Philosophical naturalism (hereafter, just “naturalism”) is a general understanding of the world and humanity’s place within it. Naturalism concludes that the only reality is the physical reality of energy/matter as gradually discovered by our intelligence using the tools of experience, reason, and science. Human experience is the ultimate source and justification for all knowledge. Experience itself has accumulated in human memory and culture, gradually producing the methods of intelligence called “reason” and “science.” Naturalism’s emphasis on the human adventure of discovery is central to its mission. If naturalism overemphasizes its current conception of reality, it risks collapsing into just another worldview that obscures its supreme commitment to intellectual methodology.
Naturalism emphasizes the progressive and expanding knowledge that observation and science provide. Science continually revises its understanding of physical reality. Today’s scientists have conceptions of energy and matter that most nineteenth-century scientists would have found incomprehensible, and the next century’s scientists will likely demand major revisions to today’s best theories about physical reality. Because science’s best ideas about reality continually improve, naturalistic philosophy requires intellectual humility. While reality is physical and discoverable by science, naturalism cannot offer any final and perfect picture of exactly what this reality is. Therefore, the primary task of naturalism is not to defend science’s current best theories about reality—science itself is responsible for reasonably justifying its own theories. Naturalism undertakes the responsibility to elaborate a comprehensive and coherent portrait of reality based on science and to defend science’s exclusive right to explore and theorize about all of reality without interference from tradition, superstition, mysticism, religious dogmatism, or priestly authority.
Science, therefore, has three close relationships with philosophy. First, when the various sciences question their ultimate principles and ponder how these principles can reasonably cohere, science becomes philosophy and intellectuals undertaking the discussions of these problems act as both philosophers and scientists. For example, the leaders of great scientific revolutions, from Aristotle, Galileo, and Newton to Helmholtz, Mach, and Einstein, are justly recognized for their major philosophical contributions. Science occasionally is naturalistic philosophy.
Second, when the sciences are under intellectual attack by jealous rivals offering nonnatural hypotheses or unnatural modes of alleged knowledge, science turns to philosophy for reasoned arguments about why nonnatural hypotheses are irrational and unnecessary and why allegedly unnatural knowledge is not knowledge at all. Philosophical naturalism explains, justifies, and improves scientific method.
Third, when the sciences are under political attack by hostile forces eager to obstruct scientific research or inhibit scientific teaching, science turns to philosophy for staunch defenses of intellectual freedom and democratic secularism. Philosophical naturalism helps to construct and maintain a liberal political order, protecting the freedom of scientific inquiry.
Since naturalism’s staunchest opponent is supernaturalism, naturalism is sometimes defined in terms of what is not included in reality: no supernatural gods or unnatural powers; no spirits; no miracles; no revelations or intuitions from a transcendent source; and no master design or plan for nature. Naturalism must not be defined, however, by what it is against. We have already defined and explained our positive idea of naturalism to make it clear that a naturalist does not believe that any hypothesis about supernaturalism enjoys sufficient reasonable support. Naturalism accepts a widely respected and commonsense view of reasonable belief that lies at the heart of science: If experience/reason/science gives sufficient reasonable support to hypothesis X, and X enjoys more reasonable support than any rival hypothesis competing with X, then I should believe that X is (fallibly) correct, and I should believe that rival hypotheses are incorrect. Where there is insufficient reasonable support for X, I ought to withhold belief from X.
A naturalist is a nonbeliever and a skeptic about any supernaturalistic hypothesis. There are two main types of nonbelievers: atheists and agnostics. Let’s begin with agnosticism. An agnostic is someone who “does not know whether supernaturalism is true or not,” according to the common definition and the roots of the term in Greek (a “gnostic” is someone who possesses knowledge, so an “a-gnostic” is someone who does not possess knowledge). In the context of religious knowledge, the agnostic about religion would, therefore, be someone who does not possess knowledge about religious claims. Based on this vague definition, there are countless agnostics: babies, most small children, adults who remain uncommitted to any particular religion, adults who have lost cognitive capacity, and so on. However, a better definition of agnostic would be “someone who has considered reasons offered for belief in supernaturalism, has not yet been convinced by those reasons, and, therefore, is presently a nonbeliever.” While more accurate, this lengthy definition needs more work, because the atheist is also someone who has considered reasons offered for belief in supernaturalism but remains a nonbeliever. What is the difference between an agnostic and an atheist?
Sometimes atheist and agnostic are defined so that an agnostic cannot also be an atheist. For example, suppose an atheist is “someone who claims to know that no god exists” and an agnostic is defined as “someone who claims that they do not know whether any god exists.” If an atheist is a gnostic—someone who claims to have knowledge—then the atheist should have excellent reasons for knowing that nothing supernatural exists. However, no one can ever know with certainty that nothing supernatural exists. Logic itself dictates that it is impossible to prove by experience, reason, or science that nothing supernatural exists. If an atheist is “someone who claims to absolutely know that no god exists,” then there could be no reasonable atheists. But there are reasonable atheists, because this is not the actual definition of an atheist. We need a more reasonable definition of atheism.
How can we accurately identify “reasonable atheists” and distinguish this group of nonbelievers from the agnostics? Let’s return to simpler, common-sense notions of these two groups. To be different from an atheist, the agnostic must be someone who may be tempted to accept supernaturalism without becoming an actual believer. On the other side, the atheist must be someone who has very little, if any, temptation to believe any supernatural hypothesis. Separating the atheist, the agnostic, and the supernaturalist from one another evidently requires that we conceive of belief as having degrees from zero to full belief—from a complete lack of belief in X, to a middle range of partial belief in X, to complete conviction about X. The agnostic is someone occupying the middle range. He or she has some sort of “partial belief” in supernaturalism, which enjoys some degree of reasonable support that is enough to tempt belief but not yet enough to convince. An agnostic would say, “Supernaturalism has some significant reasons to support it, and may possibly be correct, but I’m not yet fully convinced.” By contrast, the atheist is far less
impressed by reasons offered for supernaturalism. An atheist would instead say, “Supernaturalism has very few (or no) significant reasons to support it, and even though supernaturalism can’t be proven false, I’m so far quite unconvinced.”
Must all naturalists be atheists or can they be agnostics? Our answer depends on the particular supernaturalist hypothesis under debate. Consider the kind of supernaturalism that holds hypothesis C: God is responsible for the creation of each particular species of life. Since the naturalist believes instead that evolution by natural selection is the far superior hypothesis explaining the existence of particular species of life, the naturalist claims that C is false and disbelieves in a God that creates species. But also consider the kind of supernaturalism that holds hypothesis U: God is responsible for the Big Bang origin of the universe. Suppose a naturalist does not believe U, but not because this naturalist already believes a rival naturalist hypothesis about the cause of the Big Bang. In this case, our naturalist does not believe any hypothesis about a cause for the Big Bang (even the idea that there was a cause), because he or she believes that no currently available hypothesis, natural or supernatural, presently has sufficient reasonable support. This naturalist does not believe U, not out of a belief that U is false, but rather because he or she does not find sufficient reasonable support for U. Our naturalist is, therefore, “agnostic” about U.
A naturalist can, therefore, be an atheist concerning some hypotheses about God and can also be agnostic concerning other hypotheses about God. In either case, the naturalist is skeptical about all such supernatural hypotheses.