What is naturalism? As a worldview distinct from any form of "supernaturalism," "naturalism" is the belief that nature is (probably) all there is, and nothing supernatural exists. Of course, the word naturalism can be used in other ways. In the art world, it means one thing; as a special term in epistemology, it means something else; and so on. But as a worldview, as a comprehensive picture of the nature and content of existence, naturalism is the converse of supernaturalism. What does that mean? Attempts to distinguish the "natural" from the "supernatural" often fail on basic requirements of coherence and utility. I predict that all successful attempts will reduce to this: naturalism is the view that everything mental is fundamentally nonmental. That sounds bizarre and unexpected, but it appears to be correct.
Even though there are thousands of us who self-identify as naturalists in the broad sense, I have not found much of use in formal philosophical literature on how to define naturalism. I feel almost alone in campaigning for a coherent and useful definition of naturalism and attempting to influence other efforts in the same direction. But all of these efforts leave a lot of ambiguity in their wake. I once conducted an informal survey of naturalists in order to determine common threads that point toward a proper definition of what we actually mean when we say we're naturalists. And what I found was quite the contrary of what opponents of naturalism assume.
This should alert us to the importance of taking the effort to define naturalism seriously: if we don't do it, our enemies will. By this tactic, they stole the word atheist and the phrase "secular humanist" and infected the public with gross misperceptions of what they really mean. If you've ever encountered the bare assumption that "atheist" means "communist," then you know what I mean. That's not the only reason we should care. Just as important is the fact that to understand, articulate, develop, and even criticize and thus improve our own worldview, we need to know what exactly that worldview is—not just in the ways we may differ regarding how we construct that worldview but what all our constructions have in common that bind us together. My informal poll revealed that we shouldn't simply attempt this from the armchair; we also need to communicate with each other to debate the issue and attempt to locate what we do share in common that can provide a coherent and useful definition of naturalism. This essay is intended to serve that project by attracting constructive debate on the subject from philosophers of naturalism.
In this quest, one of the biggest problems we encounter is a nefarious tendency to "define away" all supernatural claims as untestable. This is a philosophical evil that should not persist—not only because it isn't true but also because it renders naturalism meaningless. Naturalism then becomes "whatever is empirically known" no matter what that turns out to be, which makes naturalism useless as a predictive paradigm and renders it devoid of identifiable content from the start. If a naturalist is simply "one who believes in what is empirically known," then that tells you almost nothing at all about what he or she believes that isn't conveyed by the word empiricist. If naturalism is simply a synonym for empiricism, then we should prefer the latter word, as it is much less misleading and more clearly defined.
That isn't my only objection. I must emphasize even more my first objection. For the idea that everything supernatural is empirically untestable is simply false. Of course, I attend to what people ordinarily mean when they call something "supernatural" rather than fictional definitions invented by philosophers because the latter tell us nothing useful about questions asked and claims made in the real world. To give one example, in one online article I demonstrated how a basic form of Christianity could be proven true with as much empirical certainty as anything else we believe, demonstrating that there is nothing untestable about Christian supernaturalism as such. Given that, surely if there is anything that distinguishes all naturalists, it's that we aren't Christian supernaturalists.
I find all too often that naturalists confuse efforts to make Christian (and other) supernaturalisms untestable with the assumption that they are always untestable by definition. That's a mistake. Supernaturalists now make every effort to place their beliefs beyond meaningful test precisely because their beliefs have failed countless tests—indeed, most have been decisively refuted. This is evidence of the desperation of their position but not of the untestability of more honest supernatural claims. As an obvious example, young Earth creationism is not only abundantly testable, it has decisively failed every test. Science has conclusively, and empirically, ruled out this supernatural claim. So believers retreated by constructing a new supernatural claim, old Earth creationism, which is harder to test. We should not forget where this debate began: they proposed a supernatural claim, that claim was testable, we tested it, our tests refuted it. Only then did they dig around for another supernatural claim that was harder to rule out empirically. Thus, insofar as Old Earth creationism is untestable (even if you grant that it is—I don't), it's not untestable because it's supernatural. It's untestable because believers specifically made it that way in order to avoid the risk of being trounced by the evidence.
In the same fashion, we can just as easily fabricate untestable natural hypotheses. Thus, untestability is a property of untestable theories of any kind. It is not a property of supernatural claims per se. Since this is not a property of all supernatural claims, nor even historically distinctive of them, it cannot be the definition of the supernatural. Therefore, "the empirically knowable" cannot be the definition of the natural either. If there were any true supernatural facts, we could very well have discovered them empirically by now. Science could indeed have proved Young Earth creationism true. We could have even scientifically documented conversations with God and tested and confirmed his every claim—if he were ever a willing subject. That he avoids this is not because God is supernatural but because he doesn't exist, something believers want to avoid admitting by inventing excuses for his not showing up.
So what does distinguish supernatural claims from natural ones? What demarcates naturalism from supernaturalism? Often when I encounter a layperson or even a philosopher who hasn't thought much about this, I get an answer to this question that is quickly found wanting. It either ends up incoherent or turns into an arbitrary laundry list. Get them to think a little more, and they start to work out why they are putting some things in one column and some in another and what general rule separates those columns. Then their arbitrary laundry lists start sorting themselves out into something that is no longer arbitrary but systematic. I've encountered many formulations in this direction, but I believe, on careful analysis, they all reduce to the same definition, the one with which I began, now formally stated: naturalism is true if everything that exists is causally reducible to the nonmental.
Why do I believe that? Let me briefly outline my position. A mental object is anything that exists that is distinctive of the contents or activity of a mind. The most obvious examples are thoughts, perceptions, and emotions. By contrast, the most obvious examples of the nonmental are anything that is simply matter or energy or space or time (and any arrangement of those or anything categorically similar to them). So, for example, if everything that exists, including thoughts, perceptions, and emotions, is causally reducible to different arrangements of matter-energy in space-time, then naturalism is clearly true. That all leaves open the possibility of ambiguous cases, which would have to be worked out case by case. Otherwise, when all the causes sufficient to produce a mental effect are themselves nonmental, the mental is causally reducible to the nonmental. This definition allows the possibility of irreducibly mental things (like epiphenomena), so long as those things are fully and completely caused to exist by nonmental things (like brains).
I propose that anyone who believes the above is a naturalist and that any claim to the contrary is a claim to the supernatural. The supernatural thus always involves some sort of mind over matter. Consider a love potion—that is, a drink that causes someone to fall in love. This could be an entirely natural substance. If it were an advanced nanorobotic saline solution designed by clever futuristic brain scientists, containing chemicals that entered the blood through the digestive system and thence into the brain, and of just the right chemical construction as to have all the right effects in all the right places, then the causal effects of this potion would be entirely reducible to the nonmental interaction of nonmental things.
If a love potion contained no such chemicals but was simply imbued with the property of "causing love," producing its effect directly by possessing no other structure or properties except that innate power, then this would be a supernatural potion. For the idea of "love" is a distinctly mental entity, which in this case would not be reducible to any nonmental system. That a potion would just "know" it was in a human body, just "know" where its brain was, and just "know" precisely how to affect and reorganize that brain to cause that person to fall in love would all be impossible according to any variety of naturalism without some causal mechanism (some nonmental property or arrangement of the universe) that wasn't just "love" or "love-causingness" or any similarly mental construct. So if the causal mechanism in the potion consists solely of the mental property of just "knowing" all these things and then directly causing them on contact, with no underlying mechanism, we're clearly talking about a supernatural potion. This would be measurably, detectably, objectively different from any love potion in a naturalistic universe, like that nanorobotic fluid.
This particular example can be expanded indefinitely to cover any case. Hence, I propose a general rule that covers all and thus distinguishes naturalism from supernaturalism: If naturalism is true, everything mental is caused by the nonmental, whereas if supernaturalism is true, at least one thing is not.
Richard Carrier holds a PhD in the history of ancient science from Columbia University and is the author of Sense and Goodness without God: A Defense of Metaphysical Naturalism (AuthorHouse, 2005).