I have become a major fan of historian and philosopher Richard Carrier. He has done a tremendous job developing a robust set of arguments supporting the naturalistic worldview, and I imagine that he will get regular mentions on this blog. To start out with, I’d like to point you to his posting where he provides clear definitions for natural, paranormal, and supernatural. This is important because naturalism has rightly been criticized for having vague definitions, often described as simply being “not supernatural”, which is problematic when there is no clear understanding of that concept either. Carrier offers a way out of this loop in a way that is accessible and feasible.
To start with, Carrier defines paranormal, which might be either natural or supernatural. He writes:
What makes something paranormal is the fact that it exists outside the domain of currently plausible science. As such, it could just be a natural phenomenon we don’t yet understand or haven’t yet seen. But it could also be something supernatural. Or an entirely bogus claim. We won’t know until we have enough evidence to make a determination. But either way, the category of “paranormal” can be applied to phenomena (hence the mere claim that something happens or exists can be paranormal) as well as explanations of that phenomena, i.e. paranormal hypotheses.
The key phrase there is plausible science. There are an infinite number of potential ideas that fall outside strict scientific understanding but that don’t meet the criteria for paranormality, because (depending on the state of scientific understanding when a claim is made) they might be plausible. Carrier offers two examples: alien abductions and fire that burns underwater. In the former, there is simply no scientifically plausible reason to conclude that alien abductions are happening—but that doesn’t mean it is impossible or even untrue; if enough evidence came to light, then it would be understood as a normal scientific fact. Until then, it is paranormal. The latter example is something that philosopher David Hume once said was impossible, so that any claim of underwater fire would have been to him a paranormal theory—one that we now know in practice to be absolutely true. To reiterate, based on this definition, paranormal claims can be either natural or supernatural; the defining characteristic is that they fall outside of scientific plausibility.
Now then, what makes something supernatural or natural? First, Carrier contends that these are metaphysical distinctions, not epistemological ones (i.e. they describe what things exist, not how we know they exist). This effectively rules out testability as a defining feature. That is a big deal because it is common to hear that the supernatural is, by definition, that which is not testable via scientific methods. The counterargument to this definition is simple—it is entirely possible that things exist (such as what might happen at a subatomic level) that, for whatever reason, are beyond our ability to test, but wouldn’t be thought of as supernatural (in any meaningful sense). At the same time, we can imagine supernatural forces or objects that could be scientifically examined, such as the Jedi Force or Harry Potter magic (to use Carrier’s examples).
So, in the most simple terms, Carrier defines naturalism as the view…
…that every mental thing is entirely caused by fundamentally nonmental things, and is entirely dependent on nonmental things for its existence. Therefore, “supernaturalism” means that at least some mental things cannot be reduced to nonmental things.
Within this scheme, beings such as gods, angels, demons, fairies, and so on, are supernatural insofar as they contain “any mental property or power that is not reducible to a nonmental mechanism.” This extends beyond beings as well—we can include substances and powers as well. Carrier gives some wonderful examples, such as a love potion that “knows” what love is, that it has been imbibed, and who the object of affection is; however, if the potion works entirely by the efforts of, say, a bizarre biochemical substance, no matter how strange, then it would be natural and not supernatural. Further, anything “that is the effect of a supernatural cause is a supernatural effect, even if the effect itself is not supernatural.” An example is when Gandalf causes normal wood to catch fire by speaking a magical incantation—the wood and the fire are fully natural in themselves, but their cause was supernatural, because the spoken words had a direct effect on the wood.
Naturalism understood in this way does not equate with scientism. This is because it is, in principle, possible to scientifically discover and test a supernatural being, substance, or effect, which would nevertheless remain supernatural. For this reason, it is entirely possible for science to negate metaphysical naturalism while maintaining scientific integrity. In such a world, the universe would indeed be dualistic, with natural and supernatural features—or we might even discover that the cosmos is entirely supernatural. It’s possible, although not probable.
Richard Carrier does not own the market on the definition of naturalism, of course, but he has done a superb job of articulating a coherent version of it. In fact, he has changed my mind on a key issue, that of real=natural. I have long held that if science were to discover, say, an immaterial soul, then that soul would become, by definition, natural. While I have held this opinion, I have also felt that it was somehow vacuous. If natural=real, then supernatural=unreal (or a subset of unreal), which is a trivial definition without much utility. Carrier has persuaded me to accept a conception of naturalism where the supernatural could, in principle, be real and yet not natural (although naturalism, as a worldview, would be thus disproved).
Of course, we have yet to discover any evidence of the supernatural whatsoever, and with the profound success of naturalism to date in regards to understanding reality, it is perfectly feasible and rational to adopt naturalism as a worldview. Yes, naturalism has not been “proven”; no one seriously claims otherwise—it is simply the best explanation for what we see in the universe. As Carrier writes elsewhere: “Every (I repeat: every) phenomenon we have been able to explore in the requisite detail has turned up naturalistic, without a single exception so far, in over three hundred years of concerted scientific investigation, by millions of experts, engaging the best methods imaginable, and with nearly unlimited resources.”