Varieties of Naturalism

Naturalism is a worldview that relies upon experience, reason, and science to develop an understanding of reality and humanity's place within reality. Naturalism is hence a worldview that is heavily dependent on science for knowledge about reality. One's attitude towards science and "Scientific Method" will therefore control how one thinks about naturalism.

Nowadays two versions of naturalism are frequently contrasted: methodological naturalism and ontological naturalism. Definitions of these two versions usually sound like this. Methodological naturalism holds that the only sort of knowledge available to us is the knowledge learned by applying the scientific method to all hypotheses. On this view, the scientific method itself suffices to weed out non-natural hypotheses as unscientific, by requiring hypotheses to be consistent with suitably naturalistic principles (no mysterious causes, no miracles, no violation of causal closure or conservation of energy, etc.). Such requirements, while guaranteeing the screening out on the non-natural, presupposes some basic understanding of what the natural consists of. Where could that understanding come from? Only from some already established knowledge about nature. This leads the naturalist to Ontological Naturalism, which holds that reality only consists of those things recognized by an exemplary science, such as physics, which has been satisfactorily tested by scientific method. Have you noticed yet that methodological and ontological naturalism depend on each other for substantive content? They cannot be defined independently of each other, since methodology by itself will consider any hypothesis for testing without prejudice, and ontology by itself is sheer dogmatism without a standard of knowledge. The putative contrast between methodological naturalism and ontological naturalism is spurious -- they work together or not at all. They can have a virtuous relationship by mutually enlarging each other as science progresses -- new scientific knowledge often arrives with improved methodology, which in turn extends the reach of scientific inquiry, which leads to more new knowledge, etc. You can read more about methodological and ontological naturalism in the article "Naturalism".

The label of Metaphysical Naturalism is sometimes applies to the broadest definition of naturalism (see for example wikipedia's "Metaphysical Naturalism"). Furthermore, the labels of Physicalism and Materialism are sometimes used by philosophers as synonyms for naturalism (you can read about "Physicalism" here). However, only some varieties of naturalism rely only on physics or the notion of matter, and naturalisms frequently have contentious relationships with metaphysics, so it is best to let Philosophical Naturalism stand for the broadest statement of naturalism.

Accordingly we shall persist with the definition of philosophical naturalism given at Naturalisms.org, which combines a methodological and ontological component: "Naturalism is a worldview that relies upon experience, reason, and science to develop an understanding of reality and humanity's place within reality."

Now that the genus of naturalism has been defined, what are the dominant species? We will proceed through five stages of discriminating significant kinds of naturalism. The conclusion of these stages identifies the seven viable varieties of naturalism. Which of these viable varieties enjoy superior status as the two major competitors in the race for recognition as the "genuine" naturalism? Read the concluding six stage!

Stage One: Science, Knowledge, and Reality

There are six primary options when considering whether science yields knowledge about reality:

1. Reality cannot be known at all.
2. Reality only consists of what science cannot know about.
3. Science is rarely able to give reliable knowledge about reality.
4. Science is able to give increasingly reliable knowledge about reality.
5. Science is the only source of knowledge about reality.
6. Reality only consists of what science knows about.

Each of these six options present pathways to many different worldviews. Options 4, 5, and 6 lead to varieties of naturalism. To become a worldview, each option must provide a justification for preferring it over the rest. Examples of such justifications are added below:

1. Reality cannot be known at all. All knowledge is impossible because of fatal flaws within any ways of attempted knowing. This option is usually called "Radical" or "Philosophical" Skepticism [visit a website about Philosophical Skepticism]. This option is NOT the same as the ordinary skepticism of common sense, or the scientific skepticism that demands experimental evidence to have knowledge.
    
2. Reality only consists of what science cannot know about. There is another non-scientific way of understanding reality that should be trusted instead of science. Since science's conclusions do not agree with this non-scientific way of understanding reality, science is completely untrustworthy. Three interesting varieties:
        2A. We know reality better through experience: we experience reality in some way quite different from, and better than, science. Prominent examples of this option are Religious Mysticism and F.H. Bradley's Absolute Idealism.
        2B. We know reality better through reason alone: pure reason gives us perfect knowledge of reality, not empirical science. Modern rationalists, including Spinoza, Leibniz and Kant, and Hegel have stressed reason over science, or have demanded that science be grounded on reason alone.
        2C. We know reality better through some authoritative source that commands our minds, such as scripture or a religious leader. This Fanaticism option abandons both reason and science, and probably sanity itself.
    
3. Science is rarely able to give reliable knowledge about reality. Science can occasionally provide reliable knowledge, but only about a limited portion or aspect of reality. Two interesting varieties:
        3A. There is another non-scientific way of knowing reality, such as experience or pure reason, that should be trusted more than science. Most types of Platonism, Idealism, Phenomenalism, and Phenomenology fall in this option. This option can also lead to Dualism: there are two kinds or aspects of reality, knowable through two different ways. Perhaps this non-scientific way of knowing reality is religious, so you can jump ahead to "Naturalism and Religion".
        3B. Scientific Anti-Realism: there is no alternative way of knowing reality besides science, but science itself provides little knowledge because of its own limitations. The scientific anti-realist has decided that none of arguments for Scientific Realism (the view that science does provide some genuine knowledge of reality) are convincing. In the 20th century, scientific anti-realists have preferred types of Empiricism (like Positivism's view that science can only describe patterns of phenomena), or Social Constructivism (the view that science's claims are largely caused by cultural/political forces). Another type of empiricism is Instrumentalism, which holds that science can only give knowledge about directly or instrumentally observable entities. These empiricisms can't develop into viable naturalisms, and instead collapse into 3A options such as idealism or phenomenalism.
    
4. Science is able to give increasingly reliable knowledge about reality. There may be other ways besides science for knowing reality, but those ways are not better than science. Science needs assistance from other ways of knowledge to fully understand reality. This option searches for a comprehensive worldview formed by blending together ways of knowledge. Two interesting varieties:
        4A. Ontological Dualism: there are two (or more) kinds of reality, knowable through two or more ways. For example, perhaps introspection is a non-scientific way of knowing reality, because we are consciously aware of mental realities that science can never explain -- leading to Mind-Body Dualism or Epiphenomenalism.
        4B. Synoptic Monism: there is only one kind of ultimate reality, but it is knowable through two or more ways. We consciously know of realities (perhaps mental in nature) that science cannot fully explain. Varieties include Neutral Monism and Dual Aspect Monism, and Pansychism which holds that the natural world which is explored by science is ultimately composed of entities that have a mental/spiritual aspect. Unlike option 4A, synoptic monism can be used to develop kinds of naturalisms.
    
5. Science is the only source of knowledge about reality. The only type of knowledge is scientific knowledge. However, some of reality consists of entities that cannot be known by science, simply because science is not designed to provide knowledge about these entities. Two interesting varieties:
        5A. Perspectival Realism: we are acquainted with the entities unknowable through science, because we experience these entities in some other way. For example, much of experience which provides the data for science is not itself also known by science. Specific types include Emergent Naturalism (mental entities emerge from, but are not reducible to, physical entities) and Pragmatic Naturalism, which both offer attempts to coordinate experience with science. However, unless the perspectives of ordinary experience on reality is carefully reconciled with scientific knowledge, excessive concern for ordinary experience can lead towards option 4.
        5B. Transcendent Realism: there must be entities unknowable by science, since science's own limitations suggest that some of reality is beyond scientific knowledge. This option can result from arguing that reality must in some sense be independent of knowledge, and hence not completely capturable by knowledge. You can read about the "Knowability Paradox" here.
    
6. Reality only consists of what science knows about. Only what can be known by science really exists. Two interesting varieties:
        6A. Current Scientific Exclusivism: reality only consists of what current science knows now. This option is not widely favored because science frequently revises its understanding of reality. However, on this option there is no other rational way to understand reality, so current science's worldview is the only reasonable choice. W.V. Quine is often understood as advancing this "Fallibilist Scientism" option.
        6B.  Scientific Exclusivism: reality only consists of what perfected science would know. This thesis is sometimes called Eliminative Materialism or Physicalism.  This option cannot yet depict reality accurately, since we can't know which parts of science have been perfected already, and hence this option cannot be useful for developing a concrete worldview. In practice, perfectionists usually select one or another current science (like physics) for its view of reality (like materialism), and so perfectionists quietly move over to option 6A.

Options 1, 2 and 3 will be discussed elsewhere, in "Naturalism and the Mind" and "Naturalism and Reason" [under construction] at this website. Options 4, 5, and 6 can lead towards varieties of naturalism, but unless option 3B Scientific Anti-Realism is ruled out, naturalism remains hypothetical. You can proceed to "Scientific Method" and then to "Scientific Realism and Truth" here at Naturalisms.org, and you can also go to another website on Scientific Realism.

Only three of the 6 primary worldviews can lead to kinds of naturalism. They are:

4. Science is able to give increasingly reliable knowledge about reality. There may be other ways besides science for knowing reality, but those ways are not better than science. One variety can be a kind of naturalism: 4B. Synoptic Monism (one kind of ultimate reality that is knowable in different ways).

5. Science is the only source of knowledge about reality. The only type of knowledge is scientific knowledge. However, some of reality consists of entities that cannot be known by science, simply because science is not designed to provide knowledge about these entities. Two interesting varieties: 5A. Perspectival Realism (experience is a perspective on reality but not itself knowable), and 5B. Transcendent Realism (some natural reality forever escapes science).

6. Reality only consists of what science knows about. Only what can be known by science really exists. Two interesting varieties: 6A. Current Scientific Exclusivism (reality only consists of what current science knows now), and 6B. Scientific Exclusivism (reality only consists of what perfected science would know).

Each of these three major worldviews about science leads to three significant kinds of naturalism: Synoptic Monism, Perspectival Realism, and Scientific Exclusivism. Transcendent realism may be ignored here because any transcendent natural reality, if it exists, cannot be the subject of the contest between naturalism and non-naturalism. Current scientific exclusivism may also be ignored here because the scientific exclusivist, when challenged over something that current science can't yet explain, will resort to the claim that future science will explain it.

Stage One: Three Kinds of Naturalism

Stage Two: Explaining Experiences using Science's Theories

Let us further consider ways of distinguishing kinds of naturalism. Consider these three kinds of naturalisms.

Narrow Naturalism: If some X is among those things (or among the properties of those things) which are described by science's best theories, then the existence of X is accepted; otherwise, its existence must be denied.

Fitting Naturalism: All the entities accepted by narrow naturalism exist, plus additional things as follows: If some X is successfully hypothesized as really being a Y that is among those things (or among the properties of those things) which are described by science's best theories, then the existence of X may be accepted.

Broad Naturalism: All the entities accepted by fitting naturalism exist, plus additional things as follows: If some X is successfully explained by a hypothesis about why X exists which references only those things (or properties of those things) which are described by science's best theories, then the existence of X may be accepted.

It must be made clear that in these definitions, we mean by "those things" any entities which are used by scientific theories -- from "things" in the substantive sense of material bodies, and also energies, forces, laws, and the like. Naturalists can disagree among themselves about the types of entities that are attributed existence by scientific theories (for example, do laws of nature really exist in the same way that atoms exist, or do numbers exist because science uses measured quantities, etc.). These disagreements are not relevant to the categorization of naturalisms discussed here.

Let's apply the distinction between these three naturalisms to the experience of colors. The narrow naturalist is tempted to reject the existence of colors, because no color is found among the basic properties of those things described by science's best theories. If the narrow naturalist is narrow in an additional sense because she considers physics as supplying the best scientific theories, then this naturalist will reject the existence of colors because color is not a property of any atomic or subatomic particle, or of any force of nature. The fitting naturalist can accept the existence of colors if an adequate hypothesis find a fitting place in nature for colors. For example, a successful hypothesis that a color really is the "surface spectral reflectance" from an object would permit the fitting naturalist to accept that colors naturally exist. The broad naturalist only asks that some adequate hypothesis, which uses terms from science's best theories, explains why colors are experienced. For example, a successful hypothesis that explains how colors are seen whenever sufficient visible light in certain wavelengths stimulates the optic systems of normally functioning human brains would permit the broad naturalist to accept that colors naturally exist. In summary, the narrow naturalist wants to eliminate colors from nature by comparing them against science's theories, the fitting naturalist wants to fit colors into the natural world by reducing them to scientific things, and the broad naturalist wants liberal flexibility to accept colors by explaining how colors are naturally experienced.

So far we have distinguished six kinds of naturalism, along two dimensions: (1) the degree of ontological confidence given to science, from Synoptic monism to perspectival realism to scientific exclusivism; and (2) the breadth of explanatory discretion given to science, from narrow to fitting to broad naturalism.

Stage Two: Three More Kinds of Naturalism

Stage Three: How Many Sciences Describe Reality?

There is one more dimension that further distinguishes kinds of naturalisms: (3) the number of scientific fields permitted to describe reality. Some naturalists are happy with letting many sciences know reality, while other naturalists want only one scientific field to know reality. The latter type of naturalists have typically accepted a methodological principle that may be called "reductionist universalism" -- only the smallest parts of reality really exist, and the natural laws about those parts are universally valid (they hold in all regions of the universe), exclusively valid (no other laws have independent validity), and exhaustively valid (all events are dictated by these laws). As physics is the scientific field that knows the smallest parts of reality, reductionist universalism amounts to the claim that all of reality ultimately consists solely of subatomic particles and that all events in the natural universe are ultimately dictated by the laws those subatomic particles obey. The program of reductionism amounts to the claim that any complex thing should be analyzed down into its components, and each of these components must in turn be analyzed further, etc., until the ultimate smallest parts (those studies by subatomic physics) are identified and their interact laws are discovered. Theoretical reductions to physics are therefore a series of analyses, from higher complex levels of nature to lower simple levels of nature, until the lowest (smallest) level is reached, and this series of analyses explains how the behavior of any complex thing is fully explained by the interactive behaviors of the smallest parts. If, according to reductionist universalism, subatomic particles have ontological priority and their laws have explanatory priority, then physics has naturalistic priority. The sciences could be unified in this way into physics alone. You can read more about the history of "Atomism",  "Reduction", and the "Unity of Science" movement. The naturalist who follows reductionist universalism will be the sort of materialist who puts physics first -- this naturalism is "physicalism."

Other kinds of naturalism do not agree with reductionist universalism and feel comfortable with permitting other scientific fields to describe reality with just as much legitimacy as physics. Because the biological and social sciences have traditionally used some methodological principles and modes of causality that depart from the physical sciences, many naturalists want to draw a line between trustworthy physical sciences (physics, chemistry, geosciences, astronomy, cosmology) and suspicious biological and social sciences. For example, some approaches to the social sciences have assumed the existence of social entities (that must not be treated as mere aggregates of people), and some biological and social sciences have use teleological causality (explanations that appeal to future outcomes to explain present behaviors). We will not discuss this internal dispute among naturalists here. However, the naturalists who would permit just the physical sciences to describe reality (let us call their view "scientism") do form a separate camp from those naturalists who are comfortable with all of the physical, biological, and social sciences describing reality (let us call their view "pluralism").

Stage Three: Three More Kinds of Naturalism

Stage Four: How Many Naturalisms?

The varieties of naturalism may be distinguished along three dimensions: (1) the degree of ontological confidence given to science, from Synoptic monism to perspectival realism to scientific exclusivism; (2) the breadth of explanatory discretion given to science, from narrow to fitting to broad naturalism; and (3) the number of scientific fields permitted to describe reality, from just physics to the physical sciences to all sciences.

If all combinations of these nine kinds of naturalism were created, then 27 varieties of naturalism would result. However, many of these 27 varieties are not viable because of coherence problems, and some are not practical because their principles would conflict. Let us consider the number of viable naturalisms by first considering the result of combining the breadth of explanatory discretion given to science with the number of scientific fields permitted to describe reality. The nine kinds of naturalism that result try to express nine different ways of formulating the explanatory function of science.

TABLE ONE: NINE KINDS OF NATURALISM

Let us next consider combining these nine kinds of naturalism, arrayed according to their formulation of the explanatory function of science, with the three kinds of naturalism that express the degree of ontological confidence given to science, from synoptic monism to perspectival realism to eliminative materialism. These combinations generate twenty-seven potential varieties of naturalism. Not all are viable, however.

TABLE TWO: TWENTY-SEVEN VARIETIES OF NATURALISM?

After eliminating 16 varieties of naturalism because they are either incoherent or impractical, 11 remain for consideration. We may first consider the 4 "poor fits" to understand why these varieties are such unstable combinations that they must mutate into a nearby variety.

Eliminative Scientism: can reality be only what the physical sciences say? Eliminative Scientism demands that reality only consist of those things and their properties which are described by the physical science's best theories. The various physical sciences provide theories appropriate to their "level" of reality that concerns them. Subatomic physics, atomic physics, chemistry, geosciences, astronomy, and cosmology deal with levels of nature ranging from the smallest to the grandest scales. If permitted complete theoretical liberty, these sciences would describe nature in diverse ways that are quite different from each other. For example, chemistry would assert that molecules exist, while physics would prefer to speak only of atoms (and subatomic physics renounces both molecules and atoms for the menagerie of subatomic particles). Eliminativism puts severe pressure on a worldview to finally decide what really exists -- so the awkwardness of disputes between the physical sciences over the ultimate components of reality must be resolved. During the 20th century, the needed resolution was the adoption of the principle of "reductionist universalism" -- only the smallest parts of reality really exist, and the natural laws about those parts are universally and exclusively valid. As subatomic physics is the scientific field that knows the smallest parts of reality, reductionist universalism amounts to the claim that all of reality ultimately consists solely of subatomic particles and that all events in the natural universe are ultimately dictated by the laws which those subatomic particles obey. Chemistry's molecules are just assemblies of physics' atoms, which in turn are just assemblies of subatomic particles. Furthermore, all laws of chemistry are in principle reducible to just necessary implications of the laws of subatomic and atomic physics. The awkwardness of Eliminative Scientism leads to reductionist universalism, which in turn leads to Eliminative Physicalism.

Reductive Scientism: is reality reducible to the physical sciences? Reductive Scientism demands that reality only consist of those things which are reducible to those things and their properties which are described by the physical science's best theories. Reductionism puts severe pressure on a worldview to finally decide what really exists -- so the awkwardness of disputes between the physical sciences over the ultimate components of reality must be resolved. Just as eliminative scientism had to adopt reductionist universalism, so too must Reductionist Scientism, so that it collapses into Reductive Physicalism.

Synoptic Scientism: why demand explanations when reality has multiple modes? Synoptic Scientism suggests that reality possesses multiple aspects that are not reducible to each other, but are explainable by the sciences. However, the purpose of adopting synopticism is to avoid having to explain everything in terms of just those things described by science's best theories. If it is possible to explain all realities in terms of the knowledge of just the physical sciences, then there is no need to postulate multiple modes of reality. If a philosopher tries to develop Synoptic Scientism, either the desire for synopticism will turn the philosopher towards Synoptic Pluralism, or the optimism about science will turn the philosopher towards Perspectival Pluralism.

Perspectival Scientism: can the physical sciences fully explain diverse experience? Perspectival Scientism suggests that reality has perspectival aspects which are experienced but not themselves directly knowable by the sciences. Could such experiences nevertheless be fully explained by just the physical sciences? For example, could all experiences be accounted for by brain neurophysiology? Perhaps, but appreciation for  perspectivism typically arises from skepticism about the presumptuous notion that all experiences can be explained by just the physical sciences. If a philosopher tries to develop Perspectival Scientism, either the desire for perspectivism will turn the philosopher towards Perspectival Pluralism, or the optimism about science will turn the philosopher towards Exclusivist Liberal Pluralism.

Stage Five: The Seven Viable Varieties of Naturalism

These seven viable varieties of naturalism are distinguished from each other according to their views on (1) the degree of ontological confidence given to science, from synoptic monism to perspectival realism to scientific exclusivism; (2) the breadth of explanatory discretion given to science, from narrow to fitting to broad naturalism; and (3) the number of scientific fields permitted to describe reality, from just physics to the physical sciences to all sciences. The seven varieties are listed in order from the very restrictive to the very open assertions about what reality is like.

Eliminative Physicalism: reality only is what physics says. This variety is the most austere and rigid naturalism, restricting reality most sharply. According to Eliminative Physicalism, the only realities are those that number among those things (or among their properties) which are described by physics' best theories. This eliminativism typically accepts the principle of "reductionist universalism" -- only the smallest parts of reality really exist, and the natural laws about those parts are universally, exclusively, and exhaustively valid -- and so this variety of naturalism claims that all of reality ultimately consists solely of subatomic particles (and their properties, and their laws, etc.), and that all events in the natural universe are ultimately dictated by the laws which those subatomic particles obey. According this worldview, if it rejects the existence of some X, then any belief or judgment or knowledge claim about X is probably meaningless and strictly false. Sometimes eliminative physicalists relent from this harsh treatment, saying that discourse about many condemned Xs can still be meaningful, and "second-class" practical knowledge about these Xs may be possible. But the Eliminative Physicalist who is too generous with "second-class" knowledge risks sliding over to Exclusivist Liberal Pluralism, and is under great pressure to at least admit the superiority of Reductive Physicalism.

Reductive Physicalism: reality must be reducible to physics. This variety is almost as austere as eliminative physicalism. According to Reductive Physicalism, the only realities are those of physics' best theories, plus those additional things which can be theoretically and ontologically reduced to them. Reductive Physicalism accepts reductionist universalism, but resists collapsing into eliminative physicalism by permitting the existence of things that can have their own properties, behaviors, and laws that physics itself does not investigate. The practical difference between a reductivist and an eliminativist is that an eliminativist about some X would not seek any reductive explanation of X, since it is irrational to attempt to explain the non-existent. However, most eliminativists gain their confidence in the non-existence of X after reductivists have done their work, and most physicalists are eliminativists about some things (the paranormal, the mythical) and reductivists about other things (the biological, the social). For the reductivist, chemistry's knowledge of molecules can be legitimate, so long as chemistry accepts that its entities and laws must be in principle reducible to physics' entities and laws, even if it may be difficult or seem impossible for scientists to figure out all those reductions. A harder problem for reductivism is presented by a science like geology or biology -- the reductive physicalist likewise demands that these sciences' entities and laws be reducible in principle to those of physics. How could the laws of nature concerning plate tectonics or genetic reproduction ever be reduced to those of subatomic physics? The social scientist may also be forgiven for rebelling against this imperial demand of physics. Reductive Physicalism must demand complete submission to the principle of reductionist universalism from all other sciences, so else it must admit the superiority of Exclusivist Liberal Pluralism.

Exclusivist Liberal Physicalism: physics alone supplies explanations of all reality. This variety is attractive to naturalists who are skeptical about reducing all realities to physical realities. Exclusivist Liberal Physicalism holds that reality consists of what can be explained by physics. This variety of naturalism does not adhere to the principle of reductionist universalism, keeping it distinct from its eliminativist and reductivist cousins. Exclusivist Liberal Physicalism does maintain an analogue of reductionist universalism, which can be called "explanatory universalism," which instead declares that only the things and laws theorized by physics may be referenced when explaining reality, so that explanations of things are given solely in terms of the things recognized by physics alone. The primary difficulty that confronts Exclusivist Liberal Physicalism is causality: physically explainable but irreducible things can appear to have their own causal relationships, and so one event might be described as having two sufficient causes. Unless this causal overdetermination issue is somehow resolved, Exclusivist Liberal Physicalism is under great pressure to either collapse into Reductive Physicalism, or to go in the opposite direction and mutate towards Perspectival Pluralism.

Exclusivist Liberal Scientism: the physical sciences supply explanations of all reality. Like any middle position that tries to compromise all things, this variety is highly unstable, suffering from great pressures to resign the field in favor of its exclusivist cousins. According to Exclusivist Liberal Scientism, reality consists only of those things that are explainable by the physical sciences. But why the physical sciences? The Exclusivist Liberal Physicalist will complain that purely physical explanations must in principle prevail across all the physical sciences anyways. The Exclusivist Liberal Pluralist will complain that admitting the explanatory power of the physical sciences should be extended to all of the sciences. Even worse than competition from its cousins, this variety suffers from both of the severe difficulties confronting its cousins. Like Exclusivist Liberal Physicalism, this variety must resolve the issue of causal overdetermination, lest it admit the superiority of Reductive Physicalism. Like its other cousin, Exclusivist Liberal Pluralism, this variety must also resolve the issue of incoherence between the physical sciences, which can be handled more easily by Reductive Physicalism on the one hand, or by Perspectival Pluralism on the other.

Exclusivist Liberal Pluralism: the many sciences supply explanations of all reality. This variety is attractive to naturalists who are skeptical about reducing all realities to physical realities on the one hand, and also skeptical about any naturalistic metaphysics that permits experience to yield genuine perspectives on reality that can never be fully explained by the sciences. Exclusivist Liberal Pluralism holds that reality consists of what can be explained by the sciences. Its pluralism encourages all of the sciences to draw their own conclusions about reality. But this liberality also encourages such a diversity of conclusions about reality, and such a multiplicity of entities for theorizing, that incoherence among them will inevitably result. The only way to manage this diversity is to assign each science its own task of exploring a "level" or "aspect" of reality, so that clashing scientific theories are kept apart. For example, chemistry studies the laws peculiar to interacting molecules, while subatomic physics studies the quite different laws peculiar to subatomic particles, without worrying how these entities and laws specifically relate to each other. This tactic will pull pluralism towards perspectivism -- the view that each science's knowledge is only one valid but limited perspective on reality, and no science has a monopoly or even a priority on what really exists. Explaining this coordinated and peaceful perspectivism will require great philosophical effort that surely looks like naturalistic metaphysics, since justifying such immense coordination probably can't be the work of scientific theorizing, but instead the work of higher-order principles that are not experimentally testable. How can Exclusivist Liberal Pluralism forbid experience from having any of its own independent perspectives on reality (especially since ordinary experience is not so very different from simple scientific observation)? Exclusivist Liberal Pluralism must be burdened with showing how all experiences are in principle explainable by the sciences. Any experiences not satisfactorily explained will pressure this naturalism to mutate into Perspectival Pluralism.

Perspectival Pluralism: the many sciences indicate plural perspectives on reality. This variety offers a middle path between Exclusivist Liberal Pluralism's reliance on science alone and Synoptic Pluralism's hypostatizations of multiple ways of experiencing and knowing reality. Perspectival Pluralism finds that experience and scientific knowledge presents multiple perspectives upon the same reality. Perspectival Pluralism requires some sort of naturalistic metaphysics -- a metaphysical account of reality that incorporates all experience and knowledge to construct a coherent understanding of one single natural reality having multiple but coordinated perspectives. This naturalist metaphysics is not testable by ordinary experimental methods (see "Scientific Method") because it is designed to be maximally compatible and coherent with all knowledge and experience from all sources. Since experience increases and knowledge evolves, naturalistic metaphysics must adapt to keep pace, and this adaptability serves as its test of adequacy. Perspectival Pluralism distinguishes itself from Exclusivist Liberal Pluralism by requiring that the perspectives of experience do not have to be explained by the sciences, but only by the naturalistic metaphysics. Explaining why experience cannot be fully captured by the sciences can be accomplished, for example, by emphasizing how scientific knowledge grows from experience, or by showing how experience emerges from the physical world (see "Emergent Properties") that science knows, or by showing how experience has perspectives on reality which science cannot duplicate, or by demonstrating that experience has elements not explainable by scientific knowledge. Coordinating experience with scientific knowledge in one coherent system is facilitated by Perspectival Pluralism's view that experience is not itself a kind of knowledge that competes with scientific knowledge. If any aspect of experience does deserve status as a kind of knowledge capable of competing with scientific knowledge, Synoptic Pluralism is preferable.

Synoptic Pluralism: the many sciences indicate plural modes of reality. This variety is the most open and flexible naturalism, defining reality most generously. According to Synoptic Pluralism, reality has a variety of aspects or modes as known by the many sciences, and also has aspects or modes known by experience that the sciences are incompetent to describe or explain. The simplest forms of Synoptic Pluralism include Dual Aspect Monism (the sciences deal only with reality's physical aspect while the mind only deals with reality's mental aspect), and Panpsychism (the sciences accurately but only partially describe all realities, because the sciences cannot capture the sentient or feeling aspect of these realities). Synoptic Pluralism requires some sort of naturalistic metaphysics -- a metaphysical account of reality that constructs a coherent understanding of one single natural reality having multiple aspects and known in multiple ways. Although handling such diversity is difficult, pluralism is still harder, since synopticism can more easily segregate incompatible aspects into distinct modes of reality that need not intersect. This naturalist metaphysics is not testable by ordinary experimental methods (see "Scientific Method") because it is designed to be maximally compatible and coherent with all knowledge and experience from all sources. Since experience increases and knowledge evolves, naturalistic metaphysics must adapt to keep pace, and this adaptability serves as its test of adequacy. Synoptic Pluralism can distinguish itself from Perspectival Pluralism only by concretizing and hypostatizing experienced and known entities for their classification into sharply distinct ontological categories, to forbid the merging of these entities into coordinated perspectives upon reality. On the other hand, unless Synoptic Pluralism can develop a compelling naturalistic metaphysics, its enthusiasm for multiple modes of reality can easily inspire ontological dualisms.

 

Stage Six: The Two Great Naturalisms

Each of the seven viable forms of naturalism suffer from unresolved problems requiring further intense philosophical work. Lacking resolutions to their problems so far, they are under great pressure to mutate into some other variety of naturalism. The next table diagrams the seven viable naturalisms, their most urgent issues, and the direction of pressure for mutation.

TABLE THREE: SEVEN VARIETIES OF NATURALISM AND THEIR ISSUES

As evident from the lines of pressure indicated in Table Three, two naturalisms seem to exert greater persuasive power. Because of the intense pressures of unresolved issues, most naturalisms gravitate around the two great naturalisms: Reductive Physicalism and Perspectival Pluralism. They stand opposed across a wide divide that separates the physicalists from the pluralists. The essential issue for naturalism consists of the fundamental disagreement that divides Reductive Physicalism apart from Perspectival Pluralism -- does any science's knowledge, and the reality it knows, have priority (epistemic and ontological) over all other knowledge and experience? If the answer is affirmative, then the monopoly of physics is assured and Reductive Physicalism is victorious. If the answer is negative, then the plurality of knowledge and experience is assured and Perspectival Pluralism is victorious.

The Two Great Naturalisms

How can we begin to decide this most fundamental issue over reductionism versus pluralism?

First, notice that this question, "Does any science's knowledge, and the entities it knows, have epistemic and ontological priority?" cannot be answered simply by the scientific methods of experimental testing. This question is not the statement of a scientific hypothesis, for which any experimental test can be designed. Only philosophical argument can settle this question. Likewise, this question, "Does all of the sciences' knowledge, and the entities they know, coherently fit into a coordinated set of perspectives upon a unique natural reality?" can only be answered by philosophical argument.

Second, neither the reductive physicalist nor the Perspectival pluralist should be tempted to give purely rationalistic deductive arguments in support of her variety of naturalism, since that tactic violates the spirit of physicalism (leaving him open to the charge that he appeals to a non-natural source or justification of knowledge). Furthermore, any deductive argument decisively favoring the reductive physicalist, or the Perspectival pluralist, will contain at least one premise that the rival variety must reject, so only question-begging can result. Deprived of deduction, these varieties of naturalism must resort to inductive and abductive arguments (see "Scientific Method" for explanations).

Third, let us begin with arguments for reductive physicalism. Here are two basic arguments, inductive and abductive, for reductive physicalism.

The Inductive Argument for Reductive Physicalism from Past Successful Reductions

1. There have been many successful theoretical reductions of non-physics theories and laws to the theories and law of physics.
2. These reductions have not been overturned by subsequent scientific progress, but are foundational for several scientific fields.
Conclusion: Theoretical reductions of non-physics theories and laws to the theories and law of physics should continue until all scientific fields are reducible to physics.

Both premises can be disputed. Premise 1 is not false, but it obscures the fact that the number of significant reductions, mostly among the physical sciences, is quite small when compared to the immense number of successful theoretical advances across all scientific fields that had nothing to with reductions. Proponents of this argument trot out the same old examples of significant reductions, yet they have proven quite persuasive during the 20th century, powering the "Unity of Science" movement. The rebellion from the biological and social sciences, where very few significant reductions play much of a role, came quickly as they realized that much of their success depended on departing from the atomistic and mechanistic principles of physics, and ignoring the demand for reduction. Hence premise 2 is true but again results from only a small sampling of science.

Because this inductive argument's conclusion is wildly optimistic in light of the weakness of the premises, this argument is a failure. Reductive Physicalism must ultimately reply with more successful and significant reductions to further its case and increase confidence that reductions are essential to science. Of course, Reductive Physicalism arouses much enthusiasm by holding up the diversionary complaint that Perspectival Pluralism results in an unacceptable amount of entity multiplication and causal overdetermination. This issue is considered next.

The Abductive Argument for Reductive Physicalism from Scientific Understanding

1. If the only realities are those known by, or reducible to, theories of physics, then entity multiplication and causal overdetermination can be prevented.  
2. Entity multiplication and causal overdetermination must be prevented.
Conclusion: The only realities are those known by, or reducible to, theories of physics. 

Premise 1 might be accepted with few reservations by most naturalists; its hypothetical character demands little real commitment. Entity multiplication is a problem because common sense suggests that a whole cannot count as a separate entity over and above the set of all that whole's parts. This problem can be resolved by elimination (the whole doesn't really exist), reduction (the whole exists only in the sense that it "really" is just the set of its parts), or pluralism (wholes and parts count as separate entities, so common sense should not be trusted here). Causal overdetermination is a problem because common sense suggests that if a complex thing A causes an effect E on another complex thing B, and if some or all of A's parts causes precisely that same effect E by effecting some or all of B's parts, then there really cannot be two independent causes of E, since one or the other entirely suffices for E. This problem can be resolved by elimination (neither A nor B really exist, so there is no causal relation between A and B in the first place), reduction (A and B exist only in the sense that their parts exist, so only the causal relationship between their respective parts really exists), or pluralism (wholes and parts count as separate entities, so there really are two causal relationships operating on separate levels -- common sense should not be trusted here). Because pluralism rejects elimination and reductionism, and therefore the notion that the character and capacities of the parts exhaust the characters and capacities of the whole, pluralism defends the genuinely distinct reality of wholes by favors relational properties over intrinsic properties (you can read about "Intrinsic vs. Extrinsic Properties") and holism over atomism (you can read about "Holism").

The hypothetical character of premise 1 also points to the fact that entity multiplication and causal overdetermination can be prevented by reducing realities to those of some other science besides physics. Phenomenalisms and idealisms notoriously proposed reductions of natural things to mental things, for example. Non-naturalisms aside, a sufficiently aggressive and flexible science other than physics could attempt to impose "reductionist universalism" on the rest. For example, an aggressive version of Social Constructivism would reduce the objects of scientific knowledge to outcomes of social negotiations.

Premise 2 is the far more controversial statement. Proponents of pluralism do not deny that some theoretical reductions and their clarifications of entities and causal relations can be intellectually satisfying. However, pluralists question whether theoretical reductions are, for all scientific fields, consistently the most intellectually satisfying. Pluralists would prefer to appeal to a different standard of intellectual satisfaction: scientific theories should meet the standard of producing an impressive pattern of consistent confirmations, as demanded by the scientific method (see "Scientific Method"). For example, geology's theory of plate tectonics has met this methodological standard, although a serious reduction of this theory to physics may never be developed. Even if adherence to the standard of scientific method requires the occasional serious violation of common sense (doesn't modern science do this regularly anyways?), the pluralist is more comfortable with each science taking exclusive responsibility for its own selected perspective on reality. The reductive physicalist may oppose this shifting of intellectual standards by arguing that plate tectonics is a successful theory only because its principles are entirely physical in nature (meaning, one must suppose, that its principles are consistent with those of physics proper). The reductive physicalist intends to claim that plate tectonics could in principle be explained by "lower level" theories of physics alone, even if the "higher-level" theory of plate tectonics happens to successfully explain causal relationships at that higher level. Regrettably, this counter argument only begs the question against pluralism -- why should we wait until physics alone can produce explanations for geology (especially since we might wait forever)?

The more powerful defense of premise 2 by the reductivist is raised by emphasizing the dire consequences of permitting causal overdetermination. The reductivist can first try to invoke the principle of "causal closure" -- only a physical cause can produce a physical effect. However, the pluralist can happily accept this principle, and add more -- "only a chemical cause can produce a chemical effect"; "only a biological cause can produce a biological effect"; "only a social cause can produce a social effect"; and even "only a mental cause can produce a mental effect". The pluralist's system of levels of reality would require an abundance of such principles. The reductivist's horror at the multiplication of closure principles reveals his real agenda of imposing a "physical causal closure" principle that only a physical cause can produce a natural effect. However, invoking this specific principle is just a repetition of the reductivist's overall agenda. To effectively raise alarm over permitting causal overdetermination, the reductivist must explain why any naturalist should realize that the only causes must be physical causes, by appealing to actual scientific knowledge. Now the reductivist can find a ready ally in physics itself: the law of conservation of mass/energy. If there were any other real causes of events over and above physical causes, some violation of the law of conservation of mass/energy would be necessary, according to the reductivist. But is this true? The pluralist could reply that confirmations of this law of conservation only occur in physics experiments, where physical processes are studied. At other levels of reality, other conservations may also also valid -- the conservation of chemical energy, for example. Some pluralists use the notions of "Emergence" and "Downwards Causation" to explain how higher-level processes have the own holistic properties and powers that are not capturable by anything like the law of conservation of energy. Even if a complex system's parts obey such a physical law in their interactions, must the whole system itself? If not, then perhaps the larger complex whole exerts downwards causation by affecting the behavior of its parts. Reductivists sometimes explain their position as adhering to "Supervenience" -- no change at a higher level without a change at the physical level -- but what if "subvenience" was true instead: no change at the physical level without some change at a higher level? No reductivist has made a convincing non-question-begging argument against plural levels of causal closure, conservation, and subvenience.

Conclusion? Neither the inductive nor abductive argument decisively help reductivist physicalism.

Fourth, let's proceed to inductive and abductive arguments for Perspectival Pluralism.

The Inductive Argument for Perspectival Pluralism from Past Successful Systematizations

1. There have been many successful systematizations of selected scientific and experiential perspectives on nature.
2. These systematizations have not been overturned by subsequent intellectual progress, but are essential to it.
Conclusion: A supreme metaphysical system will be developed to coordinate the entire plurality of perspectives on nature given by science and experience.

Premise 1 may be accepted with few reservations by most naturalists. Successful systematizations are numerous: some examples include the coordination of scientific knowledge about the human optic system with our visual experience using that system; the coordination of scientific knowledge about the sphericality of the earth with our practical experience navigating it; the coordination of the sciences of psychology and neurobiology together with our experience of emotions; etc. It cannot be unreasonable to search for such systematizations even for the reductivist, since these systematizations are a necessary step towards the very goal of theoretical reduction. However, the truth of premise 1 does little to support this argument, since these systematizations may not be very significant for the long-run progress of knowledge.

Premise 2 is rejected by reductive physicalism, since that variety holds that reduction to physics is far more essential to scientific progress than the  systematization of perspectives. In defense of premise 2, the Perspectival pluralist would point out that the science's primary task is to formulate successful theories that explain how and why we experience the world's processes. Scientific knowledge can never be a "view from nowhere" since it arises from, and is tested against, human viewpoints on nature. If it is objected that modern science's reliance on instruments to measure nature means that the human viewpoint has been eliminated, the pluralist would reply that humans build, calibrate, and use those machines. A measuring tool is still a human measuring tool. A scientific instrument that was truly independent from all human experiences and perspectives would not be usable for anything. The counter argument by reductive physicalism is to reply that while experiential perspectives generate and test scientific theories, the models of reality hypothesized by these theories lack any particular perspective on reality and they really do offer an entirely objective knowledge of reality. Reductive physicalism would add that physics enjoys the status of having the most objectively comprehensive knowledge of reality. Regardless, the reductivist justifiably complains that if a successful scientific theory can really provide no more than one perspective upon reality among many, in what sense does a successful scientific theory actually describe reality truthfully? How can a mere perspective, lost and crowded among innumerable others, claim to knowledge?

Because this argument's conclusion is quite premature in light of the weakness of the premises, this argument is a failure. Perspectival Pluralism must produce grander and more impressive metaphysical systematizations across the sciences and experience, and increase confidence that such systematizations are essential to science. Of course, Perspectival Pluralism arouses much enthusiasm by holding up the diversionary complaint that Reductive Physicalism refuses to permit any science other than physics to draw definitive conclusions about reality. This issue will be considered next.

The Abductive Argument for Perspectival Pluralism from Scientific Methodology

1. If a metaphysical system coordinates the plurality of perspectives on nature given by practical experience and successful scientific theories, then meeting the standard of continuous confirmations from the scientific method yields the most confidence in a scientific theory.
2. Meeting the standard of continuous confirmations from the scientific method yields the most confidence in a scientific theory.
Conclusion: A metaphysical system coordinates the plurality of perspectives on nature given by practical experience and successful scientific theories.

Even the reductivist should be able to perceive the wisdom of premise 1, although its hypothetical character does not commit any naturalist to very much. All naturalists must admit that the standard of the scientific method is entirely blind to the problem of multiple sciences drawing conflicting conclusions about reality. It could be argued that this blindness is actually a strength. Perhaps in the long run, these conflicts must subside -- as the many sciences are perfected, they will begin to merge together. The long-expected merger of subatomic physics, atomic physics, and chemistry may serve as a hoped-for example; even though this merger may not happen soon, there seems to be no barrier in principle. Such mergers, even of the hypothetical sort, inflate the hopes of reductivists. Will all scientific theories eventually merge into One Grand Unified Theory of Everything, to match the One Truth of reality? Even if this were an intelligible possibility, all of human experience would have to absorbed as well, which may be an even more improbable task. Of course, the reductivist can try to assert that most, if not all, human experience can be safely ignored as irrelevant to knowledge. Some of it may be reducible, but the recalcitrant portions of experience could be dismissed as having no informational value about reality, and perhaps not even possess any reality themselves (the last resort of the eliminativist). The pluralist must explain why practical experience is essential to successful scientific theorizing. The obvious course is to show how scientific theorizing grows out of, is tested by, and remains akin to, practical experience. Unless the pluralist can prove such continuity between science and practical experience (see the opening sections of "Scientific Method" on this continuity), the need to coordinate experience with scientific knowledge seems far less urgent. Worse, a pluralist's obstinate demand for the inclusion of experience may propel him towards Synoptic Pluralism (still a naturalism?) or outright ontological dualism (naturalism no more!).

Premise 2 is controversial as well. How can the reductivist deny it? Proponents of reductionism must be careful to duly respect the scientific method, and be understood as only proposing an additional amendment that confidence in a scientific theory is additionally enhanced by theoretical reduction to physics. This additional amendment is scorned by pluralists, who point out that it effectively segregates non-reduced scientific theories, and entire scientific fields, into "second class" status as yielding less-than-genuine knowledge about reality. This point makes an effective charge of hypocrisy against reductionism. We have heard, in discussion of the inductive argument for Perspectival Pluralism, the reductivist's complaint that perspectivism trivializes each theoretical perspective upon reality and diminishes its claim to knowledge. The reductivist will reply that at least physics can claim genuine knowledge, while no science can do so, according to perspectivism. Perspectivism's rejoinder, that only its variety of naturalism ensures that all successful scientific theories may claim some genuine knowledge of reality, only returns us to the beginning of the argument. 

Conclusion? Neither the inductive nor abductive arguments decisively help Perspectival Pluralism.

Fifth, we may summarize the results of these four arguments for the great naturalisms, Reductive Physicalism and Perspectival Pluralism. None of these arguments decisively help either variety of naturalism, as they only highlight the basic disagreements between them. What can these great naturalisms do to gain the advantage over the other? We can make some recommendations.

Reductive Physicalism must (1) display more successful and significant reductions to physics, to increase confidence that reductions are essential to science; (2) guarantee that reductive universalism is consistent with science's actual use of theoretical models and natural laws; (3) demonstrate that avoiding entity duplication and causal overdetermination is essential to scientific progress; (4) explain why its categorization of all sciences but physics as "second class" is not just as curious as perspectivism's categorization of all sciences as partial and limited; and (5) justify the view that experience is either eliminable or reducible, at least in principle.

Perspectival Pluralism must (1) produce more impressive metaphysical systematizations across the sciences and experience, to increase confidence that such systematizations are essential to the progress of knowledge; (2) show that reductive universalism is inconsistent with science's actual use of theoretical models and natural laws; (3) demonstrate that entity duplication and causal overdetermination are not detrimental to scientific progress; (4) explain why the standard of scientific method should prevail without any amendment awarding preference to reductive accounts; and (5) justify the view that scientific theorizing is continuous with ordinary experience and should be coordinated with experience in a metaphysical naturalism.

We must now leave these two great naturalisms to their work in progress. Further discussion of the disagreements between the seven viable naturalisms is provided in "Naturalism and the Mind". You may also proceed to "Scientific Realism and Truth" for a discussion of scientific antirealism's challenge to the very possibility of naturalism.

copyright 2007 by John R. Shook