Metaphysical naturalism

Metaphysical naturalism is any worldview in which nature is all there is and nothing supernatural exists. It is often simply referred to as naturalism, and occasionally as philosophical naturalism or ontological naturalism, though all those terms have other meanings as well. This article presents only a basic outline of the definition and history of metaphysical naturalism and the major arguments for and against it.

Definition
Metaphysical naturalism is most commonly distinguished from methodological naturalism. Metaphysics is the branch of philosophy concerned with what exists beyond what has already been established by science, so metaphysical naturalism refers to a belief about the totality of what exists. Methodology, however, is only the means by which knowledge is acquired. Thus, metaphysical naturalism entails the belief that nature is in fact all that exists, while methodological naturalism entails the belief that for one reason or another empirical methods will only ascertain natural facts, whether supernatural facts exist or not.

The concept of "nature" embraced by contemporary metaphysical naturalists excludes by definition gods, spirits, and any other supernatural beings, objects, or forces. There are many different varieties of metaphysical naturalism, but all can be separated into two general categories, physicalism and pluralism. Physicalism entails the claim that everything everyone has observed or claimed to observe is in actual fact the product of fundamentally mindless arrangements or interactions of matter-energy in space-time, and therefore it is unreasonable to believe anything else exists. Pluralism (which includes dualism) adds to this the existence of fundamentally mindless things besides matter-energy in space-time (such as reified abstract objects).

What all metaphysical naturalists agree on, however, is that the fundamental constituents of reality, from which everything derives and upon which everything depends, is fundamentally mindless. So if any variety of metaphysical naturalism is true, then any mental properties that exist (hence any mental powers or beings) are causally derived from, and ontologically dependent on, systems of nonmental properties, powers, or things. This means metaphysical naturalism would be false if any distinctly mental property, power, or entity exists that is not ontologically dependent on some arrangement of nonmental things, or that is not causally derived from some arrangement of nonmental things, or that has causal effects without the involvement of any arrangement of nonmental things that is already causally sufficient to produce that effect.

In lay terms, if metaphysical naturalism is true, then all minds, and all the contents and powers and effects of minds, are entirely constructed from or caused by natural phenomena, while if metaphysical naturalism is false, then some minds, or some of the contents or powers or effects of minds, are causally independent of nature, either partly or wholly caused by themselves, or existing or operating fundamentally on their own. Belief in the latter entails some form of supernaturalism (the opposite of naturalism), which is not limited to supernatural beings, but can encompass mindless things with distinctly mental properties, like magical objects (see magic and incantation) or causally efficacious Platonic Forms or the existence of Love as a cosmic force.

Finally, it should be noted that the relationship between metaphysical and methodological naturalism is not one dimensional and varies among individual thinkers. To understand this relationship, two varieties of methodological naturalism should be distinguished. Absolute methodological naturalism is the view that it is in some sense impossible for any empirical method to discover supernatural facts even if there are some, and this is compatible with (but does not entail) the view that something other than empirical methods might be able to discover supernatural facts. Contingent methodological naturalism entails the belief that, judging from past experience, empirical methods are far more likely to uncover natural facts than supernatural ones, so that it is generally an ill-advised waste of resources to pursue supernatural hypotheses, but it would not be impossible to confirm them empirically if any were true. With all this understood, every metaphysical naturalist will be either a contingent methodological naturalist or an absolute methodological naturalist, but not all methodological naturalists are metaphysical naturalists.

''This article covers metaphysical naturalism as a worldview. Another article more thoroughly discusses methodological naturalism and its role in contemporary debates.''

Ancient period
Metaphysical naturalism appears to have originated in early Greek philosophy. The earliest presocratic philosophers, such as Thales, Anaxagoras or most especially Democritus, were labeled by their peers and successors "the physikoi" (from the Greek φυσικός or physikos, meaning "natural philosopher," borrowing on the word φύσις or physis, meaning "nature") because they sought to explain everything by reference to natural causes alone, often distinctly excluding any role for gods, spirits or magic in the creation or operation of the world. This eventually led to fully developed systems such as Epicureanism, which sought to explain everything that exists as the product of atoms moving in a void, or the advanced Aristotelianism of Strato of Lampsacus, which sought to explain everything that exists as the inevitable outcome of uncreated natural forces or tendencies.

In their definition of nature, the ancient Greeks distinguished "nature" from "artifice." Anything that resulted from the innate properties of a thing was regarded as having a natural cause, regardless of whether those properties themselves were intelligently arranged or not, while anything that resulted from intelligent action was regarded as having an artificial cause, regardless of whether the intelligence itself was the product of natural causes. Thus, natural causes were partially distinguished from intelligent causes. It was often assumed that some intelligent causes were primary causes and not solely the product of natural properties, but not everyone agreed. Following the physikoi and their successors, some ancient intellectuals denied the existence of any intelligent causes that were not entirely the product of natural causes (thus reducing all intelligent causes to natural causes), and they represent the earliest metaphysical naturalists. However, only a few Greek and Roman intellectuals embraced such a view, though of those few, Epicurus and Strato of Lampsacus were the most famous.

Metaphysical naturalism is most notably a Western phenomenon, but an equivalent idea has long existed in the East. Though unnamed and never articulated into a coherent system, one tradition within Confucian philosophy embraced a view that can correctly be called metaphysical naturalism, dating back at least to Wang Ch'ung in the 1st century, if not earlier. But this tradition arose independently and had little influence on the development of modern naturalist philosophy or on Eastern or Western culture.

Middle ages to modernity
With the rise and dominance of Christianity and the decline of secular philosophy in the West, metaphysical naturalism became heretical and eventually illegal, thus making the Middle Ages a true Dark Age for the history of metaphysical naturalism. When the Renaissance reintroduced numerous lost treatises by Greek and Roman natural philosophers, many of the ideas and concepts of naturalism were picked up again, contributing to a new Scientific Revolution that would greatly advance the study and understanding of nature. But social and legal hostility continued to prevent advocates of metaphysical naturalism from coming forward, if there were any, until the political advances of the Age of Enlightenment made genuine free speech possible. Then a few intellectuals publicly renewed the case for metaphysical naturalism, like Baron d'Holbach in the 18th century.

In this period, metaphysical naturalism finally acquired a distinct name, materialism, which became the only category of metaphysical naturalism widely defended until the 20th century, when advances in physics as well as philosophy made the original premise of materialism untenable. In physics, matter was found to be a form of energy and therefore not the fundamental constituent of reality as materialists had been presuming. In philosophy, renewed attention to the problem of universals and other undeniable but "immaterial" realities further called materialism into question. These developments refined naturalism into the two forms now widely advanced (physicalism and naturalist pluralism, as explained above), both corresponding more closely to the system historians believe was articulated by Strato, rather than the system advanced by Epicurus as is commonly thought.

Currently, metaphysical naturalism is more widely embraced than ever before, especially but not exclusively in the scientific community, where acquaintance with the facts of nature is broader and more secure, though metaphysical naturalism is still a minority worldview. The vast majority of the population of the world, and of the United States, remains firmly committed to supernaturalist worldviews. However, it is likely that a substantial minority or even a majority of the population in certain European and other first world countries might embrace metaphysical naturalism in some basic, unarticulated sense. Today, noteworthy proponents are too numerous to count, but prominent defenders of metaphysical naturalism as a complete worldview include Mario Bunge, Richard Carrier, Daniel Dennett, Sam Harris, and David Mills.

Marxism, objectivism, and secular humanism
As a final note to the history of metaphysical naturalism, certain extreme varieties of politicized naturalism have arisen in the West, most notably Marxism in the 19th century and Objectivism in the 20th century. Marxism is an expression of communist or socialist ideals in a naturalist framework, while Objectivism is the exact opposite, an expression of capitalist ideals in a naturalist framework. However, today most advocates of metaphysical naturalism in first world countries reject both extremes and embrace the more moderate political ideals of Secular Humanism.

Common beliefs
Contemporary naturalists possess a wide diversity of beliefs and engage each other in healthy debate and disagreement on many issues. However, besides the basic beliefs already described above, most if not all contemporary naturalists believe the following, as a consequence of applying the core beliefs of naturalism to the findings of the sciences and their own personal experience:

Undesigned universe
The universe has either always existed or had a purely natural origin, being neither created nor designed. Either way, naturalists hold nature (rather than, say, God or Tao) to be the eternal ground of all being. Big Bang theory is now well-established and entails the observable universe had a beginning, unfolding from a process of natural laws, but this does not entail anything about what existed before, and thus does not resolve the question of whether all that exists began to exist or whether something of some sort has always existed, so most naturalists remain open to either hypothesis. Though it is neither required nor universally embraced by naturalists, multiverse theory is particularly popular now, as scientific advances have substantially increased the likelihood that the observable universe is only part of a much larger whole, and the explanatory power and relative simplicity of current multiverse cosmologies is remarkable.

Unplanned life
Life is an unplanned and accidental byproduct of blind natural processes. In our case, this meant the accidental combination of a chain of chemicals called amino acids, into a pattern that was automatically self-replicating (possibly as autocatalytic RNA or PNA), in a sheltered but energy-rich environment. The extreme improbability of such a combination has ensured that life is extremely rare. However, though life is indeed rare, the observable universe is so incredibly old and large that even very improbable events are guaranteed to happen a few times within it, and life on earth is one of those inevitable outcomes. Our existence is therefore seen as lucky rather than planned or intended, and most naturalists call upon everyone to appreciate and make the best of their good fortune rather than devaluing or squandering it.

Evolution
Evolution by natural selection is a staple of the naturalist worldview, perceived as a well-established scientific explanation for the rise and diversity of life on earth, which developed slowly and imperfectly over an extremely long period of time. Thus, our own existence as conscious animals of superior intelligence and sagacity, is explained not as the outcome of intelligent design nor as a mere accidental combination of chemicals (such as originated life), but as the product of a dynamic system that generates highly complex order on its own, without any guidance. Since this entails that the properties of living organisms like ourselves have been selected not according to a compassionate or prescient engineer, but solely according to their differential reproductive success, naturalists interpret cells, organs and species as having a "purpose" or "function" in terms of their ability to increase differential reproductive success, but do not perceive in this any moral goal that should be emulated or furthered, since nature is the cause, and nature has no compassion or plan. However, this does not exclude the possibility of true moral propositions derived from evolved facts (see Value of society and Primacy of happiness below).

Mind as brain
Human beings have no independent soul or spirit, but only a material brain, which operates to produce a conscious mind. Since our mind, and hence our identity and existence as persons, is entirely the product of a physical process, three conclusions follow. First, all mental contents (such as ideas, theories, emotions, moral and personal values, or beauty and ugliness) exist solely as the computational constructions of our brain, and not as things that exist independently of us. Second, damage to the brain (from disease, drugs, malnutrition, or injury) frequently entails damage to the self and therefore should be of great concern. Third, the death or destruction of our brain cannot be survived, and therefore all humans are mortal. That means, given present technology, death is inevitable and entails our complete extinction. Since this entails there is no present hope of an afterlife, naturalists argue humans need to accept this and make the most of what they have.

Qualified free will
Though some naturalists still disagree, most are compatibilists with regard to free will. This means they believe humans have "free will" only in the sense that humans can choose to do what they desire to do, and in many cases can even choose their desires. However, our choices are ultimately constrained by the physical and biological realities of our circumstances and our nature as human beings and as uniquely-developed individuals. For example, we cannot choose to violate the laws of physics or to be smarter than we actually are. Likewise, some of our desires are unalterable features of our nature as human beings, while others are ingrained features of our character and thus alterable only with considerable effort. Nevertheless, once we accept the limitations of our physical world and bodies, our will is not constrained by much else than our knowledge, desires, and powers of reasoning, so within the limitations of what we know, want, and can think of, we can indeed choose what to do, on the basis of a rational or intuitive analysis of the possible and likely consequences of alternative courses of action.

Utility of reason
Reason is the refinement and improvement of naturally evolved faculties, through discovering, then learning, and then employing methods and procedures that are found to increase the frequency with which we arrive at true conclusions and correct information about ourselves and the world. Naturalists thus believe that reason is superior to all the other tools available to us in ascertaining the truth, so anyone who wishes to have more beliefs that are true than are false should seek to perfect and consistently employ their reason in testing and forming beliefs. One outcome of this principle has been the discovery that empirical methods (especially those of proven use in the sciences) are unsurpassed for discovering the facts of reality, while methods of pure reason alone can securely discover only truths inherent in concepts and systems of ideas.

Value of society
Humans evolved as social animals, which is the only reason we have developed culture and civilization, and now in fact depend on them. This means that even in the neutral terms of differential reproductive success, as a species our future depends on developing and maintaining a healthy and productive culture and civilization, and any behavior contrary to that end threatens our survival and the survival of our neighbors and kin and all their descendants. Likewise, this means humans have been "designed" by blind natural forces to require a healthy society in order to flourish and feel happy and content, which means the pursuit of human happiness requires the pursuit of a healthy society to live in, interact with, and benefit from.

Primacy of happiness
There is nothing of any greater value than human happiness. Therefore, all conduct and behavior should be oriented toward that aim, maximizing its availability while minimizing opposing risks. Though happiness is an accidental product of naturally and culturally evolved attributes of the human animal, it exceeds everything else in human experience in terms of its enjoyability and desirability, and therefore it is self-contradictory to assert that there is anything humans would enjoy or want more than their own happiness. However, there are tangible differences between happiness in this sense (as a form of pleasant contentment that everyone would desire more than anything else) and mere pleasure, as also between physical and intellectual pleasure. There are also complex realities involved in pursuing happiness, most notably those created by the complexities and limitations of natural human psychology and the complexities and limitations of living within (and depending upon) a social system. When all these distinctions and complexities are understood, a system of behavioral or attitudinal principles can be deduced, which is what naturalists define as morality. It is probably a universal conclusion among naturalists that this analysis ultimately produces a commitment to the Golden Rule, with many contemporary naturalists further concluding that by cultivating the personal virtues of compassion and honesty we most effectively realize this Rule and, as a result, most effectively increase our access to happiness (in frequency, degree, duration, and probability).

Arguments for naturalism
There are many arguments for metaphysical naturalism. Only a few will be surveyed here, and only in brief. There are many others, but most involve refinements, variants or sub-arguments to the following.

Argument from precedent
For over three hundred years empirical methods have consistently discovered only natural things and causes, even underlying many things once thought to be supernatural. Meanwhile, no other methods have produced any consistent conclusions about the substance or causes of anything, much less anything supernatural. The logical inference is that since countless past gaps in knowledge have been filled by naturalism, and by nothing else, probably all remaining gaps in knowledge will be filled by naturalism as well. This simply extends a principle fundamental to science as a whole, that we should presume any new phenomenon obeys known laws of physics until we have empirically proven otherwise. Hence we should presume that any unexplained fact has a natural explanation until we have empirically proven otherwise. Therefore, since we have not found empirical proof of anything supernatural, and since we have abundant reason from past precedent to expect that natural explanations underly everything, metaphysical naturalism is most probably true.

Argument to naturalism as best explanation
Some naturalists argue that sound naturalist hypotheses about facts still scientifically unexplained outperform all other hypotheses in explanatory scope and power, relative to explanatory simplicity. If that's true, then metaphysical naturalism is the best explanation of everything we observe and experience, and is therefore probably true. This amounts to arguing that everything makes more sense if naturalism is true, many details about ourselves and the world are more probable if naturalism is true, and to explain even the most mysterious of facts naturalism has to resort to fewer ad hoc assumptions than any known alternative. For example, resorting to God as explanation typically requires a bewildering array of completely ad hoc assumptions about that God's abilities, nature, limitations, and desires, and still leaves a lot unexplained as the "mystery" of God's enigmatic will or as beyond human ken, whereas naturalism relies much more heavily on assumptions already scientifically established as precedents and principles, and makes more specific predictions about what the observed results would be if naturalism were true, which align very well with actual observations.

Argument from absence
One major way in which naturalism explains things better than alternatives is that if the supernatural exists (whether as gods, powers, or spirits), it is so silent and inert that its effects are almost never observed, despite vast and extensive searching. Even the relatively few alleged observations take place only under dubious conditions lacking in sound empirical controls or tests, and on those occasions when they are subsequently subjected to sound controls or tests, they turn out to be false. Our inability to uncover clear evidence of anything supernatural is somewhat improbable if anything supernatural exists, but very probable if nothing supernatural exists, and therefore metaphysical naturalism is probably true.

Argument from physical minds
Scientists have accumulated vast evidence that the human mind is a product of a functioning brain, which is entirely constructed from different interacting physical systems that evolved over time through the animal kingdom, and that our brain is now the most complex machine found anywhere in nature, and that our minds appear limited to our brain's physical needs and capabilities. We have discovered no clear evidence of any other kind of mind, nor any clear evidence that our minds can exceed the limitations of our physical brain, nor any clear evidence that our brains did not slowly evolve through billions of years of natural selection. This is the only way we would observe the facts to be if naturalism were true (since there is no other way to have a mind on naturalism except as the product of a slowly evolved, highly complex physical system like our brain), but if supernaturalism were true (and therefore some minds or mental content exist independently of a physical machine like our brain), what we observe is not the only way things could be (since by now we could have and likely would have observed some supernatural elements of our or other minds or observed mental powers in other things). Since this observation is less probable if supernaturalism is true, metaphysical naturalism is more likely to be true.

Cosmological argument for naturalism
If naturalism is true, life must be a very improbable accident. Therefore, the only way we would be observing life to exist if naturalism were true, is if the universe were so immensely old and big that events of such an improbability will be very rare but still likely to occur. We observe the universe to be that immensely old and big and life to be that rare. In addition, the universe is almost entirely lethal to life. By far most of what exists is a deadly radiation-filled vacuum, and by far most matter in the cosmos composes lethal environments like stars and black holes. Insofar as supernaturalism allows other possible arrangements for us to observe, such as universes more universally hospitable to life, universes far too young or small to produce life by mechanical accident, or universes in which life is far more common, what we observe to be the case is less probable given supernaturalism than given naturalism, and therefore metaphysical naturalism is more probably true.

Argument from the implausibility of alternatives
Finally, joining the first two arguments to the rest, we have an argument from implausibility. The only supernatural hypothesis that does not fall to any of the above arguments is a hypothesis wherein the proposed supernatural entity or ability is so rare, so obscure, so inert, so unrelated to human experience, and so strange and complex as to entail exactly the same observations already entailed by naturalism, that there is no reasonable argument to be made for believing it. In the absence of any reasonable argument to believe anything supernatural exists or explains anything, and in the presence of some reasonable arguments to believe the natural world exists and explains everything, metaphysical naturalism should be accepted until disproved.

Arguments against naturalism
There are certainly difficulties in formulating a sound and credible naturalist worldview, but these difficulties are no greater than those faced by all other worldviews, which encounter similar or entirely different difficulties of their own. In much the same way that theology consists largely of working out which theories of divinity are plausible and coherent (and which are not), so naturalist philosophy consists largely of working out which naturalist worldviews are plausible and coherent (and which are not). Consequently, attacking inept constructions of naturalism or caricatures of naturalism is akin to attacking inept theologies or caricatures of theology. Just as critics of the existence of God need to address the most carefully constructed and best defended theologies, critics of naturalism need to address the most carefully constructed and best defended naturalist worldviews.

That said, metaphysical naturalism has no lack of critics. It has been loathed by countless defenders of supernatural worldviews for thousands of years and has been subject to countless attacks. Most of these attacks consist of obviously fallacious or ineffective criticism or empty rhetoric. But some arguments present significant though not insurmountable challenges to naturalist philosophy. Only those arguments will be briefly surveyed here.

Argument from despair
The most commonly voiced argument against naturalism is that it leads to human despair. There are many forms of this argument. Some emphasize the fact that naturalism entails there is no cosmic meaning of life, others propose that a religion promising eternal salvation is a safer bet (as in Pascal's Wager), while others claim naturalism entails the elimination of free will, which allegedly entails there is no knowledge, hope or moral responsibility. Though there are many fallacies involved in these arguments, principal among them is that they only argue for what someone wants to be true, not what actually is true. Naturalists further respond that humans define their own meaning in life, that life contains too much potential happiness to warrant despair, and that naturalism does not eliminate free will in any sense that eliminates knowledge, hope, or moral responsibility.

Argument from religious experience
Many people claim to have seen, felt, or talked to God or any number of other spirits, and claim these experiences refute naturalism. Naturalists respond that religious experiences routinely (and often radically) contradict each other, and therefore most must be false, and if most are false, then all could be. Naturalists also observe that these experiences never reveal more than the cultural and psychological knowledge and assumptions of the experiencer, suggesting they originate entirely in the experiencer's own mind. At the same time, these experiences have scientifically known causes in human biology and psychology. Naturalists argue that for all these reasons, naturalism actually explains the content, diversity and limitations of religious experience better than supernaturalism does.

Argument from miracles
Often some miracle is offered as evidence refuting naturalism, including alleged cases of supernatural healing, fulfilled prophetic or psychic predictions, or the supposed impossibility of composing some book (like the Bible or the Koran) without divine aid. Naturalists respond that none of these claims have ever survived empirical inquiry. Most are inaccessible to such inquiry and thus amount to mere assertions rather than actual evidence against naturalism. But, naturalists argue, in every case where these claims became accessible to empirical inquiry they failed to be confirmed or were even refuted. In fact, naturalists argue, this very failure to confirm any miracle claims, despite centuries of trying, is better explained by naturalism than supernaturalism.

Argument from the necessity of God
One of the more esoteric arguments against naturalism is to claim that it is in some sense impossible for the universe to exist unless it is caused or cohabited by a supernatural person. There are several forms of this argument, some requiring a demonstration of the premise that the universe began to exist (like the Kalam cosmological argument), which, naturalists argue, cannot be demonstrated, since contemporary Big Bang theory no longer entails time began (leaving no empirical evidence to support the premise), and arguments from the alleged impossibility of infinite series fail to include a formal logical proof of that impossibility. Naturalists respond to this and other forms (like the ontological argument) that no sound argument has yet been presented that follows from demonstrably true premises to the conclusion that a supernatural person must exist.

Argument to cosmological design
Also known as the Fine Tuning argument, this is the claim that the fundamental constants of physics appear so finely-tuned to permit life that only a supernatural engineer can explain it. Naturalists respond that even apparent fine-tuning has yet to be scientifically demonstrated, and apparent fine-tuning does not entail actual fine-tuning and only actual fine-tuning argues against naturalism. Since the known and suspected interrelationships of the physical constants entails that altering one necessarily alters others, and no one has correctly worked out the consequences of any change, much less shown that it would result in a uniformly lethal universe, no one has yet demonstrated even the appearance of fine tuning. For example, altering the speed of light will likely alter the masses of all the fundamental particles, but we do not yet know to what degree such a change would have this effect, so we do not know what the actual consequences of changing the speed of light would be. It has also not been shown that a collapsing universe will not produce a new universe, yet many proposed alterations of the physical constants supposedly entail a collapsed universe. For this and many other reasons, naturalists note that multiverse theory is more scientifically plausible now than ever before, and yet consistently produces apparent fine tuning without any intelligent design. Naturalists further argue that a supernatural engineer would not be constrained to building a universe so large, so old, so needlessly complex, and so vastly lethal to life as this one, yet as far as we know this sort of universe is the only kind that we could ever find ourselves in if naturalism were true. Therefore, it is argued, naturalism actually explains the particular complexity of our cosmos better than supernaturalism does.

Argument from improbability of life
Many claim the origin of life was too improbable to have occurred without supernatural intervention and therefore naturalism fails to explain the appearance of life. Naturalists respond that this remains a mere assertion, since all attempts to demonstrate this improbability have relied on false data, fallacious methodology, or unconfirmed premises. In contrast, recent developments in the science of biogenesis suggest life could have originated in a simple enough form that its accidental formation would be well within the realm of probability (given the known size and age of the universe). In fact, naturalists argue, the extreme rarity of life throughout the cosmos, and the known fact that the first life was vastly simpler than modern life, and took at least ten billion years even to appear, is better explained by naturalism than by any theory of supernatural intervention.

Argument to biological design
Recently popular is the claim that certain structures in evolved organisms are too complex to have evolved by natural selection and can only be explained as the result of intelligent design. Naturalists respond that this claim remains a mere assertion, since no one has ever scientifically demonstrated that such structures actually exist. For example, no one has performed the necessary gene sequencing and gene splicing experiments to confirm the hypothesis that removing or altering any piece of a particular structure will eliminate all function and thus prevent the structure from arising by natural selection. Hence confirmed examples of irreducible complexity have yet to be demonstrated, and only the demonstration of such a structure's existence would challenge naturalism.

Argument from consciousness
Since no one has yet explained the qualitative nature of conscious experience, otherwise known as qualia, some argue that naturalism is therefore refuted or should not be believed. Naturalists respond that all the arguments for naturalism (especially the argument from precedent and the argument to the best explanation) entail that whatever the explanation of qualia actually is, it is far more likely to be a natural explanation than a supernatural one, and therefore naturalism is still the most credible worldview. Naturalists also point out that no supernatural theory of qualia has been produced or verified, either, and therefore supernaturalism has also failed to explain this phenomenon. And in such a case, our best bet is to follow past precedent, which in cognitive science has been a consistent and remarkable trend of confirming physicalism in almost every other aspect of the study of mind. Science has already confirmed that qualia-production has identifiable locations in the brain and requires the expenditure of energy (oxygen and nutrients), two indications that qualia have a physical cause. Since there is no scientific evidence supporting the hypothesis that qualia have a supernatural cause, their existence does not argue against naturalism.

Argument from reason
Some have argued that certain features of human reason cannot be explained by naturalism. For example, it is claimed that naturalism cannot explain intentionality, mental causation, or the existence of logical laws or abstract objects. Naturalists respond that a naive formulation of naturalism might have this failing, but robust formulations actually explain the existence of these things better than supernaturalism does. This is a complex subject that is most thoroughly discussed in a Critical Review of Victor Reppert's Defense of the Argument from Reason by Richard Carrier (2004).

Argument from physical law
Some claim naturalism cannot explain the existence of physical laws. This argument takes many forms, but the two most common are the claim that the mathematical nature of physical laws entails a supernatural mind behind them, and the claim that naturalism can provide no ontological foundation for physical laws, requiring some supernatural power or being to realize and maintain them. Naturalists respond, again, that a naive formulation of naturalism might have this failing, but robust formulations actually explain the existence of these things better than supernaturalism does. To date, no one has proven that any mathematical laws don't simply describe a physical fact of the geometry of matter-energy in space-time, and such a correlation remains the most scientifically plausible foundation for those laws, so neither the existence of physical laws nor the applicability of mathematics in describing them presents a challenge to naturalism. On the general issues involved, see Fundamental Flaws in Mark Steiner's Challenge to Naturalism by Richard Carrier (2003), and for the best recent articulation of how the laws of physics simply are a necessary consequence of the physical arrangement of matter-energy in space-time, see Victor Stenger's The Comprehensible Cosmos: Where Do the Laws of Physics Come From? (2006; ISBN 1591024242).

Argument from incoherence
Sometimes it is claimed that naturalism entails self-contradictory commitments. Naturalists respond that a naive formulation of naturalism might have this failing, but robust formulations do not. For example, see Defending Naturalism as a Worldview: A Rebuttal to Michael Rea's World Without Design by Richard Carrier (2003).

Moral argument
There are two kinds of moral argument: the claim that naturalism eliminates morality and the claim that moral facts exist that naturalism cannot explain. The first claim, that there can be no moral truth if naturalism is true, is a variety of the argument from despair already noted above, and naturalists respond in the same way here as there. In addition, naturalists argue we can derive moral propositions from actual facts about human needs and desires and the social and physical environment we inhabit. As to the second claim, naturalists respond that no one has ever demonstrated the actual existence of moral facts naturalism can't explain.

Evolutionary argument against naturalism
Finally, the most recent argument to be proposed is that of Alvin Plantinga, who claims that naturalism assumes evolution by natural selection, but evolution by natural selection may be more likely to generate unreliable faculties than reliable ones, yet unreliable faculties will lead us to false beliefs more frequently than not, and therefore if naturalism is true we have no reason to trust our belief that naturalism is true. Naturalists respond that none of the premises in this argument have any basis in fact. First, evolution by natural selection clearly tends to improve cognitive faculties over time, not the other way around. There is no scientific evidence that warrants believing it will generate hopelessly unreliable faculties. Second, unreliable faculties do not entail unreliable beliefs, because learned behavior can correct for and thus eliminate errors produced by such faculties, and it is clear humans have in fact discovered and developed exactly such behavioral tools, which, like any other technology, greatly enhance our ability to discover the facts of the world we inhabit. For more discussion of these and other problems, see Rea on Plantinga, as well as Argument from the Reliability of Our Rational Faculties and We Should Attack Rocks?.

Pro

 * Mario Bunge, 2006, Chasing Reality: Strife over Realism, University of Toronto Press. ISBN 0802090753 and 2001, Scientific Realism: Selected Essays of Mario Bunge, Prometheus Books. ISBN 1573928925


 * Richard Carrier, 2005, Sense and Goodness without God: A Defense of Metaphysical Naturalism, AuthorHouse. ISBN 1420802933


 * Daniel Dennett, 2003, Freedom Evolves, Penguin. ISBN 0142003840 and 2006, Breaking the Spell: Religion as a Natural Phenomenon, Viking. ISBN 067003472X


 * Sam Harris, 2005, The End of Faith: Religion, Terror, and the Future of Reason, W. W. Norton. ISBN 0393327655


 * Andrew Melnyk, 2003, A Physicalist Manifesto: Thoroughly Modern Materialism, Cambridge University Press. ISBN 0521827116


 * David Mills, 2004, Atheist Universe: Why God Didn't Have A Thing To Do With It, Xlibris. ISBN 1413434819


 * Jeffrey Poland, 1994, Physicalism: The Philosophical Foundations, Oxford University Press. ISBN 0198249802

Con

 * James Beilby, ed., 2002, Naturalism Defeated? Essays on Plantinga's Evolutionary Argument Against Naturalism, Cornell University Press. ISBN 0801487633


 * Phillip E. Johnson, 1998, Reason in the Balance: The Case Against Naturalism in Science, Law & Education, InterVarsity Press. ISBN 0830819290 and 2002, The Wedge of Truth: Splitting the Foundations of Naturalism, InterVarsity Press. ISBN 0830823956


 * William Lane Craig and J.P. Moreland, eds., 2000, Naturalism: A Critical Analysis, Routledge. ISBN 0415235243


 * Michael Rea, 2004, World without Design: The Ontological Consequences of Naturalism, Oxford University Press. ISBN 0199247617


 * Victor Reppert, 2003, C.S. Lewis's Dangerous Idea: In Defense of the Argument from Reason, InterVarsity Press. ISBN 0830827323


 * Mark Steiner, 2002, The Applicability of Mathematics as a Philosophical Problem, Harvard University Press. ISBN 0674009703