The basic epistemic commitment undergirding naturalism is that we should stick with science, in partnership with philosophy, as the arbiter of what fundamentally exists. Since it isn’t a worldview, science itself doesn’t make any metaphysical claims, and it certainly doesn’t presume naturalism, as advocates of teaching intelligent design sometimes argue. Rather, science is a method of inquiry that gives us generally reliable beliefs about the world. If you want reliable beliefs, then you should make the commitment to science and have little or no truck with faith, intuition, revelation and authority as grounds for belief. The commitment to science, therefore, is not a matter of faith, as is sometimes claimed by anti-naturalists (trying to tar naturalism with their own brush, see here (link is external)), but of acting rationally to fulfill the commendable desire for secure knowledge. Finally, if you make this commitment, then you’ll be led to metaphysical naturalism, the idea that only the natural world exists. This is because science tends to unify what it describes into a single interconnected whole, what we call nature. For a detailed discussion of the cognitive basis for naturalism, see Reality and Its Rivals.
Some think that making a commitment to science in deciding questions about what fundamentally exists entails scientism. Not so. Scientism is the idea that scientific empiricism is the basis for everything we know, that the only valid knowledge claims are scientific. But there are domains of human knowledge and know-how – ethical, aesthetic and political, among others – that are not science-based. Such domains aren’t primarily in the business of explaining or describing, science’s specialty, but rather doing, deciding and enjoying. This is not to say science can’t describe (or attempt to describe) regularities of human behavior in these domains, or that it can’t help settle empirical questions raised within them; it can, at least to some extent, if it suits our purposes. But deciding questions about ethical and aesthetic principles, and deciding between competing political agendas, are not empirical projects since they essentially involve normative considerations about values, not matters of fact. Science can help to tell us how things work or might turn out, e.g., what the economic ramifications are of adopting a flat tax, but it can’t tell us what’s fundamentally right, good or desirable, e.g., whether a flat tax is fair. And indeed, science and scientists generally have no such ambitions, although E.O. Wilson (link is external) sometimes seem to verge on scientism in his claims about the scope of science. It’s worth noting here that just because science can’t decide basic questions about values doesn’t mean that faith is better qualified to do so.
The epistemic allegiance to science – not scientism – takes us to the basic metaphysical position of naturalism, that what exists is the natural world. There isn’t in addition a categorically separate supernatural realm since there’s no good evidence to warrant belief in such a thing. Now, what exists in nature is for science to determine, and it may turn out that there exist very strange things indeed, we don’t know in advance. Naturalism should therefore not be equated with naïve materialism. For instance, if our best, experimentally validated and conceptually transparent explanation of consciousness turns out to include the existence of something like mental particles (“psychons”), or something ultimately representational (something neither mental nor physical as standardly conceived), so be it. If we eventually discover good evidence that a super-intelligent being created the galaxy just for its own amusement, so be it. Because it’s wedded to science, as disciplined by philosophy (or pestered, as scientists often see it), naturalism doesn’t have a priori ontological commitments about what can or cannot exist. It only insists that there be good evidential, experimental and theoretical grounds, as judged by rigorous and continuing peer review, to believe in it. Thus far there is no evidence that reality is partitioned into nature and something else or something more, whether we call it God, the supernatural, or the paranormal. The naturalist thus discounts the existence of such things, and given her desire for reliable beliefs this disbelief is by her lights rational, not faith-based or dogmatic.
In discounting the existence of a supernatural god, naturalism is therefore the basis for atheism, but it has other implications that hit closer to home. Because human beings are completely included in the natural order, which includes biology and culture, there’s nothing supernatural about us. In particular, science shows no evidence for an immaterial soul or other internal essential self that could transcend the cause and effect relationships that science finds elsewhere in nature. We are natural constructions, not supernatural essences. As persons with bodies and brain-instantiated minds, we are completely causally connected to the world via genetics and environment, both in our development and in our current experience and behavior. Although we are complex and self-modifying creatures in many respects, we don’t have a contra-causal capacity for ultimate self-creation; we are not little gods who bear ultimate responsibility for themselves. This fundamental point about our fully natural nature currently receives little attention, and indeed is actively suppressed in Western culture. But it’s nevertheless what science reveals to be the case, and has widespread ramifications for many other beliefs and attitudes. The careful elucidation of what it means, personally, socially and existentially, to be fully natural creatures is therefore central to the mission of the Naturalism.Org.
Understanding the naturalistic truth about human nature is the next big step for those who are already naturalists in their beliefs about God and the paranormal. Seeing our full causal connection to the world, knowing people are not self-created, we are more likely to adopt an interpersonal and intrapersonal ethics of compassion and acceptance, very much like the Buddha taught. This has direct consequences for how we treat each other, and it can influence our thinking in a progressive direction in many policy domains. Knowing the specific determinants of human character and behavior, we also gain greater control over ourselves, become more effective in carrying out our projects, and more adept in our relationships. The watchwords of the Naturalism.Org are therefore connection, compassion and control.
Under naturalism, our moral compass stays intact, we still love, strive, hold each other (effectively and compassionately) responsible, and seek to fulfill ourselves and create a sustainable world for our descendents and other life forms. Naturalism can also supply the basis for a satisfying approach to ultimate concerns, a naturalistic spirituality (apparent oxymoron notwithstanding). It isn’t the case, therefore, that we must keep the naturalistic understanding of ourselves under wraps, as some philosophers and scientists argue (see here). Indeed, the many benefits of worldview naturalism are there for the having, if we can manage to let go the assumption that human beings must possess some supernatural characteristic for life to be worth living. That letting go, and the concomitant access to the ethical and practical advantages of naturalism, is a long-term project of cultural change. This is the project of the Center for Naturalism and Naturalism.Org, in collaboration with its friends, allies and other organizations that take a more or less naturalistic view of the world. After 10 years we are still in the very early stages, but there are signs of progress.
 Alternatively, philosopher Owen Flanagan defines scientism as “the brash and overreaching doctrine that everything worth saying or expressing can be said or expressed in a scientific idiom,” another clear non-starter. See pp. 22-4 in his book The Really Hard Problem: Meaning in a Material World.