Naturalistic Philosophy, Physicalism, Reductionism, and the Emergence of Liberal Naturalisms
I. Introduction to Naturalistic Philosophy
Naturalistic philosophy represents a broad and influential orientation in contemporary thought, characterized by its endeavor to align philosophical inquiry closely with the methods and findings of the empirical sciences. Although the term "naturalism" eludes a single, precise definition, its modern usage draws from early to mid-twentieth-century American philosophers who advocated for a worldview where reality is exhausted by nature, devoid of "supernatural" elements, and where the scientific method serves as the primary tool for investigating all aspects of reality, including human consciousness and spirit (Papineau, 2021, citing Krikorian, 1944, and Kim, 2003). This general stance, rejecting supernatural entities and affirming science as a crucial path to understanding, is widely accepted among contemporary philosophers.
Naturalism can be understood through two principal components. The first, ontological naturalism, addresses the fundamental constituents of reality. It asserts that nature is all that exists, thereby excluding entities or forces that transcend the natural order, often described as "supernatural" or "spooky" (Papineau, 2021). Within this ontological framework, specific doctrines like moral naturalism propose that moral properties are stance-independent, natural properties, and that moral facts are themselves natural facts (Termini, 2024). The second component, methodological naturalism, pertains to the appropriate ways of investigating reality. It champions the general authority of the scientific method, suggesting that philosophical problems are best approached as if they are tractable through empirical investigation, or at least without primary reliance on a priori theorizing conducted independently of scientific findings (Papineau, 2021).
The ontological commitment that reality is "exhausted by nature" (Papineau, 2021) is a robust claim that immediately brings into focus phenomena that do not readily lend themselves to straightforward scientific explanation, such as subjective consciousness, normativity, and the status of abstract objects. This creates a philosophical tension: such phenomena must either be ultimately explicable in terms science will eventually grasp, be considered unreal, or our conceptions of "nature" or "science" must be broadened. This tension is a significant driver of debates within naturalism and motivates the exploration of concepts like physicalism and reductionism, as well as the development of more expansive "liberal" naturalisms. Furthermore, the emphasis on the "scientific method" is not uniform. The very definition of "the" scientific method and its universal applicability are subjects of ongoing philosophical discussion. Narrow interpretations tend to foster more restrictive forms of naturalism, whereas broader understandings of scientific inquiry can accommodate a wider array of philosophical approaches, including those that maintain a continuity with a more pluralistic view of science.
II. Foundational Concepts within Naturalism: Physicalism and Reductionism
Within the broad church of naturalism, physicalism and reductionism are two foundational, though often debated, concepts that attempt to provide more specific content to the naturalistic worldview.
A. Physicalism: The Thesis of Materiality
Physicalism is a strong form of ontological naturalism, positing the metaphysical thesis that everything that exists is physical, or, in a more nuanced formulation, that all facts supervene on, or are necessitated by, physical facts (Stoljar, 2021). This means that the fundamental nature of the actual world, in its entirety, conforms to the condition of being physical. Physicalists readily acknowledge the existence of entities and properties that, at first glance, do not appear to be physical—such as biological organisms, psychological states, moral values, and social structures. However, they maintain that these are ultimately either physical in nature or bear a crucial dependence relationship, like supervenience, to the physical (Stoljar, 2021). The core idea is that non-physical-seeming properties are "nothing over and above" physical properties.
Despite its centrality, formulating a precise and unproblematic definition of "physical" presents a significant philosophical challenge. Simply equating "physical" with the entities and properties described by "present-day physics" is problematic, as physics is an evolving discipline; what is considered fundamental today may be superseded tomorrow. Conversely, defining "physical" in terms of a hypothetical, future, completed physics risks rendering the thesis vacuous or empirically untestable (Hall et al., 2021). David Lewis, for instance, extensively explored the complexities of adequately defining physical properties (Hall et al., 2021). This definitional quandary has profound implications, as the coherence of physicalism hinges on a stable understanding of its core term. If "physical" remains elusive, the entire doctrine is threatened, indirectly strengthening arguments for more liberal forms of naturalism that may not tie themselves so rigidly to a specific conception of contemporary physics.
A prominent challenge to physicalism is the "knowledge argument," famously illustrated by Frank Jackson's thought experiment about Mary, a brilliant scientist who learns all the physical facts about color vision while confined to a black-and-white room. The argument suggests that if Mary learns something new upon experiencing color for the first time (e.g., "what it is like" to see red), then there are truths about consciousness that are not captured by the complete physical account, implying physicalism is false (Jackson, 1982, 1986). This argument directly targets the explanatory completeness of physicalism with respect to subjective, phenomenal experience.
B. Reductionism: Explaining Wholes Through Parts
Reductionism is a strategy often associated with physicalist naturalism. In its general sense, reductionism is the view that complex phenomena or higher-level theories can be explained by, or deduced from, more fundamental phenomena or theories, with physics often posited as the ultimate explanatory basis (Love & Hüttemann, 2023). Reductionist claims can be distinguished into ontological, epistemological, and methodological types (Love & Hüttemann, 2023).
- Ontological reductionism is the claim that every particular system or entity is constituted by, and nothing more than, its lower-level parts and their interactions. In the context of biology, this would mean that an organism is entirely composed of molecules and their interactions. This form is often considered synonymous with physicalism (Love & Hüttemann, 2023).
- Epistemological reductionism asserts that the laws, theories, and explanations of a higher-level science (e.g., biology, psychology) can, in principle, be derived from or explained by the laws and theories of a more fundamental science (e.g., physics or chemistry) (Love & Hüttemann, 2023).
- Methodological reductionism is the strategic claim that the most effective way to study complex systems is by investigating them at the lowest possible level of organization, for example, by decomposing a system into its constituent parts and studying their properties (Love & Hüttemann, 2023).
While physicalism often provides the motivation for reductionist projects, it does not strictly entail strong versions of epistemic or methodological reductionism. It is conceivable to be an ontological physicalist—believing that everything is ultimately composed of physical entities—while simultaneously holding that higher-level sciences possess autonomous explanatory power. This is because phenomena such as multiple realizability—where the same higher-level property or kind (e.g., pain, or a gene's function) can be realized by many different lower-level physical configurations—pose significant challenges to epistemic reduction (Love & Hüttemann, 2023). If a mental state can be instantiated in diverse physical substrates, a purely physical description at the micro-level might fail to capture the commonality identified by the higher-level psychological concept. Thus, ontological physicalism might be true, while strong epistemic reductionism proves false or impractical. Another challenge is context-dependence, where the causal role or properties of a lower-level entity (like a specific molecule) can vary significantly depending on the larger systemic context, making straightforward bottom-up explanations difficult (Love & Hüttemann, 2023). These complexities create philosophical space for non-reductive or more liberal forms of naturalism.
III. Grounding Reality: From Experimental Intervention to Perceptual Realism
The naturalistic commitment to an empirically grounded understanding of reality raises fundamental questions about how we justify our beliefs in the existence of both everyday objects and the unobservable entities posited by science.
A. Ian Hacking's Experimental Realism: The "Spray It" Criterion
Ian Hacking, in his influential work Representing and Intervening (1983), advanced a form of scientific realism known as "entity realism." Shifting the focus from the truth of scientific theories to the reality of the entities they posit, Hacking famously declared, "So far as I'm concerned, if you can spray them then they are real" (Hacking, 1983, p. 23, as cited in Franklin & Perović, 2023; Daston, 2023). This "spray it" criterion underscores the importance of manipulability and intervention in establishing belief in the existence of scientific entities. Hacking elaborated, "We are completely convinced of the reality of electrons when we set out to build—and often enough succeed in building—new kinds of device that use various well-understood causal properties of electrons to interfere in other more hypothetical parts of nature" (Hacking, 1983, p. 265, as cited in Franklin & Perović, 2023). For Hacking, reality is not primarily about representation or theoretical coherence, but about what entities do and what scientists can do with them as tools to investigate the world further.
This pragmatic, practice-oriented approach offers a way to ground realism in the tangible activities of experimental science. Allan Franklin, while agreeing with Hacking on the significance of manipulability, suggests that Hacking's chosen examples for the reality of electrons (such as the Peggy II experiment) came "far too late," arguing that physicists were effectively manipulating electrons and had good reasons for believing in their existence much earlier in the twentieth century. Franklin's own "conjectural realism" extends this to include good reasons for belief in scientific laws and theories, not just entities, thereby diverging slightly from Hacking's stricter entity focus (Franklin & Perović, 2023).
B. Early Modern Perspectives on Realism: The "Kicking a Stone" Analogy and Hume's Skepticism
Long before contemporary debates about scientific entities, philosophers grappled with the reality of the external world. The memorable "refutation" of idealism by kicking a large stone—"I refute it thus"—is attributed to Samuel Johnson, who performed the act in response to Bishop Berkeley's immaterialist philosophy, which denied the existence of matter independent of perception (Massin, 2023). Johnson's kick was a commonsense, pragmatic gesture intended to highlight the perceived absurdity of denying the stone's solid, material reality when it offered palpable physical resistance. Though philosophically unsophisticated compared to Hacking's criteria, Johnson's act shares with experimental realism an appeal to interaction and resistance as prima facie evidence for the reality of an external object. Both push back against views that might detach reality from tangible engagement, suggesting a deep-seated intuition that what can causally affect us, or what we can causally affect, possesses a claim to reality.
David Hume, a contemporary of Johnson, offered a far more skeptical analysis of our knowledge of the external world. Hume famously argued that "nothing is ever directly present to the mind in perception except perceptual appearances" (Hume, 1739/1978; 1748/2000, as discussed in Tucker & Silins, 2023). This establishes a "veil of perception" between the mind and any supposed external objects, meaning our access to the world is always mediated by potentially misleading sensory experiences. While we might instinctively believe in an external world (what Hume called the "vulgar" or common belief), philosophical reflection reveals the profound difficulty in justifying this belief from the content of our perceptions alone (Tucker & Silins, 2023). Hume's challenge is fundamental: if all we ever directly encounter are our own perceptions, how can we legitimately infer the existence, let alone the nature, of a mind-independent reality that supposedly causes them?
Hacking's entity realism can be seen as an attempt to circumvent Hume's general skeptical challenge, at least concerning specific scientific entities. Hacking does not aim to prove that we can pierce the "veil of perception" in a global sense. Instead, he argues that for those particular entities which we can reliably manipulate and use to intervene in other phenomena in predictable ways (like electrons), the skeptical worry loses much of its practical force for the working scientist. The success of intervention provides pragmatic grounds for belief in the reality of these entities, even if we cannot "directly perceive" them in a naive sense. The progression from Johnson's intuitive realism, through Hume's sophisticated skepticism, to Hacking's practice-based experimental realism illustrates an evolving philosophical dialogue about the nature of reality and the justification of existence claims, particularly within a naturalistic framework that values empirical grounding.
IV. Liberal Naturalisms: Expanding the Naturalistic Framework
The perceived limitations and internal challenges of more restrictive forms of naturalism, often termed "scientific naturalism," have spurred the development of "liberal naturalisms." These approaches seek a more inclusive understanding of nature and philosophical methodology while remaining committed to a broadly non-supernaturalistic worldview.
A. Critiques of Restrictive (Scientific) Naturalism
Scientific naturalism is typically characterized by two core commitments: an ontological thesis that reality consists exclusively of those entities and properties countenanced by the natural sciences (particularly physics), and a methodological thesis that philosophy should either emulate the methods of the natural sciences or operate as their abstract, reflective branch, ceding ultimate epistemic authority to them (Chudnoff, 2007, summarizing DeCaro & Macarthur; Spiegel, 2022). This form of naturalism, with its often strong reductionist tendencies, faces significant challenges in accounting for various aspects of human experience and understanding.
A central difficulty for scientific naturalism is the "placement problem": how to situate phenomena such as consciousness (especially qualia), meaning, intentionality, moral values, and rational normativity within the purely physical or scientifically described world (Mitova, 2012; Macarthur, 2008; Spiegel, 2022). The scientific image of the world, focusing on particles, forces, and objective mechanisms, appears to offer no obvious "place" for these subjective, normative, or semantic dimensions of reality. If scientific naturalism cannot adequately accommodate these crucial features, it risks presenting an impoverished and incomplete account of the world and our place within it. The perceived failures or inherent limitations of strong reductionist programs and the persistent philosophical difficulties in formulating a satisfactory version of physicalism directly motivate the search for more expansive naturalistic alternatives. Liberal naturalism emerges as a response to these perceived shortcomings.
B. The Contributions of MacArthur and DeCaro
Philosophers Mario DeCaro and David Macarthur have been pivotal in articulating and popularizing the concept of "Liberal Naturalism," notably through their edited volumes Naturalism in Question (DeCaro & Macarthur, 2004) and Naturalism and Normativity (DeCaro & Macarthur, 2010) (Mitova, 2012). Their work, and that of like-minded philosophers, aims to carve out a naturalistic position that is broader and more accommodating than restrictive scientific naturalism.
Liberal naturalism explicitly rejects the claim that "modern natural science provides... the only true picture of nature" or that its methods are the sole legitimate means of acquiring knowledge (Chudnoff, 2007, quoting DeCaro & Macarthur, 2004, p. 4). Instead, it strives for a "more inclusive conception of nature than any provided by the natural sciences" (Chudnoff, 2007, quoting DeCaro & Macarthur, 2004, p. 1). A defining feature of liberal naturalism is its rejection of either the stringent ontological claims (that only scientific posits exist) or the restrictive methodological claims (that philosophy must mimic science) of its scientific counterpart, or indeed both (Mitova, 2012). This broader framework seeks to make room for "controversial entities" or phenomena—such as moral facts, phenomenological experiences, and intentional states—and acknowledges a plurality of ways of understanding, including modes of inquiry that may be non-scientific yet remain non-supernatural (Mitova, 2012).
David Macarthur (2008), for instance, develops a form of liberal naturalism informed by pragmatism and metaphysical quietism. This approach seeks to resolve the placement problem for normativity not by trying to find an ontological "location" for norms within the physical world, but by reconceiving the issue as one concerning how best to philosophize about our existing normative practices. This involves a methodological naturalism that emphasizes understanding these practices from within, rather than reducing them to non-normative scientific descriptions (Macarthur, 2008). Mario DeCaro's work on "Varieties of Naturalism" (DeCaro, 2010) further explores the diverse landscape of naturalistic thought, distinguishing stricter scientific or reductive forms from more liberal and expansive ones. Liberal naturalism is often presented as a "middle passage" (Spiegel, 2022), navigating between the perceived austerity of scientific naturalism and any appeal to supernatural explanations.
C. Defining Liberal Naturalism: Key Tenets
Liberal naturalism, while diverse, generally upholds several key tenets. It respects the findings and methods of the sciences but does not grant them an explanatory monopoly over all domains of reality or inquiry. It acknowledges the legitimacy of non-scientific modes of knowing and understanding, for example, in appreciating art, discerning moral values, or engaging in distinctively first-personal aspects of rational agency such as deliberation and responsibility (Mitova, 2012; Drees, 2022). This perspective allows for the existence of irreducible yet perfectly natural facts, which can include normative facts (Chudnoff, 2007, referencing McDowell).
Crucially, liberal naturalism often challenges the influential Quinean thesis that a properly naturalized philosophy must restrict itself to the methods of the successful natural sciences (Macarthur, 2008). This methodological pluralism is a significant departure from many traditional forms of analytic naturalism. The aim is to reconcile the "manifest image" of the world—our everyday, commonsense understanding of ourselves as persons endowed with minds, reasons, and values—with the "scientific image"—the world as described by the sciences—without necessarily reducing the former to the latter, a philosophical project famously articulated by Wilfrid Sellars. Liberal naturalism seeks to affirm the reality and significance of the manifest image, provided it does not invoke supernatural explanations.
The "placement problem" is thus transformed. Rather than solely asking "What are norms in physical terms?", liberal naturalism often inquires into "What do norms do? How do they function indispensably in our practices of reasoning, social interaction, and even in the conduct of science itself?" If normative concepts and practices are indispensable for a coherent understanding of ourselves and the world, and function without recourse to the supernatural, they can be considered "natural" within this more liberal framework.
V. Conclusion: The Evolving Landscape of Naturalism
Naturalistic philosophy, far from being a monolithic doctrine, is a dynamic and evolving field characterized by robust internal debate. The core commitment to a reality devoid of supernatural entities and an epistemology grounded in empirical methods and scientific inquiry provides a broad framework within which diverse positions, such as physicalism and reductionism, have been developed. These positions, while offering powerful explanatory tools, also generate significant philosophical tensions, particularly when confronted with phenomena like consciousness, meaning, and normativity.
Ian Hacking's experimental realism offers a pragmatic criterion for the reality of scientific entities based on our ability to manipulate and intervene with them, providing a practical grounding for scientific claims that sidesteps some traditional metaphysical debates. This contrasts with the profound skepticism about our access to the external world articulated by David Hume, whose "veil of perception" continues to challenge naive realism, a challenge crudely but viscerally countered by Samuel Johnson's stone-kicking gesture against idealism.
In response to the perceived limitations of restrictive "scientific naturalism" in addressing these challenges, particularly the "placement problems," liberal naturalisms, championed by thinkers like Mario DeCaro and David Macarthur, have emerged. These approaches seek a more capacious understanding of nature and naturalistic methodology, one that can accommodate the richness of human experience, including moral and rational normativity, without abandoning the core anti-supernaturalist commitments of naturalism. They propose a more pluralistic view of knowledge and reality, where the manifest image of human life is not necessarily reducible to the scientific image but can coexist with it in a non-contradictory, naturalistic framework.
A recurring theme in these discussions is the inherent reflexivity of naturalism: the philosophical work required to define, defend, and refine naturalism itself cannot be simply "read off" from scientific findings but demands ongoing philosophical labor. The dialogue between restrictive and liberal forms of naturalism continues to shape contemporary metaphysics, philosophy of mind, ethics, and the philosophy of science. The overarching challenge remains to articulate a version of naturalism that is both faithful to the insights and rigor of scientific understanding and capable of providing a comprehensive and satisfying account of the full spectrum of human experience, reason, and values. The success of this endeavor has broad implications for how philosophy conceives of its own role in relation to the sciences and to human culture more broadly, aiming for an integrated intellectual landscape rather than a fragmented one.
VI. References
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