Philosophical naturalism comes in different flavors, but it is essentially the idea that nothing supernatural affects events in the world around us—or perhaps that nothing supernatural even exists. This is conveyed in the second definition of the word naturalism provided by the Oxford English Dictionary: “The idea or belief that only natural (as opposed to supernatural or spiritual) laws and forces operate in the world; (occas.) the idea or belief that nothing exists beyond the natural world.” Though the OED marks the stronger meaning as “occasional,” it is the one intended by anybody who explicitly identifies as a philosophical (or metaphysical, or ontological) naturalist. On this view, then, there are no supernatural entities, causal influences or forces, properties, laws, processes, or places. There are, likewise, no supernatural ways of obtaining knowledge about this world or another world.
In his entry titled “Naturalism” in The Stanford Encyclopedia of Philosophy, David Papineau traces current usage of the term to scientifically oriented philosophers from the first half of the twentieth century such as John Dewey, Ernest Nagel, Sidney Hook, and Roy Wood Sellars. Papineau’s article is useful, and the philosophers he names doubtless influenced the development of philosophical naturalism in twentieth-century and twenty-first-century philosophy. However, the term naturalism is older, and the concept of philosophical naturalism is much older still. The OED’s researchers have found naturalism and naturalist being employed in the sense of “nothing supernatural,” or “nothing outside the natural or material world,” as long ago as 1750. This usage seems to have been established by the turn of the nineteenth century, and to some extent naturalism was probably a euphemism for atheism.
Naturalism has a long and respectable philosophical pedigree. We can find recognizably naturalistic systems of philosophy in the Carvaka school of classical Indian thought and in ancient Greek and Roman philosophy, although the Epicurean school did make provision for the Greek gods. It explained them as natural beings, consisting of atoms like everything else, who existed in a state of detachment from human affairs. The overall effect of Epicureanism was to debunk all supernatural stories about the gods and their interventions in human affairs, and indeed it was the explicit aim of Epicurus and his followers to dispute the claims of religion.
The Epicurean worldview eventually had an immense impact. In 1417, at the height of the Renaissance, the Italian humanist scholar Poggio Bracciolini rediscovered the great Epicurean text De Rerum Natura. This was composed in the first century BCE by the Roman poet Lucretius, and it provides a comprehensive account of Epicureanism. During the Scientific Revolution of the seventeenth century, De Rerum Natura’s description of a universe composed of atoms and void profoundly affected the thinking of early modern scientists such as Galileo and Gassendi. Via their work, Epicurean physics helped shape the direction of science.
In its second definition of naturalism, the OED provides another sense of the word as used by philosophers: “the idea that moral concepts can be analysed in terms of concepts applicable to natural phenomena.” For the moment, we should note that this idea, moral naturalism, is distinct from philosophical naturalism. Many philosophical naturalists are moral naturalists, but philosophical naturalism and moral naturalism are not interchangeable. One is a set of claims about what does and does not exist: in summary, it is a claim that there are no supernatural entities or causal influences. The other is a cluster of metaethical theories. It deals—roughly speaking—with moral language and concepts. I’ll return to moral naturalism toward the end of this article.
Meanwhile, it is also standard to distinguish between philosophical naturalism, as a set of claims about what exists, and methodological naturalism, as a thesis about the limits of scientific inquiry. While philosophical naturalists deny (or at least do not believe in) the existence of supernatural entities or causal influences, methodological naturalists contend that science should not appeal to their existence in formulating hypotheses—and perhaps that it should not or cannot investigate them at all. Let’s look more closely at these positions.
A fundamental point to get clear about philosophical and methodological naturalism is that neither position subsumes the other. It is possible to be a philosophical naturalist without insisting on methodological naturalism in the practice of science. It is also possible to insist that science avoid framing supernatural hypotheses—which sufficiently qualifies as a form of methodological naturalism—without thereby embracing philosophical naturalism as a worldview. As one illustration, I might, as a philosophical naturalist, think it vanishingly unlikely that science will ever discover any supernatural entities acting on the world. I might not believe in the existence of these entities, and I might see no realistic prospect of evidence that would persuade me to change my mind. And yet, I might not deny that science could frame and test hypotheses involving supernatural entities. Even if I considered this futile for the purpose of advancing scientific knowledge, I might discern a social benefit in investigating (and possibly debunking) a wide range of supernatural as well as natural claims.
Conversely, I might, as a methodological naturalist, consider it good scientific practice to avoid framing supernatural hypotheses—and I might even believe that science lacks the resources to investigate supernatural claims—but I might personally believe in certain supernatural phenomena. If so, I would be a methodological naturalist in respect to science but certainly not a philosophical naturalist. For example, I might believe in the existence of God, gods, ghosts, demons, and/or a spiritual afterlife. Given my understanding of science and its limits, these beliefs of mine will inevitably be grounded in faith or in some other nonscientific “way of knowing” to which I subscribe.
What I’ve just described is not a rare approach to making sense of the world and the human condition. Some of the most explicit and vocal self-identifying methodological naturalists are religious adherents, while others are nonbelievers who are keen to avoid public conflict between science and religion. Either way, the effect is to isolate science and religious belief from each other.
Natural and Supernatural: Definitional Issues
An obvious problem for this entire debate is working out just where and how we should draw a line between supernatural and natural phenomena. If we don’t know what we mean when we refer to the supernatural, we don’t really know what philosophical naturalism and methodological naturalism amount to (this also applies to moral naturalism as it is usually understood). Without that knowledge, it is impossible to form reasonable judgments as to whether science is equipped to investigate supernatural claims. Of course, we usually recognize supernatural claims when we see them—for example, we classify gods and ghosts as “supernatural” entities, while classifying goats as “natural”—but that is not an intellectually satisfying response.
Sometimes human societies must get by without sharp definitions of concepts. In enforcing vaguely understood and ever-changing social standards, courts of law often work with fuzzy concepts such as reasonableness and proportionality. Even the most carefully drafted statutes include phrasing that is open to a range of interpretations or relies on vaguely understood boundaries. Notoriously, the Supreme Court Judge Potter Stewart once said of hard-core pornography that it is difficult to define but “I know it when I see it” (Jacobellis v. Ohio, 1964). Such statements can sound silly, but they’re really not all that absurd in their original contexts. Of necessity, part of the law’s function is enforcing a society’s implicit and imprecise ideas of tolerable behavior.
We might, by contrast, hope to make sharp distinctions in specifying what sorts of phenomena act upon the world or in defining good practice for scientific research. Accordingly, we might hope that philosophical and methodological naturalism would be precise ideas with straightforward criteria for their application to particular cases. Unfortunately, this seems to be a forlorn hope. The concept of the supernatural may not be as vague as that of reasonableness or proportionality, but, much like the concept of hard-core pornography, it can be disappointingly imprecise.
Consider just one example—although there are many others. Imagine a world in which people survive their bodily deaths, return as ghosts, and continue to interact with living human beings and with everyday objects. In a world like that, it would make sense to view ghosts as part of the natural order of things. Experts could study ghosts’ behavior much as relevant experts in our own world study the behavior of goats and other animals. Spectrology—the study of ghosts—would be a legitimate scientific discipline, and its practitioners would attempt to work out what abilities ghosts have, what drives or purposes motivate them to behave in certain ways in certain circumstances, and so on. The reason that institutionally respectable science does not attempt any such study of ghosts is not so much that they are classified as supernatural as that educated people are pretty sure they don’t exist.
One way to try to pick out the supernatural entities is to note that they are not observed—they are unseen (and unheard, unsmelled, etc.) entities—but the same applies to many things that are merely separated from us in time or space. For example, no human being has ever seen a non-avian dinosaur or a trilobite, although we have seen what we confidently interpret as their fossilized remains. And yet, although they have never been observed by anyone, no scientist, or anybody else, considers the great dinosaurs of the Mesozoic Era to have been supernatural creatures.
A philosopher who wants to tie the concept of the supernatural to a concept of human observation might reply that there’s a sense in which dinosaurs were at least observable. That is, they were creatures of a kind that could have been observed if humans had been around at the same time and in their proximity. But many narratives about gods such as Zeus and Apollo portray them as observable in this very generous sense. Even the Abrahamic god was observable when he walked in the Garden of Eden. Likewise, many people claim to have seen ghosts. At least sometimes, we imagine ghosts as directly observable things provided that we happen to be around when they are. By contrast, many of the micro-entities postulated by science are not directly observable even within the most generous understanding of observability. Observability may have some role in our concepts of the natural and supernatural. Surely, the unobservability of Heaven and Hell is part of what makes us classify them as supernatural places. But this alone does not establish a clear boundary.
In practice, our understanding of the supernatural may not be based on a single specifiable characteristic, such as which entities are (in one or another sense) observable. Rather, we’re inclined to regard imagined influences on the world as lying outside the natural realm if they violate commonsense ideas, perhaps supplemented by science, of what kinds of phenomena are encountered in ordinary life. We’re more inclined to classify these influences as supernatural if they resemble those originally described in prescientific narratives found in mythology, holy books, and folktales. For scientists and other empirical investigators such as historians, stories about these sorts of influences on the world are no longer taken seriously. There is no prospect of institutionalized empirical inquiry returning to explanations based on gods, ghosts, demons, magic, or the law of karma. All these stand as discarded and discredited explanations of the world’s phenomena.
On this account, the category of the supernatural is blurred, but it is not totally meaningless. It certainly does not follow from definitional difficulties that scientists are incapable of testing recognizably supernatural claims. Rather, science has moved on from them. Hypotheses invoking, for example, the causal powers of ghosts and demons could be investigated scientifically if someone relied on their existence during public discussion and there was social utility in putting them to the test. All that’s required is that (1) the ghosts and demons are alleged to act on the world in ways that would leave physical traces and (2) either their powers and motivations are sufficiently specified in advance to make their actions predictable or they are alleged to have acted in specific ways on specific past occasions.
Indeed, far from lying beyond the reach of scientific testing, many supernatural claims have been directly or indirectly falsified. This includes some straightforwardly religious claims, such those about the age of the Earth. If our planet had turned out to be about six thousand years old, this would have supported many of the claims made by biblical literalists. Unfortunately for them, however, the Earth is between four and five billion years old. Even where science has not straightforwardly falsified religious claims, many have come to seem implausible as a result of science’s advance. (To be clear, the erosion of religion’s authority has not come only from the sciences, narrowly defined. For example, reputable textual-historical scholarship now makes sense of foundational religious texts in ways that reject traditional beliefs about their origins and historical reliability.)
The history of science provides contemporary scientists with more than adequate reasons to avoid taking the lead in invoking beings such as gods, ghosts, and demons. This is a sensible institutional norm controlling what will be regarded—for example, by peer-reviewed journals and funding agencies—as legitimate science. It is, however, no more than that. It does not preclude investigating claims about supernatural entities as and when there is a public interest in doing so. It does not entail that claims made about supernatural entities lie beyond science’s investigative resources. Thus, methodological naturalism can be justified up to a point—as a useful institutional norm—but it does not provide a wall to isolate supernatural claims from any and all scientific scrutiny. It does not rule out scientific investigation of the supernatural, where socially appropriate, and it does not undermine the plausibility of a robust form of philosophical naturalism.
Naturalism and Post-Religious Philosophy
Much contemporary academic philosophy is post-religious. As the debates within philosophy have proceeded since the middle decades of the twentieth century, many philosophers who view themselves as taking a “naturalistic” approach are not especially interested in debunking or defending religious beliefs. This gives their work a rather different emphasis from that of colleagues who are involved in more popular debates about religion’s authority and social role. For many philosophers, then, the debate has reached a point where they are no longer concerned about a traditional category of supernatural entities and influences. Their focus is, instead, on issues that might seem obscure to others outside the academy.
For example, contemporary naturalistic philosophers sometimes ask themselves how mathematical abstractions, such as numbers and sets, fit into the best philosophical account of what sorts of things exist in the world. In what sense (if it at all) does, say, the number 2 exist, since it is obviously not a material entity? This is puzzling, but it might not be a topic of concern for traditional philosophical naturalists. Why not? For them, the point was not necessarily to establish a complete, airtight description of the world’s ontology. Rather, it was to dispute the existence of unseen entities and forces that were portrayed in prescientific narratives as interfering in human endeavors. Whatever we ultimately want to say about the ontological status of numbers and sets, mathematics has played an indispensable role in the advance of science. Numbers and sets do not belong on the reject pile with gods, ghosts, and demons.
Contemporary post-religious philosophers may not care about the Epicurean goal of liberating human beings from supernatural terrors through rational understanding and inquiry. They may, however, be impressed by evidence for the causal closure of the physical world. This means that there is no room for causal influences on physical events from anything lying outside the fundamental particles, structures, and forces recognized by physics. It follows that our own bodily movements and the processes within our brains have entirely physical causes. In fact, as Papineau describes in his Stanford Encyclopedia article, much of the evidence for causal closure arises from twentieth-century physiological research that detected no unusual influences or causes operating within living cells.
Our current evidence for causal closure of the physical world appears to rule out, among other things, the existence of potentially immortal minds or souls that interact with our bodies. This tends to encourage scientists and naturalistic philosophers away from ideas such as Cartesian dualism and toward materialist accounts of the mind and consciousness. From a suitably post-religious perspective, methodological naturalism can be repackaged as merely an institutional rule about deference to physics. It forbids scientists (except, perhaps, cutting-edge physicists themselves) from postulating causal influences that cannot ultimately be reconciled with—or that clearly add another layer to—those identified in well-established and fundamental physics. Note, however, that any such rule is a socially chosen response to scientific experimentation in the past. It is not an inherent limitation on the power of scientific methods of inquiry. As with the investigation of ghosts and demons, science is not incapable of investigating claims about, say, a special “life force.” Rather, the track record of such claims is so poor that they will no longer be taken seriously by peer reviewers, journal editors, funding agencies, and other gatekeepers.
As naturalistic academic philosophy progresses, it increasingly concerns itself with technical and esoteric issues. There’s not necessarily anything wrong with that, although we might worry about the increasingly hyper-specialized nature of philosophy, and we might suspect that the discipline is becoming remote from the existential worries of ordinary people. This may not be healthy. As its questions and methods of analysis become increasingly specialized, philosophy becomes less attractive to prospective students—especially if rival humanities disciplines speak more clearly to their cultural and political concerns. Nonetheless, there are legitimate questions that fall outside any of science’s disciplines and subdisciplines: questions relating to the nature and methodology of science itself; science’s justification and role; and the relationship of the specialized sciences to each other. These are within the scope of philosophy, and they should be studied with as much intellectual rigor as possible.
But philosophical naturalism should not be viewed as merely a technical approach to science and philosophy. It has deep roots in the Western tradition of inquiry, going back at least as far as Epicurean philosophy with its principled hostility toward religion and its imperative of liberating us from religious terrors. This tradition must continue: it remains important and intellectually persuasive.
Moral Naturalism and Its Prospects
As promised in my opening paragraphs, I turn now to the issue of moral naturalism. Unfortunately, this has a confusing history. We can identify ancient theories of ethics that were broadly naturalistic, among them the Epicurean theory based on achieving aponia (absence of pain) and ataraxia (serenity, or absence of anxiety). There are also, of course, ancient and modern theories of morality based on the idea of a social contract. All of these theories avoid sources and explanations for morality that fall outside of a broadly naturalistic worldview. Often they depend on one or another conception of human nature. Conceptions of human nature can, of course, be completely naturalistic, treating us as a particular mammalian species—alongside goats and gorillas—with distinctive capacities and an evolved psychological profile.
However, the modern debate over various kinds of moral naturalism, explicitly so called, deals with something narrower and more specific. The debate can be traced to G. E. Moore’s work in the early years of the twentieth century. Moore’s identification of a supposed “naturalistic fallacy” was influential, but it also produced confusion. He denied that terms such as good and bad (in their specifically moral senses) can mean anything such as, in the case of goodness, “produces pleasure” or “maximizes social benefit.” However, he also denied that good could mean something like “commanded by God.” The alleged fallacy was really one of defining moral terms in such a way as to substitute a natural or a recognizably supernatural property. Instead, Moore viewed moral goodness (and likewise moral badness) as a unique, irreducible, non-natural property.
For over a century, there has been much argument as to whether Moore’s arguments are successful. His key point was that it seems possible to ask questions such as “Taking drugs produces pleasure, but is it morally good?” or “Slaughtering the Canaanites was commanded by God, but was it morally good?” (For the sake of argument, let’s entertain the fiction that God exists and did, indeed, command the slaughter of the Canaanites.) Notice that the argument does not depend on whether or not taking certain drugs really does, on balance, produce pleasure or whether slaughtering the Canaanites was somehow justified by its consequences. Moore is noting that questions such as these would not even make sense to us if the moral term good simply meant “produces pleasure” or “commanded by God.”
While the argument seems to work for simple cases, it’s not obvious that it works for more complex analyses of the meanings of moral terms. Nor does it work, at least by itself, to resist some of the more sophisticated attempts to show how moral goodness could, after all, be a natural property—even though not by definition. For example, the word water does not (or at least did not originally) mean the chemical compound H20. It turned out, however, that water—the watery stuff that we’re all acquainted with—is H20. This was a substantive scientific discovery, not something that could have been revealed by analyzing the word w
ater and how it is used by English speakers.
By analogy, some philosophers accept that good (in the moral sense) does not mean “produces pleasure” (for example), but they argue that moral goodness might nonetheless turn out to be the property of (for example) producing pleasure. These metaethical issues become devilishly complicated to unravel, so I won’t attempt the task here. I can sense some readers’ eyes already glazing over. (But for those who are interested, I try to sort it all out in chapter 4 of my 2016 book, The Mystery of Moral Authority.)
Despite all the confusion, including the confusion introduced by Moore’s explanation of his point, moral naturalism now has a reasonably clear meaning. It is usually taken to be the idea that moral properties are natural properties—whether by definition or otherwise. Moral naturalists typically wish to claim something additional: that moral properties are not subjective, end-relational, or institutional properties. For example, moral naturalists typically claim that an action’s moral goodness or badness does not depend on someone’s attitude toward it, or how it is classified within a social institution such as the mores of a particular culture.
In this approach, when I claim that torturing kittens is morally bad or morally wrong I don’t mean something like “torturing kittens is one of the actions that I disapprove of” or “torturing kittens will not advance our agreed goals” or “torturing kittens is forbidden by the local mores.” These are all strictly naturalistic claims—nothing supernatural or nonnatural is involved—but moral naturalists are usually hoping for a bit more than that. They want morality to make claims whose truth is not hostage to attitudes and social institutions. In that sense, they are hoping that morality will turn out to be something more “objective.”
There are obvious reasons why philosophical naturalists might be attracted to moral naturalism, especially in a form that avoids analyzing moral properties as subjective, end-relational, or institutional properties. Moral naturalism in this form will provide an account in which moral goodness, badness, rightness, wrongness, and so on are all empirically tractable. At the same time, moral properties will have a kind of objectivity in not being hostage to anyone’s ends and attitudes or to local and historically mutable social institutions.
Given the attractions of this kind of moral naturalism, we might wonder whether philosophical naturalists should be moral naturalists. Moral naturalism is certainly consistent with philosophical naturalism. It attempts to understand morality without appealing to anything supernatural (or to any unique, rather mysterious, Moorean nonnatural properties). However, moral naturalism is not the only account of morality that is consistent with philosophical naturalism. For a start, noncognitivist accounts of morality, in which moral judgments do not assert facts but perhaps prescribe our own or others’ conduct, or perhaps express the speaker’s feelings, are equally consistent with philosophical naturalism. So are many theories that find some kind of error in the practice of morality—usually theories that see moral judgments as involving claims that cannot be literally true. And of course, theories that analyze moral properties as subjective, end-relational, or institutional properties can be entirely naturalistic. They need invoke nothing supernatural or otherwise metaphysically mysterious.
For reasons well beyond the scope of this article, I am unconvinced by any form of moral naturalism (again, see the full story in The Mystery of Moral Authority). If I’m right, however, this may entail that no plausible theory of morality is capable of giving us everything we’d ideally want: moral judgments as factual claims capable of being true or false; objectivity in the sense that I’ve briefly described; and nothing supernatural or mysterious involved. As I understand morality, following the philosopher J. L. Mackie among many others, it is a social technology that assists human beings to function cooperatively in groups. However, social groups then (fairly much inevitably) rationalize it on one or another false basis as something more metaphysical. On this account, there is no single “true morality,” but some moral systems are better than others when judged against widely agreed-upon social goals and given certain facts about our nature as social animals.
My account of morality is naturalistic; it is fully consistent with philosophical naturalism. But for many people—even for many atheists and philosophical naturalists—it is likely to be disconcerting. It does not deliver complete moral objectivity, and it suggests that there is pervasive misunderstanding of morality’s true nature. Nonetheless, it provides an intellectually attractive paradigm for the future study of morality from a naturalistic viewpoint. This should, I suggest, take place in conjunction with the study of human nature itself. As David Hume saw, commonalities of human nature place constraints on morality’s content. This rules out the more extreme and worrisome varieties of cultural relativism, but it is highly unlikely to constrain us to a single true moral system.
From my viewpoint, this is where the action is for philosophical naturalists. We have available to us an attractive, even if disconcerting, paradigm for something much more like a science of morality than anything that has existed in the past. The new paradigm is capable of open-ended development and elaboration as we gain more knowledge about moral systems, our own needs as human beings, and ourselves.