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Multiple realizability

Multiple realizability is a foundational thesis in the philosophy of mind, positing that a single mental or psychological state type—such as pain or belief—can be instantiated by diverse physical state types, including different neural configurations across individuals, species, or even artificial systems, thereby establishing a one-to-many relationship between mental kinds and their physical realizers.^[1] This concept challenges strict type-identity theories, which propose a one-to-one correspondence between mental and physical states, by emphasizing the functional autonomy of mental properties defined by their causal roles rather than specific material substrates.^[2] The idea of multiple realizability emerged in the mid-20th century as part of broader debates on reductionism and the nature of scientific explanation, with Hilary Putnam first articulating it in his 1967 paper "Psychological Predicates," where he argued that psychological states like hunger could have different physical-chemical correlates in mammals versus octopuses, undermining brain-state theories of mind.^[1] Jerry Fodor further developed the argument in his 1974 essay "Special Sciences (or: The Disunity of Science as a Working Hypothesis)," extending it beyond mind to the autonomy of all special sciences by contending that higher-level laws, such as those governing psychological processes, are multiply realized in physical systems and thus not reducible to fundamental physics.^[3] These formulations aligned with the rise of functionalism, viewing mental states as abstract computational or causal functions that could be implemented in varied hardware, including biological brains or silicon-based machines.^[2] Empirical support for multiple realizability draws from neuroscientific evidence of neural plasticity and cross-species comparisons, such as studies showing that functions like response inhibition in rats can be recovered through different brain regions after lesions, or that visual processing in rewired ferrets occurs in auditory cortex with functional adaptations.^[2] Critics, however, including Lawrence Shapiro and Thomas Polger, contend that such cases often involve variations within a single physical kind rather than true distinct realizers, or that multiple realization is compatible with a nuanced form of identity theory if differences are not taxonomically significant.^[1] Despite these challenges, the thesis remains central to nonreductive physicalism, affirming that psychology maintains explanatory independence from neuroscience while remaining compatible with materialism.^[3]

Core Concept

Definition

Multiple realizability is a central thesis in the philosophy of mind, asserting that a single mental state or property can be instantiated by diverse physical states or properties across different individuals, species, or even hypothetical systems.^[4] This view posits that mental types are not strictly identical to any particular physical type, thereby undermining type-type identity theories, which claim that each mental kind corresponds one-to-one with a specific physical kind, such as identifying pain exclusively with the stimulation of C-fibers in the human brain.^[5] Instead, the thesis emphasizes the possibility of one-many correlations between mental and physical types, allowing the same mental kind to emerge from varied physical bases.^[4] The concept is closely tied to functionalism, a theory that defines mental states in terms of their functional roles—their causal relations to sensory inputs, behavioral outputs, and other mental states—rather than any intrinsic physical makeup.^[6] Under this approach, as originally articulated by Hilary Putnam, a mental state like belief or desire is realized whenever a system exhibits the appropriate functional organization, regardless of the underlying physical substrate, whether biological or artificial.^[5] This functional characterization enables multiple realizability by decoupling mental content from specific material implementations, permitting the same psychological function to be performed by neurons in one organism and silicon circuits in another.^[4] A classic illustration involves the mental state of pain, which is realized in humans through the activation of C-fibers but could be realized by entirely different neural structures in octopuses, given their divergent evolutionary biology and decentralized nervous systems.^[4] Extending this, philosophers have considered hypothetical silicon-based aliens, where pain might arise from computational processes unrelated to organic neurology, yet fulfilling the same functional role of aversion and protective response.^[5] These examples highlight how multiple realizability accommodates empirical diversity in nature while preserving the unity of mental kinds.^[4] Multiple realizability forms a cornerstone of non-reductive physicalism, a position that affirms the physical basis of mind—mental properties supervene on physical ones—without reducing mental states to particular physical realizations, thus preserving the causal efficacy of the mental in physical processes.^[7] By rejecting strict psychophysical reductions, this framework allows psychological explanations to retain autonomy, as mental types can exert downward causation through their multiple physical vehicles without being eliminable or identical to any single brain state.^[4]

Key Distinctions and Examples

Philosopher Gualtiero Piccinini distinguishes multiple realizability from related concepts within mechanistic frameworks, clarifying its scope in the philosophy of mind and cognitive science. Variable realizability refers to cases where the same higher-level property is realized by slightly varying lower-level mechanisms that differ in minor details but share core structural features.^[8] In contrast, multiple realizability involves the same higher-level property being instantiated by disparate physical bases that lack significant commonality beyond performing the functional role.^[8] Medium independence, a stronger notion, occurs when a property can be realized across fundamentally different media, such as non-biological substrates like silicon chips, entailing multiple realizability but not vice versa.^[8] An illustrative example of variable realizability is neural plasticity in humans, where cognitive functions like language processing can be supported by alternative neural pathways after brain injury, adapting through slight reorganizations of existing circuitry rather than entirely new mechanisms.^[9] For multiple realizability, color vision in humans provides a biological case: the same perceptual property of trichromatic hue discrimination is realized by varying opsin protein variants across individuals, such as differences in cone pigments (e.g., Ser180 vs. Ala180 alleles in the red opsin) that have slightly different absorption spectra but enable equivalent functional outcomes despite molecular differences.^[10] Hypothetical scenarios further highlight these distinctions. Philosopher Hilary Putnam proposed that pain could be multiply realized in hypothetical Martians through silicon-based or chemical processes entirely unlike human neural C-fibers, emphasizing disparate physical bases for the same mental state. Similarly, computational functionalism, as articulated by Jerry Fodor, suggests that mental states like belief could be realized in software simulations on digital hardware, independent of biological substrates, aligning with medium independence.

Historical Development

Early Precursors

The concept of multiple realizability, though formalized later in philosophy of mind, finds early precursors in philosophical and scientific thought that highlighted how certain functions or processes could be achieved through diverse mechanisms without strict dependence on specific physical implementations. Influences from 19th-century biology further prefigured the idea, particularly in Charles Darwin's evolutionary theory as outlined in On the Origin of Species. Darwin observed that natural selection could produce different anatomical structures in closely related species to perform the same function, addressing an objection to his theory by noting cases where allied forms exhibit "widely different" structures serving identical ends, such as pollination mechanisms in orchid genera like Coryanthes and Catasetum.[^[11]] This variability in anatomical realization for functional equivalence across species underscored how biological adaptations could diverge while preserving core purposes, laying groundwork for understanding functional independence from specific forms.[^[11]] By the mid-20th century, Alan Turing's work on computation provided another key precursor, independent of direct applications to mind. In his 1950 paper "Computing Machinery and Intelligence," Turing described how digital computers could mimic the behavior of any discrete-state machine, including theoretical models like the Turing machine from his earlier 1936 work, provided sufficient storage and speed.[^[12]] This universality demonstrated that the same computational processes could be realized on diverse hardware substrates—such as mechanical Turing machines versus electronic computers—emphasizing medium independence in executing functions.[^[12]]

Formulation in Philosophy of Mind

The concept of multiple realizability emerged prominently in the 1960s within analytic philosophy of mind as a challenge to type-identity theories, which posited that mental states are strictly identical to specific physical or brain states. Hilary Putnam introduced the idea in his 1967 paper "Psychological Predicates," where he argued that psychological predicates, such as those denoting pain or belief, do not refer to particular physical states but rather to functional roles that can be fulfilled by diverse physical mechanisms across different systems.[^[5]] For instance, Putnam illustrated this with the example of pain, suggesting it could be realized by C-fiber stimulation in humans, analogous neural processes in other mammals like cats or dogs, or entirely different biochemical structures in hypothetical extraterrestrial beings or even robotic systems, thereby undermining the necessity of a one-to-one correspondence between mental types and physical types.[^[5]] He extended this to simpler cases, such as a thermostat regulating temperature through electronic circuitry, which performs a functional role akin to more complex regulatory processes in living organisms, highlighting the non-reductive nature of such states.[^[5]] Building on Putnam's foundation, Jerry Fodor elaborated multiple realizability in 1974 to defend the autonomy of the special sciences, including psychology, from reduction to fundamental physics or neuroscience. In his essay "Special Sciences (or: The Disunity of Science as a Working Hypothesis),"[^[13]] Fodor contended that higher-level scientific predicates, like those in psychology describing cognitive processes, are multiply realizable in a wide array of physical substrates, making strict type-type reductions implausible and justifying the explanatory independence of psychological laws. He drew examples from economics, such as the predicate "price increase," which could be instantiated through various physical events like shifts in supply chains or market behaviors, and paralleled this with mental states that might be realized differently across species or artificial systems without losing their psychological identity. This framework positioned multiple realizability as a cornerstone for non-reductive physicalism, emphasizing that while mental states supervene on physical ones, they do not reduce to them in a type-identical manner. Saul Kripke's 1980 work Naming and Necessity further bolstered the case against identity theories through a modal semantic analysis, influencing the philosophical reception of multiple realizability.[^[14]] Kripke critiqued the type-identity thesis by arguing that mental state terms like "pain" and physical descriptors like "C-fiber stimulation" function as rigid designators, referring to the same entities in all possible worlds where they exist, yet pain is not necessarily identical to any specific brain state, as one can conceive of worlds where pain occurs without that physical process. This necessary a posteriori distinction supported the contingency of mental-physical relations implicit in multiple realizability, reinforcing Putnam and Fodor's arguments by showing that even if mental states are physically realized, such realizations are not metaphysically necessary identities. By the late 1980s, Putnam himself critiqued aspects of the functionalist framework he had helped develop, using multiple realizability to question whether mental states could be adequately captured by computational or machine-based realizations. In Representation and Reality (1988),[^[15]] Putnam argued that while multiple realizability allows for diverse physical substrates, it also reveals limitations in functionalism's reliance on Turing-style computation, as mental phenomena exhibit a "compositional plasticity" and environmental embedding that transcend purely syntactic functional roles realizable in machines. This shift, further explored in The Threefold Cord: Mind, Body, and World (1999),[^[16]] highlighted how multiple realizability, originally a tool against identity theory, could problematize overly abstract functionalist accounts of the mind.

Significance

Role in Debates on Mind and Reductionism

Multiple realizability has played a central role in philosophy of mind by challenging reductive accounts that seek to identify mental states strictly with physical brain processes. The type-identity theory, advanced by U.T. Place in 1956 and J.J.C. Smart in 1959, maintained that each type of mental state, such as pain or belief, is identical to a specific type of brain state, implying a one-to-one correspondence between psychological and neurophysiological kinds.^[17]^[18] This view faced significant opposition from the multiple realizability thesis, which argues that the same mental state can be instantiated by diverse physical mechanisms, such as similar functional responses to stimuli in human C-fiber activation and analogous nociceptor firings in octopuses or aliens, thereby refuting the necessary type-type identity across all instances.^[5] By highlighting these one-to-many relations, multiple realizability bolstered functionalism, a theory emphasizing that mental states are defined by their causal roles in producing inputs and outputs rather than their material basis, as elaborated by Hilary Putnam and Jerry Fodor in the late 1960s and 1970s.^[5]^[19] This approach supports non-reductive physicalism, positing that mental properties supervene on physical ones—such that any change in the mental requires a physical change—but remain distinct higher-level kinds due to their realizability in varied physical substrates, preserving the autonomy of psychological explanation from neuroscience.^[19] Multiple realizability further critiques eliminativism, the position championed by Paul Churchland in the 1980s, which contends that folk-psychological concepts like beliefs and desires form a false theory destined for elimination by maturing neuroscience. Proponents argue that the robust realizability of mental states across biological and potentially artificial systems demonstrates their empirical reality and explanatory utility, undermining the need for wholesale rejection in favor of purely physicalist replacements.^[5] This debate unfolded in the 1960s and 1970s, a period marking the decline of behaviorism—which dismissed internal mental states in favor of observable behaviors following critiques like Noam Chomsky's 1959 review of B.F. Skinner's Verbal Behavior—and the rise of computational metaphors portraying the mind as an information-processing system akin to digital computers.^[20] Within this shift toward cognitive science, multiple realizability emerged as a foundational argument against reductionism, influencing ongoing discussions on the mind's place in the physical world.

Broader Implications

In cognitive science, multiple realizability underpins the modularity of mind hypothesis by positing that cognitive processes can operate through diverse neural architectures, thereby permitting psychological laws to function independently of specific neurobiological implementations. This perspective allows researchers to investigate higher-level cognitive functions, such as perception or decision-making, without being tethered to the particulars of brain structure across species or individuals, fostering a taxonomic approach that emphasizes computational roles over physical substrates.^[21]^[22] For instance, the same learning algorithm might be realized in mammalian cortical circuits or avian forebrain regions, supporting interdisciplinary models that integrate behavioral data with varied neural evidence.^[23] The ethical ramifications of multiple realizability extend to assessments of sentience in non-mammalian animals and the potential for machine consciousness, challenging anthropocentric biases in moral consideration. In animal ethics, it counters arguments requiring a mammalian neocortex for pain or awareness, suggesting that cephalopods or birds could exhibit sentience through alternative neural pathways, thus broadening welfare standards to include diverse taxa. This perspective has influenced recent policies, such as the UK's Animal Welfare (Sentience) Act 2022, which legally recognizes sentience in vertebrates and certain invertebrates including cephalopods and decapods, and the New York Declaration on Animal Consciousness (2024), which asserts strong evidence for consciousness in mammals, birds, and many cephalopods and fish, with reasonable possibility in other taxa like reptiles, amphibians, insects, and crustaceans.^[24]^[25]^[26]^[27] Similarly, for artificial intelligence, the thesis implies that consciousness might emerge in silicon-based systems via functional equivalence to biological minds, raising questions about rights for advanced machines that simulate qualia or self-awareness without organic hardware.^[28]^[29] This view prompts ethical frameworks that prioritize behavioral and functional indicators over material composition, influencing debates on AI accountability and autonomy.^[30] In metaphysics, multiple realizability probes the nature of property identity and realization relations, complicating reductive ontologies by illustrating how higher-level properties supervene on yet are not identical to their physical bases. It invites analyses of realization as a non-reductive relation, where mental or special science properties achieve type identity across heterogeneous realizers without collapsing into disjunctive physical kinds, thereby sustaining non-reductive physicalism.^[31] This has implications for general ontology, as it questions whether realization entails determination without identity, influencing theories of emergence and causal powers in the special sciences.^[32] From a policy standpoint in neuroethics, multiple realizability advocates caution against overly reductive approaches to mental health treatments, emphasizing that psychological disorders may not map straightforwardly onto brain states due to their realizability in varied physiological contexts. This justifies integrated therapeutic strategies that address functional disruptions across individuals, rather than solely targeting presumed neural correlates, thereby mitigating risks of one-size-fits-all interventions in psychiatry.^[33]^[34] Such considerations inform ethical guidelines for brain-targeted therapies, promoting pluralism in mental health policy to account for the autonomy of psychological kinds.

Arguments in Favor

Conceivability Argument

The conceivability argument for multiple realizability maintains that it is conceivable for a given mental state, such as pain, to be instantiated without relying on any particular physical state, like the firing of C-fibers in human brains, and that this conceivability indicates a metaphysical possibility, thereby establishing that mental states can be realized by diverse physical mechanisms.^[4] This approach leverages modal reasoning to challenge reductive identity theories in the philosophy of mind, positing that the absence of a necessary connection between specific mental and physical states allows for multiple physical realizations of the same mental kind.^[4] A seminal illustration of this argument comes from Hilary Putnam's 1967 thought experiment, which envisions hypothetical entities—such as silicon-based androids, electronic robots, or Martians with pulsating green slime—capable of experiencing pain or other mental states through physical structures entirely dissimilar to earthly biology, yet fulfilling analogous functional roles.^[4] Putnam argued that these scenarios are coherently imaginable, underscoring that psychological predicates like "pain" are not confined to human-specific neurophysiology but can apply across varied physical substrates, thus supporting the multiple realizability thesis against type-identity physicalism.^[4] To address potential objections concerning necessities, proponents of the conceivability argument distinguish between epistemic conceivability (scenarios imaginable without apparent contradiction, based on current knowledge) and metaphysical conceivability (scenarios possible in the broadest sense, across possible worlds), asserting that under ideal rational scrutiny, the former reliably entails the latter, thereby validating the modal possibilities essential to multiple realizability.^[35]

Likelihood Argument

The likelihood argument for multiple realizability posits that empirical evidence from biological diversity makes it probable that mental or functional kinds are realized by multiple distinct physical structures, rather than uniquely identified with any single physical type. Across the animal kingdom, similar behaviors such as learning or pain response are achieved through disparate neural architectures; for instance, associative learning occurs in vertebrates via synaptic plasticity in the hippocampus, while in invertebrates like the sea slug Aplysia, it relies on different cellular mechanisms involving sensory-motor neurons.^[36] This variation suggests that psychological kinds are not tied to specific physical realizations, as evolutionary adaptations produce functional equivalence through diverse substrates.^[5] A key piece of evidence comes from homoplasy, or convergent evolution, where unrelated species independently evolve similar functions using non-homologous structures. Echolocation, for example, enables prey detection in bats through laryngeal production of ultrasonic pulses processed by auditory cortex specialized for Doppler shifts, whereas in dolphins, it involves nasal clicks modulated by the melon organ and analyzed via a distinct thalamic pathway.^[37] Despite these structural differences, the functional outcome—precise spatial mapping—is equivalent, illustrating how selective pressures can yield the same higher-level capacity via multiple physical routes.^[38] Such cases underscore the non-uniqueness of realizations in nature. From a statistical perspective, given the vast array of evolutionary pathways and environmental pressures, it is unlikely that mental states would map one-to-one with physical states across all instances. Hilary Putnam argued that functional states like pain could be realized in innumerable ways, from carbon-based organisms to hypothetical silicon-based systems, making unique type-identity improbable.^[5] Similarly, Jerry Fodor emphasized in his account of special sciences that psychological generalizations, such as those governing learning or belief formation, hold across diverse physical bases without requiring reduction to a single neurological kind, as the heterogeneity of realizers ensures the autonomy of higher-level laws.^[19] This probabilistic reasoning supports multiple realizability as a default expectation in biologically complex systems.

A Priori Argument

Hilary Putnam developed an a priori argument for multiple realizability through his theory of functionalism, positing that mental states are defined by their functional roles rather than their intrinsic physical composition. Specifically, Putnam introduced the concept of functional isomorphism, according to which a mental state, such as pain, is characterized by its causal relations to sensory inputs, behavioral outputs, and other mental states within an organism's overall functional organization.^[39] Any physical system that realizes this isomorphic structure—exhibiting the same input-output patterns and internal causal transitions—would thereby instantiate the same mental state, regardless of the underlying physical mechanisms. For instance, Putnam argued that any system capable of passing a Turing test, by demonstrating equivalent behavioral responses to environmental stimuli, would possess the corresponding mental states, underscoring the independence of psychological properties from particular physical substrates.^[39] This reasoning proceeds a priori from the semantic nature of psychological predicates, which denote functional roles without presupposing any specific physiological realization. Putnam contended that terms like "pain" or "belief" are not analytically equivalent to descriptions of particular brain processes, as their meanings are exhausted by the abstract causal and relational properties they pick out.^[40] Consequently, the identity of a mental state with a physical state cannot be established a priori, since psychological concepts are tied only to the satisfaction of functional conditions that multiple physical kinds could fulfill. This structural argument establishes multiple realizability as a conceptual necessity inherent in the way mental states are defined, independent of empirical investigation into actual biological or artificial systems.^[40] An instructive analogy in Putnam's framework compares mental states to software programs, which can execute the same computations on diverse hardware platforms, such as different computer architectures, as long as the hardware supports the required functional operations. Just as the program's identity resides in its algorithmic structure rather than the silicon chips or circuits implementing it, mental functions remain invariant across varying physical media that preserve the relevant causal roles.^[39] This software-hardware distinction highlights the a priori detachment of psychological properties from specific material bases, reinforcing the potential for multiple physical realizations. Building on Putnam's ideas, Ned Block and Jerry Fodor advanced machine functionalism, viewing psychological states as abstract states in a machine defined by its state transitions and inputs/outputs, akin to a Turing machine table. They argued that such abstract machines admit of multiple physical implementations, as any device—biological, electronic, or otherwise—that duplicates the machine's functional table realizes the same psychological states. For example, a thermostat's functional state of "regulating temperature" can be embodied in mechanical, electronic, or pneumatic systems, illustrating how the same functional description applies across heterogeneous physical realizations without empirical contingency. This approach solidifies the a priori case by emphasizing the definitional priority of functional specifications over concrete implementations.

Arguments Against

Reductionist Counterexamples from Science

One prominent scientific counterexample to multiple realizability arises in thermodynamics, where temperature seems to be realized differently across physical media—for instance, as the average molecular kinetic energy in gases and as the mean vibrational energy of atoms in solids—yet these diverse realizations are unified under a single physical property: the average kinetic energy of microscopic particles in thermal equilibrium.^[41] This reduction demonstrates that apparent multiplicity at a higher level does not preclude a unified lower-level explanation, as the kinetic theory of gases and extensions to solids via statistical mechanics provide a cohesive account without disjunctive bridge laws.^[41] Similar patterns appear in other sciences. In biology, gene expression occurs through varied mechanisms, such as prokaryotic transcription without introns versus eukaryotic splicing, but these processes reduce to shared principles of DNA as the informational substrate, with expression governed by universal molecular interactions like base pairing and polymerase activity. Likewise, in chemistry, the kind "water" manifests in diverse contexts—liquid at room temperature, solid as ice under pressure, or vapor in steam—yet it is invariantly realized as H₂O molecular aggregates, with macroscopic properties derivable from quantum mechanical and statistical descriptions of those molecules.^[42] Philosopher Joseph Levine has critiqued appeals to multiple realizability by arguing that such multiplicity often reflects superficial functional descriptions rather than irreducible diversity, as deeper scientific analysis reveals underlying physical unity that bridges the explanatory gap between higher- and lower-level properties.^[43] These examples suggest that for mental states, neuroscience may similarly uncover unifying physical realizations, potentially enabling type-identity reductions akin to those in thermodynamics and chemistry, rather than sustaining non-reductive autonomy.^[44]

Disjunctive Realization Problem

The disjunctive realization problem arises in the context of multiple realizability when a mental property, such as pain, is implemented by a heterogeneous array of physical states across different systems, resulting in a disjunctive physical base (e.g., C-fiber stimulation in humans or analogous neural activity in octopuses or silicon-based processes in hypothetical robots).^[45] Philosopher Jaegwon Kim, in his critiques from the late 1980s and 1990s, argued that this disjunction renders psychophysical laws—correlations between mental and physical properties—too heterogeneous and non-nomic to function effectively in scientific explanation.^[45] For instance, a law stating that "pain is realized by Nh ∨ Nr ∨ Nm" (where Nh, Nr, and Nm represent distinct realization bases in humans, reptiles, and Martians, respectively) lacks the homogeneity needed for projectibility, as the causal powers of these diverse bases may vary unpredictably, preventing unified generalizations like "pains cause avoidance behavior."^[45] This heterogeneity undermines the explanatory power of psychological laws, as disjunctive predicates fail to support the predictive and nomological structure required for a mature science.^[45] Kim likened such disjunctions to the property "jade," which encompasses both jadeite and nephrite microstructures but does not yield robust laws due to their differing physical properties, suggesting that multiply realized mental kinds similarly disqualify psychology as an autonomous discipline.^[45] Without homogeneous bases, mental properties cannot inherit consistent causal roles from their realizers, leading to fragmented explanations that cannot generalize across instances.^[45] In response, Jerry Fodor advocated for genus realism, positing that higher-level mental kinds like pain function as genuine scientific categories (genera) despite their multiple physical realizations (species), allowing projectible laws based on shared functional roles rather than disjunctive physical descriptions.^[46] Fodor distinguished multiply realized properties from mere disjunctions by emphasizing their cross-world applicability and relational nature, arguing that this preserves the autonomy of special sciences like psychology.^[46] However, Kim countered that this defense begs the question, as it assumes the legitimacy of mental kinds without addressing why disjunctive realizations should not render them explanatorily inert in the same way physical disjunctions are.^[45] Ultimately, the disjunctive realization problem challenges the independence of special sciences by implying that their laws lack the unity and explanatory efficacy needed to stand apart from more fundamental physical theories, potentially requiring reduction to species-specific local laws rather than broad generalizations.^[45]

Causal Closure of the Physical

The principle of the causal closure of the physical, a cornerstone of physicalism, asserts that every physical event has a sufficient physical cause occurring at the same time.^[47] Formulated prominently by Jaegwon Kim in the 1990s, this principle implies that no non-physical causes are needed to explain physical occurrences, thereby closing the physical domain to external influences.^[48] This principle underpins Kim's exclusion argument against mental causation in non-reductive physicalism, where mental states are distinct from but supervenient on physical states.^[48] According to the argument, if a mental event M (such as an intention to raise one's arm) causes a physical event P (the arm rising), and M is realized by a physical event R (neural firing), then causal closure ensures R sufficiently causes P.^[49] The exclusion principle further states that no event can have more than one sufficient cause at the same time, leading to either causal overdetermination—where both M and R independently cause P, which is deemed implausible for systematic cases—or epiphenomenalism, rendering mental events causally inert.^[48] Multiple realizability intensifies this challenge by allowing a single mental property, like pain, to be instantiated by diverse physical realizations across different organisms (e.g., C-fiber stimulation in humans or analogous processes in octopuses).^[45] In such scenarios, the varied physical bases R1, R2, etc., each fully account for the physical effect P under causal closure, making the higher-level mental property appear causally irrelevant, as it adds no explanatory power beyond its disjunctive physical realizations.^[45] This exacerbates the exclusion problem, as the mental property cannot be reduced to a single physical type, yet its causal efficacy seems preempted by the heterogeneous lower-level causes.^[48] Philosophers have proposed responses to preserve mental causation. Compatibilist approaches argue that mental properties can be causally efficacious as higher-level descriptions that inherit or specify the causal powers of their physical realizers, without violating exclusion by treating mental and physical causation as aspects of the same process.^[50] Alternatively, advocates of downward causation suggest that mental events can influence physical realizations, thereby affecting outcomes without overdetermining them, often framed within emergentist views where higher-level properties exert selective control over lower-level dynamics.^[50]

Limits of Generalizability

Critics of the multiple realizability thesis argue that it overgeneralizes from a narrow set of cases, failing to account for the varied constraints on mental properties across different levels of analysis. While the thesis posits that mental states can be realized by diverse physical mechanisms, particularly across species, evidence suggests that this does not extend universally to all psychological kinds. For instance, broad functional properties like general pain responses may appear multiply realized in coarse-grained comparisons between humans and octopuses, but such examples rely on abstracted descriptions that overlook underlying physiological specificities.^[51] Philosopher Barry Loewer, in collaboration with Ernest LePore, has highlighted that multiple realizability applies selectively, holding for certain higher-level functional properties but not for all mental states, particularly those with fine-grained structures. Fine-grained mental states, such as specific qualia or nuanced intentional contents, may necessitate particular neural architectures or types, limiting their realization to specific biological substrates rather than arbitrary physical systems. This selectivity challenges the thesis's claim of broad autonomy for psychology, as not every mental property exhibits the disjunctive realizations assumed in interspecies analogies. The evidential base for multiple realizability further weakens under scrutiny, as it predominantly draws from coarse examples like "pain" without addressing finer distinctions that may be species-unique or even individual-specific. Qualia, the subjective experiential aspects of mental states, for example, appear tied to human-like neural processing in ways that resist realization in non-mammalian systems, suggesting insufficient empirical support for universal generalizability. Moreover, the thesis's scope is primarily demonstrated at interspecies levels—such as shared behaviors across vertebrates—but falters at intraspecies or individual scales, where neural variability within a single organism or population does not consistently yield distinct realizations of the same mental kind.^[52]^[53] Recent analyses of neuroplasticity reinforce these bounds on generalizability. Amber Maimon and Meir Hemmo (2022) examine cases of brain reorganization following injury, such as cortical remapping in stroke patients, and conclude that while plasticity allows some functional recovery, it operates within strict physical and nomological constraints that prevent unbounded multiple realizations. Neural plasticity demonstrates variability in realization but remains tethered to the underlying physical laws of the brain, undermining claims of radical independence for mental states from their neural bases. This bounded variability indicates that multiple realizability, even where present, is far more restricted than the thesis typically portrays.^[54]

Empirical and Modern Perspectives

Evidence from Biology and Neuroscience

Neuroplasticity in the brain has been cited in debates on multiple realizability, illustrating how cognitive functions can be maintained through rewiring of neural pathways following injury or adaptation, though its support for the thesis remains contested. For instance, after damage to specific brain regions, such as in stroke patients, remaining intact areas can reorganize to compensate and restore functions like motor control or language processing, indicating that the same psychological property can be realized by different neural configurations.^[54] However, critics argue that such plasticity does not necessarily support multiple realization, as the rewired pathways may still involve type-identical mechanisms at a finer-grained level, potentially aligning more with identity theories rather than disjunctive realizations.^[54] Comparative neuroscience further bolsters the case for multiple realizability through examples of homologous functions achieved via non-homologous structures across species. Spatial memory, for example, is realized in mammals primarily through the hippocampus, which encodes place cells for navigation, whereas in birds, a functionally equivalent hippocampal formation handles similar tasks despite lacking direct homology to the mammalian structure, relying instead on analogous circuits for cache recovery and orientation.^[55] Similarly, long-term memory consolidation exhibits multiple realizations at the biochemical level: in the sea slug Aplysia, it involves distinct protein kinases and synaptic growth mechanisms compared to the cAMP-PKA pathways in mammals, yet both sustain analogous associative learning and retention.^[56] These interspecies variations highlight how cognitive kinds can be implemented by diverse physical substrates while preserving functional outcomes. Empirical methods like fMRI and lesion studies offer direct tests of functional equivalence across neural variants, supporting multiple realizability within species. Lesion research, tracing back to Broca's 1861 observations, shows that language production persists after damage to Broca's area through recruitment of perisylvian regions, with recovery patterns varying by individual neural architecture.^[23] Complementing this, fMRI studies reveal distributed activations for visual object recognition, where distinct extrastriate pathways in different individuals achieve equivalent perceptual processing despite anatomical differences. Challenges to these findings arise from debates over homology versus homoplasy in neural structures, where homologous brain regions (e.g., conserved across mammals) often realize functions similarly, undermining claims of multiple realization, while homoplasious traits (convergent evolution in distant species) provide stronger evidence.^[38]

Applications in AI and Computation

Multiple realizability manifests in artificial intelligence through diverse architectures that achieve analogous cognitive functions, such as pattern recognition and decision-making. For instance, deep neural networks (DNNs) process language and visual tasks via distributed, data-driven representations, while symbolic AI employs rule-based logic and explicit knowledge structures to accomplish similar outcomes like theorem proving or planning. This divergence illustrates how the same psychological or computational states—such as inference or categorization—can be realized without shared physical mechanisms, supporting the thesis that cognitive capacities are substrate-neutral rather than tied to specific implementations.^[28] Substrate independence further underscores multiple realizability in AI, positing that computational processes underlying intelligence can operate across varied hardware without altering functional outputs. Large language models (LLMs), developed post-2020, exemplify this by simulating reasoning and natural language understanding on graphics processing units (GPUs), which excel in parallel matrix operations, yet could theoretically run on quantum hardware to handle probabilistic computations more efficiently.^[57]^[58] Debates surrounding multiple realizability in AI often center on whether such functional equivalence implies consciousness, with John Searle's Chinese Room argument critiquing the notion that syntactic symbol manipulation—common in both neural and symbolic systems—equates to genuine understanding or sentience. Searle contends that even perfectly simulating cognitive tasks on non-biological substrates fails to produce intentionality, challenging functionalist views of multiple realizability as a pathway to machine minds. Counterarguments, drawing from functionalism, assert that integrated causal roles across architectures could yield emergent understanding, as seen in LLMs' contextual responses, though empirical verification remains elusive.^[59]^[60] From 2023 to 2025, AI ethics discussions have increasingly addressed multiple realizability's implications for sentience in non-biological systems, prompting calls for governance frameworks to protect potentially conscious AIs as of November 2025. Surveys indicate growing public attribution of sentience to advanced models, with 20% of U.S. adults in 2023 believing some AI systems possess it, amid ongoing ethical concerns over welfare and rights. Reports from organizations like Effective Altruism highlight gaps in international AI policies, advocating recognition of substrate-independent moral status to mitigate risks like exploitation of sentient machines. These debates emphasize precautionary principles, balancing innovation with safeguards against unintended harms to non-human entities.^[61]^[62]^[63]

References

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