalism is an extremely good bet given all the available evidence thus far, although it’s an empirically well-substantiated conjecture about what is ultimately the case, we can’t take it as a foregone, a priori conclusion about reality. To admit this keeps naturalism consistent with its own epistemic foundations in science, and makes naturalists non-dogmatic fallibilists about their worldview. We must remain cognitively humble, not arrogant, given the possibility of someday being proven wrong by our own standards of evidence.
But supernaturalists should be humble too, at least if they grant science a decisive role in judging what’s factually true (many of course don’t, see here). For the supposition that the anomaly is supernatural can only be provisional. Why? Because we can’t rule out the possibility that as science progresses it might discover laws, relationships and principles that end up incorporating the apparently supernatural phenomenon into its theories. Should this come to pass the anomaly is no longer; a newly minted natural phenomenon takes its place.
There’s perhaps yet another possibility. Say we come across something that fits the ordinary description of a supernatural god: an entity with all sorts of amazing powers and a god’s typical immunity from being subject to natural laws. We discover we are its creations, and it literally lords it over us, exerting a one-way causal power that frustrates any attempts on our part to influence its actions. But further, let’s say we come to understand its nature and modes of action (not inconceivable: suppose the being deigns to fully disclose its nature to us). We learn that it partakes of a very different set of laws and processes than what characterize all previously known phenomena. These laws and relationships satisfactorily explain to us why it can lord it over us and the rest of nature. If there’s no mystery about its causal and creative powers, would we then continue to say that that it’s supernatural, or even a god?
Perhaps. After all, it fills the supernatural bill in terms of its causal superiority and immunity to influence, forcing us to conceptually divide reality into upper and lower realms, what we might want to call nature and supernature. On the other hand, in this imagined scenario we understand the characteristics and operations of both realms and why it is that one is causally privileged over the other. If there’s no mystery about this being, if it operates according to comprehensible laws, we’d see it as simply another denizen of existence, somewhat in the way the Greeks conceived their Olympian gods. This militates against it being supernatural in the sense of being beyond our ken.
What’s happened is that our understanding has incorporated this being into a unified model of reality, albeit a reality which turns out to have causally superior and inferior realms. There are not two sciences, once concerning nature and the other supernature, but rather a comprehensive theory that explains why and how one realm exerts unreciprocated influence over another. By virtue of this theory we see how these distinct realms participate in a single reality.
But isn’t the idea of a single reality encompassed in our understanding, however disparate the phenomena it contains, just what we mean by nature? It seems that in understanding what we would otherwise call supernatural (because of its superior causal powers) we end up naturalizing it. For our god to really count as supernatural, it has to remain inscrutable in its nature and operations, it has to stand outside the known world.
The permanent possibility of naturalization
As a last variation on this thought experiment, let’s say our hypothetical god does not disclose itself to us, so that its nature remains a mystery. Even so, as we’ve seen above, the possibility of its incorporation into our understanding always remains. Any purportedly supernatural phenomenon is potentially naturalizable in that its nature, however esoteric, might someday be revealed to us. Should we have an encounter of the 4th kind, we can’t conclude that what looks to be supernatural necessarily exists as something categorically apart from what we might someday come to understand. It too might come to be tamed, at least in our conceptual grasp of reality.
As things currently stand, if you stick with science as your guide to what’s real, naturalism is the best bet about the nature of existence since there’s no evidence for anything supernatural. But naturalism is potentially falsifiable since we can’t know for sure that all phenomena we encounter will fall to scientific explanation and thus be naturalized. We can't rule out the possibility that there might come a time when, for good evidential and conceptual reasons, we will divide reality into the natural and supernatural. Still, in the unlikely but conceivable event we encounter what we judge to be supernatural, we also can’t rule out the possibility of eventual naturalization. Neither naturalists and supernaturalists can responsibly claim to know in advance, as inquiry proceeds, what existence must encompass, so they should have at least that much in common.
TWC, August 2010
 This essay owes a great deal to productive and cordial exchanges with J. Ash Bowie on defining the supernatural, and it draws from a NPR blog co-authored with Ursula Goodenough, Is scientific inquiry restricted to nature?