Worldviews define themselves partially by way of contrast with their competitors. In defining naturalism, it helps to consider the differences in assumptions and conclusions between it and non-naturalistic worldviews. Below are a few of the most salient. If you’re undecided about where to place your bet about the ultimate nature of things, considering these differences can help you decide. Naturalism isn’t for everyone, at least not immediately, since it requires revising some conventional wisdom and giving up some comforting beliefs. But it has its advantages, not the least of which is consistency with our most reliable way of grasping reality.
One fundamental difference, mentioned in the first section above, is the naturalist’s commitment to scientific empiricism as the basis for reliable beliefs about what’s ultimately there in reality. This contrasts with contemporary theistic theology (there are non-theistic versions), which admits other sorts of knowledge claims into the mix, for instance those based on faith (although faith gets relatively little emphasis these days in academic theology), on first person experience and intuition, and on what theologian John Haught calls a “richer” empiricism (see here). This is to say that naturalism and theistic theology differ with respect to their epistemic criteria for belief. The naturalist, of our stripe at any rate, thinks that any departure from science likely results in beliefs that aren’t sufficiently insulated from prior assumptions about what must exist, for instance God, the soul, paranormal powers, a driving cosmic intention, or contra-causal free will.
Since wishful thinking is such a powerful determinant of belief, we can’t be too vigilant in setting up procedures that systematically eliminate its influence if we want the truth about reality. Science is, among other things, the culturally-evolved distillation of procedures that do exactly this. Theists, and more broadly supernaturalists and anti-naturalists of various sorts, might reply that such methodological strictures are themselves a bias. They might say (some do say, see here) that a truly reasonable and rational approach to understanding the world should admit there are other and better ways besides science of deciding what’s ultimately real. The very interesting open question, which forms one nexus of debate between naturalists and anti-naturalists, is whether there are criteria both sides can accept that could help decide the question of what grounds for belief are rationally acceptable.** There may be no agreement on such criteria forthcoming, in which case this question (among other disputes between naturalism and anti-naturalism) ultimately gets decided on a pre-rational, political basis, which is to say it will likely always be contested. This is why naturalists and anti-naturalists must learn to live together in an open society that accepts the variability of worldviews and their epistemic commitments.
** Since this writing, I've concluded that there are indeed worldview neutral criteria of epistemic adequacy that anyone should respect if they claim to be representing reality objectively, which is of course what all worldviews, including religions, claim to do. Most basically, these criteria require that we should 1) insulate our beliefs from various sorts of bias, e.g., perceptual distortions, uncorroborated subjective intuitions, arbitrary authority, revelatory religious traditions, and other influences not responsive to the way the world actually is; and 2) validate our beliefs using publicly available data and observation. Non-empirically based worldviews claim objectivity but generally fail to meet these requirements, so we shouldn't trust them as guides to reality. Papers that develop this line of thought are here, here, here and most comprehensively and recently here.
Another major contrast between naturalism and anti-naturalism is the relative emphasis placed on explanatory transparency. The obvious example here is intelligent design. Supernaturalists at the Discovery Institute, for instance, think the God hypothesis about the origin of species is a perfectly adequate explanation, even though the characteristics of the intelligent designer aren’t specified, nor are the processes by which he (she, it) created new species. They want public schools to teach intelligent design in science class on an equal footing with natural selection, even though ID manifestly fails to explain the target phenomenon according to widely accepted criteria of explanatory adequacy. This failure is apparently of no concern to the Discovery Institute, and more importantly they don’t even see it as a failure. The requirement of explanatory transparency, in which a theory shows clearly how phenomena are interrelated by various causal processes and organized on hierarchical levels, is very far down on their list of cognitive virtues, perhaps missing altogether.
Another example, at a more abstract level, is metaphysical dualism. Anti-naturalists are often href="/philosophy/epistemology/differences-between-theism-and-naturalism#dualism" > dualists, not a surprise since their world is split from the beginning between the natural and supernatural, or between nature and God, where God is conceived as something transcending, subtending or immanent in nature, but not nature itself. The problem for anti-naturalists is to explain how two categorically separate realms of existence interact and influence one another, which they must to some extent. This problem presents itself whether the dualism is one of God vs. nature, soul vs. body, or immaterial thought vs. neural processes. If a clear explanation of how the two sides interact is forthcoming, this threatens to unite them into a single category of existence, not good news for dualists. On the other hand, insisting on their categorical metaphysical separateness guarantees that no transparent explanation of their interaction will be forthcoming. Anti-naturalists, therefore, not only seem less interested in explanatory transparency, they are barred from achieving it given their investment in metaphysical dualism. Of course, some anti-naturalists will deny having any such investment, and say that their dualism is a conclusion of their unbiased investigations. This brings us back to the issue of their epistemic and methodological commitments, see above. In any case, dualists necessarily have a tougher time uniting the world as it appears to them within a satisfying, transparent explanatory scheme. So if explanatory transparency is high on your wish list, as opposed to your wishful thinking list, chances are you’ll end up a naturalist. Of course naturalists candidly admit that their explanatory scheme has major lacunae, such as how it all began and how life got started. But that’s fine since lacunae make life interesting for natural-born problem solvers such as homo sapiens.
Perhaps as a result of their differing epistemic commitments and differing desires for explanatory transparency, naturalists and supernaturalists often end up with very different ideas about the basic nature of human beings, the mind and ultimate reality. The fundamental contrast gets expressed in ideas about persons. Anti-naturalists often suppose that human beings possess something indivisible and essential, something like a soul or non-physical mental essence which constitutes their core identity. Naturalists are more likely to think that people and their minds are in all respects constructions, made up of physical constituents none of which are in themselves personal or mental. Similarly, on the anti-naturalist understanding, the ultimate nature of reality includes some non-compositional, intention-bearing core or stratum, often thought of as God, while on the naturalist understanding it doesn’t. For naturalists there are just the various phenomena science shows to exist, which as far as we can tell don’t include anything like such a core or stratum. As mentioned in the introductory section, the ultimate characteristics of natural phenomena are for scientific investigation and theory to determine, so for the naturalist it isn’t necessary to claim that the root nature of existence (if there is such a thing) must be material or physical. Further, if the best naturalistic explanations end up in some sort of essentialism, so be it. But for the nonce the naturalist sees no evidence for mental or personal essences, either on the local or cosmic level.
For some anti-naturalists, usually of the faith-based religious variety, the very reality and worth of people and minds is tied to their being or having a permanent, indestructible essence. If persons turn out to be perishable organic constructions without souls, then they lose existential significance – they are merely constructions, superficial ephemera without intrinsic value, in a word, dead. Similarly, if human experience and subjectivity turn out to be products of what the brain does, that would effectively explode them as real, respect-worthy existents. People and their minds must have an essential, spiritual core in order to possess dignity and value, otherwise we are mere robots. Existence as a whole, if it turned out not to have an intentional core, would likewise necessarily be dead, valueless, meaningless – an impersonal existential horror.
Oppositely, naturalists find that the constructed nature of things, including people and their minds, is no bar to their reality or worth. Life, value and meaning arise in and are made available by certain types of natural (and perhaps eventually artificial) constructions, among which are notably ourselves. We are really alive, really conscious, even if none of our smallest parts is alive or conscious. That they are constructions, or as it’s often put, emergent phenomena, doesn’t diminish the rather spectacular natural achievements of life and consciousness, indeed, that’s what makes them achievements. Nor does it undercut the self- and other-ascribed worth of being a sentient creature with intentions and purposes that make life meaningful. We are built by nature out of simple elements, but built such that we can’t help but accord value and dignity to one another, and be gripped by the life-project. For naturalists, understanding its constructed, emergent nature doesn’t devalue our valuing, it doesn’t rob people of their worth, or life of the meaning we give it. It does for anti-naturalists, however, which is perhaps one reason they resist reductionist and emergentist explanations of ourselves in which essences play no role.
One of the most important contrasts between naturalism and anti-naturalism concerns the basis for morality. Because naturalism discovers no god or governing intention in reality, it’s at a rhetorical disadvantage to theism when it comes to justifying our moral intuitions. The complaint against naturalism by anti-naturalists is that it can’t supply a foundational, binding reason why we must obey moral commandments, and why any particular set of commandments should hold force. Science can perhaps explain our moral proclivities as a function of evolution and culture, but it doesn’t show why we should be moral, according to these particular rules. The theist can appeal to God as the source of moral authority and moral law: moral injunctions are binding because they issue from divine judgment, which of course is infallibly right and good.
The standard naturalist reply derives from Plato’s Euthyphro dilemma (link is external), which aims to show that appealing to God’s authority can’t establish the rightness and obligatoriness of moral laws. Whether or not this argument persuades theists (it usually doesn’t), the fact remains that for the naturalist there is no foundation for morality outside of human nature and culture. There is no ultimate, external source of moral authority which makes a particular set of moral rules binding upon us. Yet we can’t help but feel when making moral judgments that the rules are binding and universal (about which see here and here). Unable to appeal to God’s command, naturalists have a tougher time providing clear cut, easily grasped reassurances that we are in fact justified in our ethical intuitions. We don’t have a simple one-liner about why we should be good analogous to the supernaturalist’s easy but problematic answer: “God commands us to be good.”
That we can be good without belief in God is an empirical fact; even most theists acknowledge that atheists can be moral exemplars. So clearly people can be bound by moral laws whether or not they are thought to have external back up. The scientific investigation of moral intuitions and altruistic behavior suggests that they played, and continue to play, a key role in the formation of stable groups and communities within which individuals can survive and reproduce. Being good – altruistic, kind, generous, fair, protective – was therefore adaptive and naturally selected, which means most of us are innately inclined to act ethically (see for instance Steven Pinker’s recent New York Times Magazine article (link is external) on the natural basis for morality). The “moral instinct” is part of who we are, it’s a defining characteristic of being human. We don’t, therefore, need commanding to be good; it happens quite naturally as a function of our evolved nature. We are, overall, happier, more productive, more secure and more interpersonally fulfilled when we act according to the golden rule and other ethical maxims. So the naturalist’s one-liner in response to the question “Why be moral?” might be simply: acting morally is essential for human happiness and flourishing within a community.
This won’t satisfy those, theists included, who think that to be binding morality can’t merely be a means to an end (happiness and flourishing), but must have it’s own independent obligatory force independent of the consequences of good behavior. Being motivated to be moral, as we obviously are, isn’t enough: we have to obey moral rules because it’s right to do so, not because we get something out of the deal. But the naturalist wonders what could possibly establish rightness or goodness that isn’t a function of what we humans want or need. The demand for a non-motivational basis for obligatory rightness and goodness seems – is – inhuman. We are fundamentally motivated creatures (as are all sentient beings), so to require that goodness have a foundation external to our motives asks the impossible; it alienates us from morality. If this is true, then not only is God not needed for us to be good, his commandments distract us from appreciating the actual source of moral obligation: our natural need for others.
Naturalism therefore understands morality, like persons and minds, as a construction, in this case a bio-social construction built of non-moral elements. It is not for this reason any less real, valuable or binding, since moral rules and felt obligations are the robust outcomes of the construction itself. We can’t help but feel the pull of morality, and we can now see that the pull needs no justification external to human needs, and indeed couldn’t have one. We are, as Robert Wright and others have argued, moral animals (link is external) who can’t escape the force of moral obligation precisely because it’s essential for our flourishing. Naturalists can therefore be confident in our natural capacity for goodness, and we have a reply to the anti-naturalist who thinks the only alternative to theism is nihilism: we’ve been naturally selected to take moral laws as binding upon us. Morality is a natural phenomenon. So not to worry, we won’t run amok without God.
 In an excellent New York Times Magazine article (link is external) on the natural basis for morality, Steven Pinker expresses the dilemma thusly: “Does God have a good reason for designating certain acts as moral and others as immoral? If not — if his dictates are divine whims — why should we take them seriously? Suppose that God commanded us to torture a child. Would that make it all right, or would some other standard give us reasons to resist? And if, on the other hand, God was forced by moral reasons to issue some dictates and not others — if a command to torture a child was never an option — then why not appeal to those reasons directly?”