In the philosophy of mind, the multiple realizability thesis contendsthat a single mental kind (property, state, event) can be realized bymany distinct physical kinds. A common example is pain. Manyphilosophers have asserted that a wide variety of physical properties,states, or events, sharing no features in common at that level ofdescription, can all realize the same pain. This thesis served as apremise in the most influential argument against early theories thatidentified mental states with brain states (psychoneural, ormind-brain identity theories). It also served in early arguments forfunctionalism. Nonreductive physicalists later adopted this premiseand these arguments (usually without alteration) to challenge allvarieties of psychophysical reductionism. The argument was even usedto challenge the functionalism it initially was offered to support.Reductionists (and other critics) quickly offered a number ofresponses, initially attacking either the anti-reductionist oranti-identity conclusion from the multiple realizability premise, oradvocating accounts of the reduction relation that accommodatedmultiple realizability. More recently it has become fashionable toattack the multiple realizability premise itself. Most recently thefirst book-length treatment of multiple realizability and itsphilosophical import has appeared.

This entry proceeds mostly chronologically, to indicate the historicaldevelopment of the topic. Its principle focus is on philosophy of mindand cognitive science, but it also indicates the more recent shift inemphasis to concerns in the metaphysics of science more generally. Itis worth mentioning at the outset that multiple realizability has beenclaimed in physics (e.g., Batterman 2000), biochemistry (Tahkoforthcoming) and synthetic biology (Koskinen 2019a,b). After more thanfifty years of detailed philosophical discussion there still seems tobe no end in sight for novel ideas about this persistent concern.

• 1. Multiple Realizability Arguments
  • 1.1 What is Multiple Realizability?
  • 1.2 Initial Multiple Realizability Arguments Against “Reductive” Mind-Brain Identity Theories
  • 1.3 Early Multiple Realizability Arguments for Functionalism
  • 1.4 The Multiple Realizability Argument from Psychological Explanations
  • 1.5 Appeals to Multiple Realizability in a Token System Over Time
• 2. Replies to Multiple Realizability Arguments: The First Forty Years
  • 2.1 One-Way Bridge Principles in Nagel Reductions
  • 2.2 Domain-Specific Identities and Reductions
  • 2.3 Appeals to Neuroscience’s Successes
  • 2.4 Challenging the Individuation of Mental Kinds
  • 2.5 Reduction and Identity Theory Reconceived
  • 2.7 Lower Level Reductive Unity?
• 3. The Shifting Status of Multiple Realizability Early in the New Millennium
  • 3.1 The Empire Strikes Back
  • 3.2 New Attacks on Multiple Realizability
• 4.The Multiple Realization Book and Its Initial Critical Reception
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Multiple Realizability Arguments

1.1 What is Multiple Realizability?

The multiple realizability contention about the mental is that a givenpsychological kind (like pain) can be realized by many distinctphysical kinds: by different brain states in earthly biologicalpain-bearers, by electronic states in properly programmed digitalcomputers, by green slime states in imagined extraterrestrials, and soon. Correctly characterizing the realization relation remains acontentious matter in analytic metaphysics (Gillett 2003; Polger 2004)and this issue quickly reaches detailed technical depths. But whateverthe correct account of realization turns out to be, about whateverkinds turn out to be related by realization, the multiplerealizability contention about the mental holds that a givenpsychological kind (like pain) can stand in that relationship to manydistinct physical kinds. Further discussion of this issue withnumerous references will arise insection 3 below.

1.2 Initial Multiple Realizability Arguments Against “Reductive” Mind-Brain Identity Theories

Hilary Putnam introduced multiple realizability into the philosophy ofmind. Challenging the “brain state theorists”, who heldthat every mental kind is identical to some yet-to-be-discoveredneural kind, Putnam (1967) notes the wide variety of terrestrialcreatures seemingly capable of experiencing pain. Humans, otherprimates, other mammals, birds, reptiles, amphibians, and evenmollusks (e.g., octopi) seem reasonable candidates. But then, for the“brain state theory” to be true, there must be some“physical-chemical kind” common to this wide variety ofpain-bearing species, and correlated exactly with each occurrence ofthe mental kind. (This is a necessary condition of the hypothesizedtype-identity.) Biological considerations speak intuitively againstthis requirement. Neuroanatomy differs across terrestrialbrain-bearing creatures, especially at systems and circuit levels;function, especially sensory functions, are increasingly“corticalized” as cortical mass increases across species.Convergent evolution generates similar phylogenic traits in organismsnot closely related due to their having to adapt to similarenvironments or ecological niches. Such realizers of the same functionwill highly likely be distinct “physical-chemical”kinds.

In addition, Putnam (1967) points out that early mind-brain identitytheorists insisted that these identities, while contingent, hold byvirtue of natural (scientific) law. So then any physically possiblepain-bearer must also be capable of possessing that physical-chemicalkind. This point marks the introduction of the crucial distinctionbetween multiple realization and multiplerealizability into the discussion, as well as the well-knownphilosophers’ imagined metaphysical possibilities. Silicon-basedandroids, artificially intelligent electronic robots, and Martianswith green slime pulsating within their bodies all seem to be possiblepain realizers. But they lack “brain states” comparable toours at any level of physical description. Further still, thesemind-brain identity theories were supposed to be completely general.Every mental kind was held to be identical to some neuralkind. So the critic needs to find only one mental kind, shared acrossthese structure-types yet realized differently at thephysical-chemical level. Putnam (1967) acknowledges that the earlyidentity theories were being offered an empirical hypothesis. But thisconsequence was “certainly ambitious”, and almostcertainly false.

One quick word on this distinction between multiplerealizability and realization, for it marks a pointwhere treatment of this topic in metaphysics and cognitive sciencediverged. Metaphysicians, many of whom came quickly to reject thecontingent identity claims of the early mind-brain identity theoristsin favor of the necessity of identity claims, focused on multiplerealizability, since the possibility of distinct physicalrealizers of the same psychological kind was sufficient to block anysuch identity between realized kind and any one of its possiblerealizers. Philosophers who took their cue from the cognitive andbrain sciences focused on proposed instances of multiplerealization, of actual instances of the relation amongexisting cognizers. The emphasis in this entry will be on the latterissue, although some discussion of the metaphysical issues will ariseinsection 2 andsection 3 below.

Stated in canonical form, Putnam’s original multiplerealizability argument draws an anti-identity theory conclusion fromtwo premises:

1. (the multiple realizability contention) (At least) some mentalkinds are multiply realizable by distinct physical kinds.
2. If a given mental kind is multiply realizable by distinct physicalkinds, then it cannot be identical to any one (of those) specificphysical kind.
3. (the anti-identity thesis conclusion) (At least) some mental kindsare not identical to any one specific physical kind.

In this simple form, this argument is deductively valid.

Jerry Fodor (1974) extended Putnam’s initial argument by arguingthat reductionism imposes too strong a constraint on acceptabletheories in special sciences like psychology and economics. Accordingto Fodor, reductionism is the conjunction of “tokenphysicalism” with the claim that there are natural kindpredicates in an ideally completed physics corresponding to eachnatural kind term in any ideally completed special science. Hecharacterizes “token physicalism” as the claim that allevents that science talks about are physical events. Token physicalismis a logically weaker thesis than reductionism or type-typephysicalism. To illustrate why, consider the following string ofnumerals:

1 1 2

This string contains two types of numerals (1 and 2), but three tokensof the two types (two tokens of the numeral type 1 and one token ofthe numeral type 2). Mental states are similarly ambiguous. When youand I both entertain the belief that Fodor advocated a Language ofThought, one type of mental state is entertained, but two tokens ofthat type occur (your belief state and my belief state). Type-typephysicalism insists that each mental state type is identical to somephysical state type, i.e., some specific brain state type on themind-brain identity theory. This view runs afoul of multiplerealizability. But token physicalism only insists that each tokenoccurrence of each type of mental state is identical to some tokenoccurrence of some physical state type—not necessarily a tokenoccurrence of the same physical state type on each occasion, e.g.,when you and I, with our brains differentiated by our separate lifehistories, entertain the same belief about Fodor’s favoritephilosophy of mind.

Fodor gave reductionists the best-known account of reduction at thattime, the one at work in Paul Oppenheim and Hilary Putnam’s(1958; Fodor subtitles his 1974 as “The Disunity of Science as aWorking Hypothesis”) and culminating in Ernest Nagel’s(1961) “derivability” account of intertheoretic reduction.According to Fodor, this “classic” account“connects” disparate elements of the reduced and reducingtheories’ vocabularies by way of “bridge laws” (notNagel’s term!) and claims a reduction when the laws of thereduced theory are derived from the laws of the reducing theory plusthe bridge laws. According to Fodor (1974), if reductionism is toestablish physicalism, these cross-theoretic bridge laws must assert(contingent) identities of reduced and reducing kinds. But givenmultiple realizability, the only way this can obtain is if thephysical science constituent of a psychophysical bridge law is adisjunction of all the terms denoting possible physical realizationsof the mental kind. Given the extent and variety of actual (not to saypossible) physical realizations, Fodor insists that it isoverwhelmingly likely that the disjunctive component will not be akind-predicate of any specific physical science. It is alsooverwhelmingly likely that the disjunctive component will not appearin any genuine law of some specific physical science. Multiplerealizability thus demonstrates that the additional requirementimposed by reductionism, beyond that of token physicalism, isempirically untenable. (For a recent argument that multiplerealization is consistent with Oppenheim and Putnam’s unity ofscience via reductions, see Shapiro 2018)

1.3 Early Multiple Realizability Arguments for Functionalism

The multiple realizability premise was also part of early argumentsfor functionalism. Functionalism in the philosophy of mindindividuates mental states in terms of their causes and effects. Pain,for example, is caused by tissue damage or trauma to bodily regions,and in turn causes specific beliefs (e.g., that one is in pain),desires (e.g., that one relieves the pain), and behaviors such ascrying out, nursing the damaged area, and seeking out pain relievingdrugs. Any internal state that mediates a similar pattern of causesand effects is of the mental type pain, regardless of the specificphysical mechanisms that realize that pattern of causes and effects inany given case. Ned Block and Jerry Fodor (1972) note that themultiple realizability of mental types on physical types shows thatany physicalist type-identity hypothesis will fail to be sufficientlyabstract. Functionalism, on the other hand, seems to be at the correctlevel of abstraction up from explanations of behavior based onphysical mechanisms. In particular, it seems sufficiently abstract tohandle multiple realizability. Block and Fodor (1972) also note thatmultiple realizability at the level of physical description is acommon characteristic of ordinary functional kinds, like mousetrapsand valve lifters. So it appears to be a reasonable empiricalhypothesis that characterizing mental kinds as functional kinds is atthe correct level of abstraction.

Some contemporary nonreductive physicalists deny that mental kinds arecorrectly identified with functional kinds. A popular criticism ofthis sort hinges on issues about individualism in psychology. ButPutnam later used multiple realizability itself to argue againstfunctionalism, arguing that mental kinds are both“compositionally” and “computationally”plastic. The first point is his familiar contention of the multiplerealizability of the mental on the physical. The second contends thatthe same mental kind can be a property of systems that are not in thesame computational state (see Putnam 1988 for details). In thisargument, multiple realizability strikes back against the very theoryof mind it initially was used to defend.

1.4 The Multiple Realizability Argument from Psychological Explanations

Psychologist Zenon Pylyshyn (1984) appeals to multiple realizabilityto ground a methodological criticism of reductionism about psychology.He describes a pedestrian (in the U.S., say), having just witnessed anautomobile accident, rushing into a nearby phone booth and dialing a 9and a 1. (For readers unfamiliar with phone booths, suppose insteadthat he pulls out his cell phone and punches those numbers.) What willthis person do next? With very high probability, he will dial (push)another 1. Why? Because in the U.S., 911 is the universal emergencynumber and there is a systematic generalization holding between theevent he witnessed, his background knowledge, resulting intentions,and that subsequent action (all intentionally described).

We won’t discover that generalization, however, if we focus onthe person’s neurophysiology and resulting muscularcontractions. That level of explanation logically is too weak, for itcannot tell us that the prior sequence of neural events and muscularcontractions will lead to the subsequent action of dialing another 1.A physiological explanation for any given instance will only link oneway of learning the emergency phone number to one way of coming toknow that an emergency occurred to one sequence of neural events andresulting muscular contractions producing the behavior(nonintentionally described). However, the number of physical eventsconstituting each of these cognitive classes—the learning, thecoming to know, and the action of dialing—is potentiallyunlimited, with the constituents of each class often unrelated to eachother at the physiological level of description. (This isPylyshyn’s appeal to multiple realizability.) So if there is ageneralization at the higher level of description available forcapturing—and in the example sketched above, surely thereis—an exclusively reductionist approach to psychologicalexplanation will miss it. Thus because of multiple realizability,reductionism violates a tenet of scientific methodology: seek tocapture all capturable generalizations. Such methodological argumentsdid not end with Pylyshyn’s. Fodor (1975: Chapter 5) and TerenceHorgan (1993) raise related methodological caveats about reductionismbased on multiple realizability. Bickle (1998: Chapter 4) respondsdirectly to these.

1.5 Appeals to Multiple Realizability in a Token System Over Time

Anti-reductionists quickly came to stress the more radical form ofmultiple realizability mentioned insection 1.1 above, in a token system over times. As far back as the late 1970s,Ned Block (1978) insisted that the narrowing of psychological kindsrequired to render them identifiable with specific physical (i.e.,neural) kinds due to the more radical type of multiple realizabilitywould render psychology incapable of capturing most allgeneralizations across realizers. Ronald Endicott (1993) givesBlock’s point empirical backing by noting detailed facts aboutplasticity in individual human brains. The capacity for distinctneural structures and processes to subserve a given psychologicalfunction owing to trauma, damage, changing task demands, development,and other factors is extensive. These facts count further against anyproposed reduction of, or identities between psychological andphysical kinds. Horgan (1993) clearly appeals to this radical sense ofmultiple realizability when he writes:

Multiple realizability might well begin at home. For all we now know(and I emphasize that we really donot now know), theintentional mental states we attribute to one another might turn outto be radically multiply realizable at the neurobiological level ofdescription,even in humans; indeed, even inindividual humans; indeed, even in an individual humangiven the structure of his central nervous system at a singlemoment of his life. (1993: 308; author’s emphases)

This radical sense of multiple realizability was widely adopted amongnonreductive physicalists, whose solution to the mind-body problemprobably remains the most popular position in Anglo-Americanphilosophy of mind. Putnam’s original multiple realizabilityargument against the mind-brain identity (see§1.2 above) remains central to this view, with the second premise nowreplaced with:

(2′)

If some mental kinds are multiply realizable in this radicalsingle-token-system-over-time sense, then psychology cannot be reducedto any physical science;

and Putnam’s original conclusion replaced with:

(3′)

Psychology cannot be reduced to any physical science.

2. Replies to Multiple Realizability Arguments: The First Forty Years

2.1 One-Way Bridge Principles in Nagel Reductions

Robert Richardson (1979) argues that Fodor’s multiplerealizability challenge to reductionism (see§1.2 above) rests on a misunderstanding of Ernest Nagel’s“classic” account of intertheoretic reduction. AlthoughNagel’s (1961) detailed examples of historical cases ofintertheoretic reduction in science all involve biconditionalcross-theory “conditions of connectivity” (what Fodor andothers called “bridge laws”), one-way conditionalconnections expressing sufficient conditions at the reducing level areall that his “principle of derivability” requires.Richardson cites passages from Nagel (1961) indicating that Nagelhimself saw the force of this point. Multiple realizability onlychallenges necessity (and nondisjunctive) reducing conditions, and sois not a challenge to a projected, distinctively Nagelian reduction ofpsychology to some physical science. However, this response seems notto dispose of the original argument’s force against mind-brainidentity theory.

2.2 Domain-Specific Identities and Reductions

David Lewis (1969) argues that the inconsistency between the identitytheorist’s thesis and multiple realizability evaporates when wenotice a tacit relativity of the former to contexts. A common senseexample illustrates his point. The following three claims appearlogically inconsistent:

(1)

There is only one winning lottery number.

(2)

The winning lottery number is 03.

(3)

The winning lottery number is 61.

These three similar claims likewise seem inconsistent:

(1′)

(the identity theorist’s thesis) There is only onephysical-chemical realization of pain.

(2′)

The physical-chemical realization of pain is C-fiber firing.

(3′)

The physical-chemical realization of pain is … (somethingelse entirely).

(2′) and (3′) reflect the multiple realizabilitycontention. But there is no mystery in how to reconcile (1)–(3).Append “per week” to (1), “this week” to (2),and “last week” to (3). Similarly, append “pernervous system-type” to (1′), “in human nervoussystems” to (2′), and “in mollusk nervoussystems” to (3′). Inconsistencies evaporate. Lewis’spoint is that identity claims are always specific to a domain. Onemight wonder whether Lewis’s point generalizes to capture thescience fiction examples Putnam hinted at in passing (see§1.2 above). Lewis was certainly aware of Putnam’s (1967)argument’s reach (Lewis offers this argument as part of a reviewof the book in which Putnam’s piece first appeared). So we canassume Lewis thought domain-specific reductions generalize tothese metaphysically possible cases as well. (See Lewis’s ownelaboration of this argument in his 1983.)

Many reductionist philosophers subsequently elaborated onLewis’s point with the help of scientific examples. PatriciaChurchland (1986: Chapter 7), Clifford Hooker (1981), Berent Enç(1983), and other philosophers of science have described historicalintertheoretic reductions where a given reduced concept is multiplyrealized at the reducing level. A common example is the concept oftemperature from classical equilibrium thermodynamics. Temperature ina gas is identical to mean molecular kinetic energy. Temperature in asolid, however, is identical to mean maximal molecular kinetic energy,since the molecules of a solid are bound in lattice structures andhence restricted to a range of vibratory motions. Temperature in aplasma is something else entirely, since the molecular constituents ofa plasma have been ripped apart. Even a vacuum can have a(“blackbody”) temperature, though it contains no molecularconstituents. “Temperature” of classical thermodynamics isthus multiply realized microphysically in a variety of distinctphysical states. Yet this is a “textbook” intertheoreticreduction in the history of science. It is just that the reduction,and the subsequent ontological identifications across theories, arespecific to the domain of physical state.

Lewis’s original response also underlies Jaegwon Kim’s(1989, 1992) appeals to structure-specific “localreductions”. Kim agrees that multiple realizability rules out ageneral reduction of (structure-independent) psychology to somephysical science. But it permits, and even sanctions, a localreduction of psychology to a theory of the physical mechanisms ofgiven structure-types. (Kim admits that the relevant structure-typeshere will probably be narrower than biological species.) Localreductions involve “structure-specific bridge laws” wherethe mental-physical biconditional occurs as the consequent of aconditional whose antecedent denotes a specific structure-type (e.g.,“ifX is a member of structure typeS, thenX is in mental stateM if and only ifX is inphysical stateP”). Conditionals whose antecedents denotedifferent structure types will typically have biconditionals asconsequents whose mental term-constituents are co-referential butwhose physical term-constituents denote different physical events.Multiple realizability forces this much revision to the bridge laws ofclassical reductionism. But according to Kim, domain-specific localreductions are the rule rather than the exception in sciencegenerally, and are sufficient for any reasonable scientific orphilosophical purpose.

2.3 Appeals to Neuroscience’s Successes

Jaegwon Kim (1992) suggests, and John Bickle (1998: Chapter 4)emphasizes that guiding methodological principles in contemporaryneuroscience assume continuity of underlying neural mechanisms acrossthe nervous systems of different species. This assumption informs manyneuroscience experimental techniques and conclusions drawn fromexperimental results. Continuity is assumed both within and acrossspecies. If radical multiple realizability of the sort championed byanti-reductionist philosophers actually obtained among existentspecies, contemporary neuroscientific experimental techniques andpractices built upon this continuity assumption should bear littlescientific fruit. Why experiment on the macaque visual system toinvestigate human visual processing, if we can’t safely assumesome fairly strong continuity across these species? Why shouldpositron emission tomography (PET) and functional magnetic resonanceimaging (fMRI) reveal common areas of high metabolic activity duringpsychological task performance, both across and within individualhumans—now down to less than a millimeter of spatial resolutionin fMRI—if multiple realizability were as prevalent orwidespread as many nonreductive philosophers assume? (see§1.5 above). Standard neuroscientific experimental procedures, and evenclinical diagnostic tools widely used by practicing neurologists andneuropsychologists, would seem hopelessly naïve in the face of commonphilosophical assertions about the extent of multiple realizability.But these procedures and tools do work, and are not hopelesslynaïve.

So Kim and Bickle draw the modus tollens conclusion, and insist thatthese neuroscientific successes are evidence that psychological kindsare not as widely or as radically multiply realized in brains as manyfunctionalists and nonreductive physicalists assume. It turns out thateven celebrated neural plasticity is systematic within and acrossnervous systems. It has a regular progression following damage to aprincipal structure; there are underlying neural mechanisms thatsubserve it, evolutionarily conserved across species. Furthermore, thenewly realized function following brain damage is typically seriouslydegraded compared to initial performance. Persons can still talk,manipulate spatial representations, or move their extremities, buttheir performances are typically qualitatively and quantitativelyworse than normals. This fact gives rise to tricky questions aboutindividuation of psychological function. Are these alternative neuralstructures realizingthe same psychological function,thesame mental kind, as before, as the philosophical uses ofmultiple realizability require? (This last response has been furtherdeveloped more recently; see§2.4 and§3.1 below.)

William Bechtel and Jennifer Mundale (1999) provide extensiveempirical details about hypothesized or assumed brain type-identitiesacross species in neuroscientific practice. Their explicit target is amethodological consequence sometimes drawn from the multiplerealizability premise: if psychological states are multiply realizedacross biological species, then neuroscience will be of little usetoward understanding cognition. But as details of the neuroscience ofvision demonstrate, neuroscientists have successfully usedunderstanding of the brain to decompose visual function. Theneuroscientific goal has been to

show how functional considerations get built into developing thestructural taxonomy and how that taxonomy in turn can be a heuristicguide in developing information-processing models. This project hasnot been impaired by multiple realization of psychological states;rather, it relies on the assumption that there is a common realizationof mechanisms for processing visual information across species. (1999:201)

It is difficult to argue against contemporary neuroscience’sempirical successes. So even if one accepts multiple realizability inits original type-type form advocated by Putnam (see§1.2 above), Bechtel and Mundale’s argument cautions against drawingstrong consequences from it about psychology’smethodological autonomy from neuroscience.

2.4 Challenging the Individuation of Mental Kinds

Over the past two decades philosophers have been more willing thanpreviously to challenge the truth of the multiple realizabilitypremise itself (see§3.2 below.) One approach of this kind challenges the way proponents ofmultiple realizability individuate mental kinds.

An early version of this argument is from Nick Zangwill (1992), whoconcludes that multiple realizability across biological species has“never been proven”. The multiple realizability contentionassumes a type-identity of mental kinds across species. According toZangwill this assumption is problematic, given that the obvioussensory and motor differences across species by themselves yielddifferent cause-and-effect patterns at all but the grossest level ofdescription. If successful, Zangwill’s challenge undercuts themultiple realizability argument by denying that the same mental kindsobtain across species, to be realized by different physicalmechanisms.

Lawrence Shapiro (2000) also contends that philosophers are too quickto claim that a single given kind is multiply realized. Someproperties of the realizers are relevant to the purposes, activities,or capacities that define a given functional kind, but otherproperties are not. Consider corkscrews. That functional kind can be“multiply realized” in two tokens that differ only intheir color. That physical difference does not make them genuinelydifferent realizations of corkscrew, however, because that one makesno relevant difference, that is, no difference to their performancesas corkscrews. Similarly for two corkscrews that differ only in thatone is made of aluminum and the other of steel. Although thatcompositional difference might matter for some functional kinds, itdoesn’t matter for corkscrews. As Shapiro notes,

steel and aluminum arenot different realizations of awaiter’s corkscrew because, relative to the properties that makethem suitable for removing corks, they are identical. (2000: 644)

Establishing relevant multiple realizability requires pointing toproperty differences in the realizers that make for a functionaldifference for that kind of thing.

Shapiro then points out that this requirement sets up a dilemma forproponents of multiple realizability. Consider what appears to be agenuine case of multiple realizability, that is, two objects that“do the same thing” but in very different ways. Either therealizing kinds genuinely differ in their causally relevant propertiesor they do not. If they do not, then we don’t really have a caseof multiple realizability (like the corkscrews that differ only incolor or metal composition). If they do so differ, then they genuinelyare different kinds. But then functionally they are not the same kind,and again we don’t have an instance of multiplerealizability—of asingle functional kind with distinctrealizations.

The usual justification for grouping distinct realizers under a singlefunctional kind is that such a classification reveals interestingsimilarities, of the sort we expect to be captured by laws orgeneralizations of some higher level science. But according toShapiro, when the realizing kinds differ significantly in theircausally relevant properties for the function at issue, any sharedlaws or generalizations are “numbingly dull” (2000: 649),e.g., all realizers of mouse traps can be used to catch mice, or bothcamera eyes and compound eyes have the function of enabling theorganism to see. Shapiro remarks:

If [functional kinds] share many causally relevant properties, thenthey are not distinct realizations … If they have no or onlyfew causally relevant properties in common, then there are no or justa few laws that are true of both of them. (2000: 649)

The first horn acknowledges a single functional kind but denies thatit is multiply realized. The second horn undercuts the principalreason for grouping genuinely different physical kinds under a singlefunctional kind. Shapiro concludes that taken together these two hornsblunt any claim ofmultiple realizations ofthe samefunctional type. His argument has been central to more recent disputesas concerns about multiple realization shifted to the metaphysics ofscience (see§3).

Shapiro’s dilemma is in the spirit of an earlier “causalpowers” argument from Jaegwon Kim (1992) Kim insists thatscientific kinds are individuated by their causal powers, and thecausal powers of each instance of some realized kind are identical tothose of its realizer. From these principles it follows that instancesof a mental kind with different physical realizations are distinctkinds because of the distinct causal powers of their realizers. Astructure-independent mental kind is not a causal kind, and hence isdisqualified as proper scientific kinds. Multiple realizability yieldsthe failure of structure-independent mental kinds to meet a standardof what makes something a scientific kind.

Mark Couch (2004) presses a related dilemma. Defending a claimedmultiple realization involves two steps. Proponents must show

1. that the physical states (of the realizers) are type distinct,and
2. that the functional properties are type identical.

Challenges to claimed multiple realizations can attack either step,and most importantly, the step challenged can differ from case tocase. Successfully challenging either step blocks any multiplerealizability argument appealing to that kind. As we saw in theprevious section, Bechtel and Mundale (1999) describe cases in whichcognitive neuroscientists treat the physical realizers (brain states)as type-identical across species, thus attacking step (i). In othercases—Couch’s example is primate versus octopuseyes—one can appeal to easily-found differences in functionalproperties, attacking step (ii). The two types of eyes have differentvisual pigments in their photoreceptors, different retinas, anddifferent ways of focusing light. These physical differences lead tostraightforward input-output (functional) differences: in the opticstimuli the two eyes respond to, in reaction times, and more. Theirfunctions may be similar, but similarity isn’t identity and themultiple realization argument since Putnam’s original version(see§1.2 above) requires identity at the realized functional kinds.Cross-species functional similarities are often quite superficial,especially across species from widely differing taxa (a point stressedby both Couch and Shapiro). In actual scientific practice, discoveredphysical (neural) differences typically incline psychologists tosearch for functional differences. Couch’s point is that theindividuation of psychological states, like the individuation of brainstates, is an empirical issue; Shapiro and Couch both suggest thatclaims about multiple realizability often rely heavily on“folk” psychological intuitions about individuating mentalkinds.

Bechtel and Mundale (1999) note that proponents of multiplerealizability often appeal to different amounts of“granularity” in how they individuate mental versusneurobiological kinds. Proponents often analyze psychological statesat a coarse-grained level, in which only the loosest input-outputsimilarities across species are deemed sufficient for mental kindidentities. Yet they insist on very fine-grained individuation forbrain states, in which small differences across species are sufficientfor neural type-differences. But psychological ascriptions admit offiner grains and neural ascriptions admit of coarser grains. Bechteland Mundale insist that when a common grain is chosen for both,mental-neural type-identities holding across species are found.

These responses quickly attracted critical counters. Carl Gillett(2003) argues that Fodor and other proponents of multiplerealizability assume a “Dimensioned” view of realizationthat allows realizer/realized properties to be instantiated in thedistinct individuals that bear part-whole relations to oneanother. Shapiro instead assumes a “Flat” view ofrealization, which demands that realizer/realized properties beinstantiated in thesame individual. Gillett argues thatShapiro’s arguments fail under the Dimensioned view ofrealization, and that he never defends the Flat view over theDimensioned view. Gillett concludes that failing to directly addressthe nature of the realization relation more generally vitiatesShapiro’s critique, leaving it as simply begging the questionagainst Fodor’s and others original defenses of the multiplerealization premise. These more general accounts later emerged todominate more recent discussions (see§3 below).

2.5 Reduction and Identity Theory Reconceived

The more radical type of multiple realizability seems to forceincreasingly narrower domains for reductions to be relativized; at theextreme, to individuals at times. This much “localreduction” seems inconsistent with the assumed generality ofscience. To avoid this problem, some philosophers of science havesuggested significant changes to the “classical”(Nagelian) account of (intertheoretic) reduction to enable the accountto handle multiple realizability of reduced kinds, a feature of anumber of “textbook” cases of scientific reductions.

Following suggestions by Clifford Hooker (1981) and Berent Enç(1983), John Bickle (1998: Chapter 4) argues that even the radicaltype of multiple realizability (in the same token system over timesense) is a feature of some widely-accepted historical scientificreductions. For example, it occurs in the reduction of classicalequilibrium thermodynamics to statistical mechanics and microphysics.For any token aggregate of gas molecules, there is an indefinitenumber of realizations of a given temperature: a givenmeanmolecular kinetic energy. Microphysically, the most fine-grainedtheoretical specification of a gas is its microcanonical ensemble, inwhich the momentum and location (and thus the kinetic energy) of eachmolecule is specified. Indefinitely many distinct microcanonicalensembles of a token volume of gas molecules can yield the samemean molecular kinetic energy. Thus at the lowest level ofmicrophysical description, a given temperature is vastly multiplyrealizable in the same token system over times. Nevertheless, the caseof temperature is a textbook case of scientific reduction. So eventhis radical type of token-token multiple realizability is no barrierin principle to reducibility. An actual case in science exemplifiesit.

To accommodate multiple realizability, Hooker (1981: Part III)supplements his general theory of reduction with an account of“token-to-token” reductions. He notes explicitly that thissupplement builds the possibility of handling multiple realizabilitydirectly into his revised definition of the reduction relation. LetS be the predicate, “satisfies functional theoryF”,T be the class of systems to which the tokensystem in question belongs,S′ be an appropriatepredicate in some lower level theory of T-system causal mechanisms,andT* be the class of systems to whichS′applies. Then, according to Hooker,

systems of typeS of classT are contingentlytoken/token identical with systems of typeS′ in classT* =_df every instance (token) of a typeS system externally classified as in classT iscontingently identical with some instance (token) of a typeS′ system externally classified as in classT*. (1981: 504)

By “externally classified”, Hooker refers to the sort ofcross-classification that holds across differentdeterminable/determinate hierarchies. To address some self-assessedshortcomings of Hooker’s own formulation of his general accountof reduction, Bickle (1998) reformulates Hooker’s insights,including his token-token reduction supplement, within aset-theoretically characterized “semantic” account oftheory structure and relations. Bickle’s account draws directlyon formal work from “structuralist” philosophy of science.More recently Klein (2013) advocated for a semantic “sets ofmodels” account of theory structure as a strategy foraccommodating multiple realized reduced kinds.

Other reconceptions of both reduction and the mind-brain identitytheory have been proposed explicitly to handle multiple realizability.Elliott Sober (1999) insists that a reductionist thesis actuallyfollows from the multiple realizability premise. He begins byattacking Putnam’s (1967) “objective” account ofsuperior explanation, namely that one explanation is superior toanother if the former is more general. According to Putnam, superiorexplanations “bring out the relevant laws”. But Soberreminds us that explanatory generalizations at lower levels bring outmore details. Science “aims for depth as well as breadth”and there is no “objective rule” concerning which endeavoris “better” (1999: 550). Both reductionists andanti-reductionists err in privileging one aim at the expense of theother. Sober then notes that multiplerealizabilitypresupposes some form of asymmetric determination: the lower levelphysical properties that are present at a given time determine thehigher level properties that are present. But this assumption commitsits proponents to the causal completeness of physics, the thesis thatphysical effects have only physical causes (a doctrine that Sobersketches toward the end of his 1999). If one is also concerned withcausal explanation—that is, if one holds that individualoccurrences are explained by citing their causes—then the causalcompleteness of physics in turn commits multiple realizabilityproponents to physics’ possessing an important kind ofexplanatory completeness that all other sciences lack. This is“reductionism of a sort” (Sober 1999: 562).

William Bechtel and Robert McCauley (1999) develop a version of“heuristic” mind-brain identity theory (HIT) and defend itexplicitly against multiple realizability. HIT insists that identityclaims in science typically are hypotheses adopted in the course ofempirical investigations, which serve to guide subsequent research.They are not conclusions reached after empirical research has beenconducted. Concerning the multiple realizability of psychological onbrain (physical) states, cognitive neuroscience’s heuristicidentity claims assert type-commonalities in comparative studiesacross species, not type-differences. Bechtel and McCauley illustratetheir hypothesis with case studies: Brodmann’s early twentiethcentury work mapping the brain into functionally relevant areas;Ferrier’s late-nineteenth century work employing electricalstimulation to cortex; and more recent detailed maps of visualprocessing regions in the primate brain. All of these landmarkfunctional anatomical studies used multiple species. As Bechtel andMcCauley remind us,

when they consider theories of mind-brain relations, philosophers seemto forget that the overwhelming majority of studies have been onnon-human brains. … Although the ultimate objective is tounderstand the structure and function of the human brain,neuroscientists depend upon indirect, comparative procedures to applythe information from studies with non-human animals to the study ofthe human brain. (1999: 70–71)

Heuristic psychoneural type-identity claims across species are keycomponents of these standard neuroscientific procedures.

When reduction or identity theory gets reconceived in ways built toaccommodate multiple realizability, are reductionists/identitytheorists and proponents of multiple realizability then simply talkingpast one another? Perhaps not. It is worth reminding ourselves thatmany nonreductive physicalists have employed multiple realizability toargue againstall forms of psychophysical reductionism. Ifbetter general accounts of scientific reduction or identity theorymake room for multiple realizability, these demonstrations countagainst this broader challenge. And if “nonreductive”physicalists were to reformulate their position to oppose only aspecific brand of psychophysical reductionism or identity theory, thatwould weaken their position significantly, perhaps to the point ofshowing that it targets a straw person, but also making it compatiblewith forms of “reductive” physicalism. In fact, thisreconception move on the part of reductionists and identity theoriststraces back to Fodor’s (1974) essay. While Fodor’sarguments there explicitly targeted a reductionism built on theclassical Nagelian account, he suggests in footnote 2 that

what I shall be attacking is what many people have in mind when theyrefer to the unity of science, and I suspect (though I shan’ttry to prove it) that many of the liberalized versions of reductionismsuffer from the same basic defect as what I shall take to be theclassical form of the doctrine.

The problem was, even back then, new work on (intertheoretic)reduction in the philosophy of science was attempting explicitly tomake room for multiple realizability of reduced on reducing kinds.Fodor’s “shan’t trying to prove” thegenerality of his attack against even the earliest of thosereconceptions begged explicitly one of the questions they wereaddressing.

2.7 Lower Level Reductive Unity?

In searching for reductive unity underlying the variety of cognitivesystems, Paul Churchland (1982) once recommended descending“below” neurobiology and even biochemistry, to the levelof nonequilibrium thermodynamics. He insisted that finding reductiveunity there was more than a bare logical possibility because of someparallels between biological processes, whose multiply realized kindsfind reductive unity there, and cognitive activity, especiallylearning.

Concerning Pylyshyn’s (1984) attack on reductionist methodology(see§1.5 above), Patricia Churchland (1986: Chapter 9) suggests thatfunctional theories are constructed in lower level sciences. Newlevels of theory thus get inserted between those describing thestructure of the lower level kinds and those of purely functionalkinds: between, for example, the physiology of individual neurons andcognitive psychology. We might find a common neurofunctional propertyfor a given type of psychological state across a wide variety ofdistinct brains (or even the same brain over time). And if the scopeof the macro-theory doesn’t extend beyond that of itsmicrofunctional counterpart, then reduction will be achieved despitevast multiple realizability at the microstructural level.Neurocomputational approaches that have blossomed since the early1990s give real empirical credence to Churchland’ssuggestion.

Bickle (2003) claims that if we leave our neuroscientificunderstanding at the systems level, psychoneural multiplerealizability seems obvious. Neural systems differ significantlyacross species. But neuroscience does not stop its inquiries at thesystems level. As it moves further down, into cellular physiology andincreasingly into the molecular biology of nervous tissue, identitiesof mechanisms across species have been found. Many molecularmechanisms of neural conductance, transmission, and plasticity are thesame, from invertebrates through mammals. This matters for psychologybecause mechanisms of cognition and consciousness are increasinglybeing found at these levels. Bickle’s key example is memoryconsolidation, the conversion of labile, easily disrupted short-termmemories into more durable, stable long-term form. Work with fruitflies, sea slugs, and mice has revealed the role of the cyclicadenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMPresponsive-element binding protein (CREB) signaling pathway in keyforms of experience-driven synaptic plasticity. Across these verydistinct taxa, this molecular intracellular signaling pathways hasbeen implicated experimentally in memory consolidation. By altering asingle protein in this cascade (using biotechnology and moleculargenetics), experimenters have built mutant organisms whose short-termmemory remains intact (as does their sensory, motor, and motivationalcapacities), but which cannot consolidate these short-term memoriesinto long-term form. Bickle quotes with approval statements like thefollowing, from insect biologists Josh Dubnau and Tom Tully:

In all systems studied, the cAMP signaling cascade has been identifiedas one of the major biochemical pathways involved in modulating bothneuronal and behavioral plasticity. … More recently,elucidation of the role ofCREB-mediated transcription inlong-term memory in flies, LTP and long-term memory in vertebrates,and long-term facilitation inA. californica [a sea slug]suggest that CREB may constitute a universally conserved molecularswitch for long term memory. (1998: 438)

Memory consolidation is just one psychological phenomenon, and so its“ruthless” reduction to molecular events doesn’testablish a general claim about unitary mechanisms across widelydivergent taxa for other shared cognitive kinds. For that argument,Bickle turns to principles of molecular evolution. The first principleholds that evolution at the molecular level—changes to the aminoacid sequence of a given protein—is much slower in functionallyimportant (“constrained”) domains than in functionallyless important ones. The second principle is that molecular evolutionis much slower in all domains of “housekeeping” proteins,especially in ones that participate in cell-metabolic processes inmany tissue types. These two principles imply that these molecules,their domains, and the intracellular processes they participate inwill remain constant across existing biological species that share thecommon ancestor that first possessed them. (This is what Dubnau andTully refer to above as a “universally conserved”molecular switch.) In the end, any psychological kind that affects anorganism’s behavior must engage the cell-metabolic machinery inindividual neurons. In the brain, causally speaking, that’swhere the rubber meets the road. But that’s the machineryconserved across existing biological species—changes to it,especially its functionally constrained domains, have (almost)inevitably been detrimental to an organism’s survival. So weshould expect that the molecular mechanisms for any causallyefficacious cognitive kind be “universally conserved”. Thediscovery of these shared mechanisms of memory consolidation is notsome isolated case, but follows from the core principles of molecularevolution. As “molecular and cellular cognition” proceeds,we should expect more examples of unitary realizers—ruthlessreductions—across species for shared psychological kinds

3. The Shifting Status of Multiple Realizability Early in the New Millennium

Over the first decade of the twenty-first century, nonreductivephysicalism retained its status as the most popular position inAnglo-American philosophy of mind. Its proponents continued to appealto the standard multiple realizability arguments (see§1 above) to challenge all versions of psychophysical reductionism andidentity theory. However, the challenges to multiple realizability andthe arguments using it, scouted insection 2 above, began to attract wider notice. New versions of type-identitytheory and reductive physicalism began to be formulated more widely(Gozzano and Hill, 2012). The nonidentity of mental content propertieswith any physical properties no longer seemed to be the“practically received wisdom”, as Ernest LePore and BarryLoewer (1989) called it thirty years ago, although it has clearlyremained an entrenched view.

Some of these arguments continued lines introduced insection 2 above. But they warrant separate treatment, not just to keep mypresentation historically accurate, but also because more generalconsiderations from the metaphysics of science increasingly came tomotivate specifics of the new discussions, although some of these moregeneral features had been scouted previously in works focused onphilosophy of mind (see especially William Lycan 1987).

3.1 The Empire Strikes Back

Expectedly, proponents of multiple realizability quickly responded tothe myriad challenges outlined insection 2 above. Carl Gillett and Ken Aizawa were perhaps the most vocaldefenders of multiple realizability against a number of them. Gillett(2003) develops a precise framework for understanding compositionalityrelations in science generally, and uses this framework to defineproperty realization and multiple realization, and to distinguishfurther “flat” from “dimensioned” realization.Recall fromsection 2.4 above that “flat” realization involves both realized andrealizing properties inhering in a single object.“Dimensioned” realization involves realized and realizingproperties inhering in distinct individuals that stand in acompositional relationship to one another. For Gillett (2002, 2003),this distinction is important for two reasons. First, scientificexplanations employ dimensioned realizations, because inter-levelmechanistic explanations relate distinct individuals. Second, Fodorand other proponents of the standard multiple realizability argumentswere assuming a dimensioned account, while the arguments of Shapiroand other recent critics (see§2 above) challenge the existence of multiple realizability only byassuming a flat realization. According to Gillett, no recent proponentof multiple realizability has assumed or defended flat realization asthe correct account involved in any of the scientific cases atissue.

Explicitly applying Gillett’s precise framework for realizationand related notions, Ken Aizawa and Gillett defend the existence ofmultiple realization in a variety of sciences (2009a). They also arguefor the “massive multiple realization” of properties atevery level of organization, from the structure and function ofproteins in neurons to those of human cognition to those of socialinteractions (2009b). Their detailed scientific example in the latteressay is visual processing. They contend that neuroscientists, unlikephilosophers, are unfazed by massive multiple realization. Multiplerealization has been so contentious in philosophy of mind, theyinsist, because philosophers tacitly assume flawed or scientificallyunfounded accounts of realization, like the flat view. According toAizawa and Gillett (2009b), philosophers uncritically accept anarrative that links multiple realization to the strict methodologicalautonomy of psychology from neuroscience. They also contend that theempirical details of vision research show that a co-evolutionaryresearch methodology is not just consistent with, but explicitlymotivated by massive multiple realization. So this uncriticallyaccepted philosophical narrative not only blinds philosophers to factsthat scientists recognize as unproblematic; it is also empiricallyfalse.

Soon afterwards, Aizawa and Gillett (2011) distinguish two strategiesscientists might adopt to deal with putative cases of multiplerealization. One strategy is simply to take multiple realization atface value and live with multiply realized kinds. The other is tosplit the higher-level multiply realized kind into a variety ofsub-kinds, one for each of its distinct lower level realizers, andthen eliminate the original higher-level kind, at least for thepurposes of further scientific investigation. Do scientists alwaysfavor the second strategy, as recent philosophical critics of multiplerealizability would seem to recommend? With its well-knowndistinctions between different types or systems, memory research wouldseem to be a clear instance of using such an“eliminate-and-split” strategy. Yet, Aizawa and Gillettargue, such an assessment oversimplifies the actual scientific detailsin even this much-discussed case. Here too they sense an importantgeneral methodological lesson: psychology took account of neurosciencediscoveries, so even when taking multiple realization at face valuescientists do not advocate strict methodological autonomy. But theactual details of how psychology takes neuroscientific discoveriesinto account depends both on the nature of the psychological kinds inquestion and the needs of theorizing specific to psychology.

Aizawa has also replied to a number of the challenges to the standardmultiple realizability argument scouted insection 2 above. After separating three distinct arguments in Bechtel andMundale (1999;§2.3 above), Aizawa (2009) sets his critical sights on their CentralArgument, which argues against multiple realization from the existenceand continued success of brain mapping studies. He argues that Bechteland Mundale misrepresent the actual nature of these studies, and moregenerally of the methods employed in functional localization studies.Working with exactly the scientific examples Bechtel and Mundalediscuss (mostly from the functional neuroanatomy of vision), Aizawaargues that claims about psychological functions do not play thespecific role in these studies that Bechtel and Mundale insist. So thesuccess of these studies does not imply the falsity of multiplerealization. Later in that paper Aizawa challenges two of Bechtel andMundale’s key assertions. He denies that if psychologicalproperties were multiply realized, then functional taxonomy of thebrain would have to be carried out independently of psychologicalfunction. And he denies that multiple realization rules outcomparisons of brains across different species. Hence all the premisesof Bechtel and Mundale’s Central Argument are false. Aizawa(2007) criticizes Bickle’s (2003) argument that a unitaryrealization of memory consolidation across species has been found atthe level of molecular mechanisms, despite widespread neuraldifferences in these brains at higher levels of neuroscientificdescription (§2.3 above). According to Aizawa, the protein components of theseevolutionarily conserved molecular mechanisms, and the NNA sequencescoding for them, are themselves multiply realized across species.Aizawa (2013) presented numerous scientific examples of multiplerealization by “compensatory differences”. In such cases,changes to one or more properties that jointly realize a realizedpropertyG are compensated for by changes in others of thejointly realizing properties. Although his overall goal in this paperis to bring this form of multiple realization to wider recognition andstudy by philosophers of science, he uses the broader“Gillett-Aizawa framework” to argue that highly specificdeterminate properties, not just generic determinable properties, aremultiply realized in this specific fashion. Such multiply realizeddeterminate properties are indeed exactly similar across distinctrealizations, and so answer the dilemma posed by Shapiro and others (§2.4 above).

Carrie Figdor (2010) challenges criticisms of multiple realizationthat appeal to neuroscience, citing explicitly the works discussed insection 2.3 andsection 2.7 above. She undertakes to demonstrate not only the empiricalplausibility of mind-brain multiple realization, but also to clarifythe terms of the empirically-focused debate. Her scientific focus ison “localizationism”, especially the “brain mappingprojects” using state-of-the-art functional neuroimaging (2010:420). Concerning the terms of the empirically-focused debate, Figdorargues that none of the multiple realization hypotheses common to thephilosophical literature is appropriate to this scientific endeavor.Instead she appeals to “degeneracy”, a concept fromgeneral biology meaning “the ability of elements that arestructurally different to perform the same function or yield the sameoutput”, a characteristic of complexity ranging from the geneticcode to immune systems to populations (e.g., Edelman and Gally 2001).This is the notion, Figdor claims, that has been co-opted by cognitiveneuroscientists to mean

for a given cognitive functionF, there is more than onenonisomorphic (nonidentical) structural element that can subserveF, either within an individual at a time, across individuals,or within an individual across times. (2010: 428)

A major reason for cognitive neuroscientist co-opting degeneracy inthis fashion is the frequency of anomalous results in functionalneuroimaging studies that show activations of brain areas that differacross subjects, or even in a single subject across performances ofthe same experimental task,. Figdor discusses numerous examples fromrecent cognitive neuroscience, and carefully separates cases ofdegeneracy that count as multiple realizations from others that donot. She concludes that her “sketch of degeneracy in cognitiveneuroscience suffices to show that [multiple realizability] is aviable empirical hypothesis (2010: 237). And she concludes byaddressing specifically Bechtel and Mundale’s (1999) empiricalarguments challenging multiple realizability from functional imagingstudies (see§2.3 above). Bechtel and Mundale’s claim that neuroscience has beenuseful for psychology is true, Figdor asserts, but does not challengea degeneracy-based empirical case for multiple realizability. AndBechtel and Mundale’s stronger claim, that neuroscientistsimplicitly assume the falsity of multiple realization as reflected intheir accepted practices, is itself shown to be false by the use ofdegeneracy in cognitive neuroscience.

3.2 New Attacks on Multiple Realizability

Critics of multiple realizability and the standard arguments it hasfigured into also extended their reach early in the twenty-firstcentury. Lawrence Shapiro (2008) raises some methodologicaldifficulties involved in testing whether a given psychological kindactually is multiply realized. (For a related argument see ThomasPolger 2009.) Shapiro reminds us of the crucial role that auxiliaryassumptions play in hypothesis testing generally (within a broadlyhypothetico-deductive model), and considers a collection of explicitauxiliary assumptions that might be implicitly used to establish amultiple realization hypothesis. He presents a recent ferretbrain-rewiring experiment as a scientific example, in which axonalinputs from the primary visual tract were redirected in ferret embryosto project to primary auditory cortex (see Sharma, Angelucci, &Sur 2000 for the scientific details). An auxiliary hypothesisrequiring multiply realized higher-level (in this case, psychological)kinds to be “exactly similar”—identical—acrossdistinct realizers won’t help the proponent of the standardmultiple realizability argument with this purported case. It is easyto measure better visual performance in the normally-wired controlferrets compared to the re-wired experimental animals. While theexperimentally re-wired animals have some visual function, it isdiminished significantly compared to controls. “Exactsimilarity” (identity) of visual function is thus not presentacross these groups. On the other hand, one might argue for themultiple realizability premise in this ferret re-wiring case using anauxiliary hypothesis that only requires similarity in multiplyrealized higher level properties, yet still requires that differencesacross the realizers should not be limited only to the differencesthat cause differences in the realized (in this case, visual)properties. Shapiro speculates that this auxiliary assumption seemsbest to capture the sense of multiple realization stressed byproponents of the standard argument. But if we adopt it, again theferret re-wiring case seems not to provide an empirical instance ofmultiple realization. Shapiro remarks: “the differences inferret brains explain nothing more than differences in ferret visualproperties” (2008: 523). Shapiro also argues that his detaileddiscussion of hypothesis testing difficulties for any multiplerealization hypothesis reveals a flaw in Bechtel and Mundale’s(1999) influential criticism (discussed in§2.3 above). Bechtel and Mundale’s examples, drawn from thecomparative functional neuroanatomy of vision, only comparedhomologous brain structures. But these only have differences that makea difference in their visual properties, nothing else. Instead ofthese examples, Shapiro insists, “one should be looking atdifferent brains that reveal similar visual properties despite theirdifferences” (2008: 524)—exactly the kinds of evidencethat Bechtel and Mundale’s emphasis on homologies doesn’tconsider.

Shapiro and Polger (2012) build upon Shapiro’s account of thecomplexity of actually testing for scientifically-justified multiplerealization. They insist that these actual details render thesignificance of multiple realization far more dubious thanphilosophers of mind typically suppose. They introduce explicitcriteria to capture the common assumption that multiple realizationrequires not merely differences between realizing kinds, but“differently the same”: the features of entitiesAandB that lead them to be classified differently by therealizing scienceS₂ “must be among those thatlead them to be commonly classified” by the realized scienceS₁ (2012: 282, criterion iii). This explicitcriterion rules out the popular assertion that camera eyes versuscompound eyes, with different photoreceptive chemicals in theirretinal cones, is a genuine (empirical) instance of multiplerealization. Considered coarsely, such eyes are doing the same thingin the same way, so they’re not “differently thesame”. Considered finely, the two kinds of eyes are sensitive todifferent ranges and peaks of spectral stimulation, so they’re“differently different, not differently the same” (2012:283–284).

Shapiro and Polger’s final explicit criterion captures the“differently the same” intuition in terms of quantitativedifferences. The relevant variation between entitiesA andB in realizing scienceS₂ “must begreater than” the individual differences betweenA andB recognized by the realized scienceS₁(2012: 282, criterion iv). The variation recognized by the realizingscience must not merely map onto individual differences betweenA andB recognized by the realized science. The demandsin actually establishing multiple realization are thus quite strict.Not any old variation will do. According to Shapiro and Polger, thesestrict demands show both that multiple realization in the senserequired to fund the standard argument is “a relatively rarephenomenon”—despite the vast variability everywhere in theworld—and that a “relatively modest” mind-brainidentity theory has little to worry from it (2012: 284).

Similar in some ways to Couch’s arguments (discussed in§2.4 above), Colin Klein raises a challenge to the scientificcontributions made by multiply realized kinds. Noting the variety ofthings that materials science classifies as “brittle,”Klein (2008) notes that few to none of the many scientific discoveriesabout realization-restricted brittle things—for example, aboutbrittle steel—generalize to other realization-restrictedtypes—such as brittle glass. Klein insists that generalizationsabout genuine scientific kinds should be projectable across instancesof those kinds, so this requirement seems not to be met by asignificant class of multiple realized kinds, namely, therealization-restricted ones. Applying this point to psychologicalkinds, instead of supporting a scientifically-backed nonreductivephysicalism, it appears rather that special sciences should abandonmultiply realized kinds. Klein notes that proponents ofscientifically-based multiple realizability can find terms in specialsciences that figure in legitimate explanations, and so appear torefer to projectable multiply realize kinds. But close investigationof some paradigmatic examples reveals these to be idealizations ofactual kinds. Special-science kind-terms are thus typically ambiguous.Sometimes a given term refers to an actual but realization-restrictedkind. Other times it refers to features of explanatory but non-actualidealized models. (Klein 2008 illustrates this ambiguity with hisdetailed example from materials science.) Neither suffices to providea kind of actual multiple realization that the standard argumentrequires. However, he insists that his argument isn’t entirelynegative for non-reductive physicalism. Idealizations can function inexplanations that are autonomous in an important sense from lowerlevel sciences. And furthermore, Klein argues, Jaegwon Kim’s(1992) assumption thatall explanatory work in science must appeal to realization-restrictedkinds and properties (discussed in§2 above), is simply incorrect. Still, Klein insists, there appear to beno actual and projectable—hence genuinelyscientific—multiple realized kinds.

Bickle (2010) questions whether the “second-wave” ofcriticisms of the standard argument, those that challenge the multiplerealization premise itself (discussed in§2.4 and this section above), give aid and comfort to psychoneuralreductionists. Since psychoneural reductionism was one of the explicittargets of the standard multiple realization argument, one mightplausibly assume that they do. Yet none of those who offer these“second-wave” challenges (Shapiro, Polger, Couch) identifythemselves as “reductionists” (with the possible exceptionof Shapiro, and more recently Bechtel 2009, although his response tothe multiple realization argument figures nowhere in his plumb for“mechanistic reduction”). Some of these“second-wave” critics, such as Polger 2004, are explicitlyanti-reductionist. (Though Polger 2004 is also explicitlyanti-anti-reduction. He argues that multiple realization has little ifanything to do with reduction.) This fact alone should give apsychoneural reductionist pause. Second, the direction that thesecond-wave debates have developed, starting with Gillett’s(2003) criticisms—deeply into the nature of the realizationrelation, and so deeply into the metaphysics of science rather thaninto science itself—should prompt the psychoneural reductionistwith a metascientific bent to simply tell the second-wavers, thanksfor nothing! Does that leave psychoneural reductionism back on itsheels, in light of the standard multiple realization argument? Not atall, Bickle (2010) insists. For the “first-wave”actual-scientific-history challenge to the first premise of thestandard argument, and the initial critical discussions (see§2.1,§2.2, and§2.5 above) turns out never to have been rejoined by anti-reductionists.Why not? Bickle speculates that metaphysics was the culprit.Non-reductive physicalists seem to have assumed that by rejoiningarguments of metaphysicans like Kim (1992), they thereby dismissed theentire first-wave of challenges. Many of the challenges discussed insection 2 above shows that this is not the case. There are numerous examples ofmultiply realized kinds that are components of scientific theorieswidely acknowledged to having been reduced to other theories. Somultiple realization alone is no barrier to actual scientificreduction. The detailed scientific cases that illuminate many of those“first-wave” challenge to the standard multiplerealizability argument remain unanswered to this day.

So well into the second decade of the new millennium, a renewedcritical interest in multiple realizability continued to generate newdiscussions and arguments on both sides, pro and can. The assumption,unfortunately still too common in philosophy of mind, that multiplerealizability “seals the deal” against reductivephysicalism and mind-brain identity theory, was mistaken back in the1970s and 1980s (see§2 above), and grew even more discredited by the full scope of publishedliterature on the topic that has followed since. But equallyunfortunately, the literature on multiple realizability had taken offin numerous varied directions, and following out any one of theseleads one quickly into detailed complicated and technical discussions,in both philosophy and science, and often at quite a distance fromthose who follow out arguments in other directions. And the fate ofone of the most influential arguments in late-twentieth centuryAnglo-American philosophy hangs in the balance, in all of thesedebates. By the second decade of the new millennium, concerns aboutmultiple realizability arguments had grown beyond their philosophy ofmind origins, into more general questions about relationshipsobtaining between sciences addressing higher and lower levels of theworld’s organization. What the issue needed now was a singlework that captured both the full scope of writings on this topic, andwith a focus unifying all of these diverging literatures. Fortunately,such a work appeared, and it and its critical reception is the focuson the next section.

4.The Multiple Realization Book and Its Initial Critical Reception

Thomas Polger’s and Lawrence Shapiro’sThe MultipleRealization Book (2016) provided what the increasinglyfragmenting literature on multiple realizability needed. It is thefirst book-length treatment of the topic, including many of thebroader arguments the topic had become part of. The book pullstogether much of the history of work on the topic, plus the morerecent work by both proponents and opponents, and usefully organizesall of this around numerous key themes that had come to frame themyriad debates. Polger’s and Shapiro’s goal is not simplyto review or clarify the various positions, however. They have aposition to push. They end up developing and defending a mind-brainidentity theory that explicitly recognizes a sense of the autonomy ofpsychology from neuroscience. And as one might expect, critics oftheir view quickly responded. The unifying focus that this bookoffers, and its initial critical reception, make a detailed discussionof it a useful focus for where the broader issues about multiplerealizability stand now, at the end of the second decade of thetwentieth century.

Polger and Shapiro begin by focusing on the key contention thatfunctionalists and “realization physicalists” haveemphasized to support their views over identity theories. That thesisis “Generality”, namely that

psychological states are general rather than species-bound, such thata range of creatures may possess the same psychological state. (2016:15)

And the key premise offered in support of this contention, going backto Putnam’s work (presented in§1 above) is multiple realizability. This contention sets the agenda forthe book: Polger and Shapiro will argue that there is not the sort ofwidespread multiple realization of mental states and processes thatwould be needed to support functionalism or realization physicalismover identity theory. Importantly, Polger and Shapiro do not deny thatcases of multiple realization exist. Rather, they insist that thedebate between functionalists and mind-brain identity theorists shouldbe guided primarily by the question,

does the best overall model of psychological and neuroscientificprocesses make substantial and important use of identities? (2016: 34)

Their goal is to defend an affirmative answer.

Polger and Shapiro’s book confirms that multiple realization hasof late come to be fought in the metaphysics of science literature. Inkeeping, they begin by offering an account of the ontologicalrealization relation. And following the lead of their (Shapiro &Polger 2012) paper (discussed in§3.2 above) they use this account to scout the types of evidence andconcerns relevant to deciding between realization physicalisms andidentity theories. They restrict the scope of their arguments toactual proposed cases of realization, eschewing at least initiallyconcern with philosophers’ imagined cases; they are solelyconcerned at the outset with multiple realization as an empiricalhypothesis. They are also careful to point out that multiplerealization is logically narrower than mere variation. Workingdirectly with Putnam’s (1967) discussion, Polger and Shapiroarticulate the “Basic Recipe” for finding occurrences ofmultiple realization: “Multiple realization occurs if and onlyif two (or more) systems perform the same function in differentways” (2016: 45). While this characterization requires furtherrefinement, even it is enough to rule out some kinds of“sameness” that are not evidence for multiple realization,i.e., sameness of behavioral output. Two systems exhibiting the samebehavior in the same circumstances, Polger and Shapiro insist, doesnot establish sameness of underlying psychological function generatingthat behavior.

Further refinements introduce “relevance” into Polger andShapiro’s account. The very function shared by the two kindsthat classifies them as the same in the realized science must be whatis realized by different mechanisms that classify them as different inthe realizing science. Further, the relevant differences in therealizers contribute to the sameness of function; the kinds are“not just differently different, they are differently thesame” (2016: 75). Polger and Shapiro show that simple (camera)and compound eyes meet their fully revised “OfficialRecipe” for genuinely multiply realized kinds. They write:

Multiple realization requires a special pattern of variation:relevantly the same function performed in relevantly different ways,where the differences contribute to the sameness in function and notjust to the differences in function. (2016: 77)

Armed with their Official Recipe, Polger and Shapiro next seek todismiss the evidence commonly put forward by proponents of multiplerealization as arguments for functionalism or realization-physicalismover mind-brain identity theory. They first consider “directevidence”, empirical cases presented as genuine instances ofmultiple realization of the mental on the physical. One type is“cortical functional plasticity”, in both“common” and “radical” varieties. A“common” kind is cortical reorganization, where neurons ina specific cortical region take on a new function following damage tonerves or axons providing their normal input. A more“radical” variety is exemplified by the well-known work indeveloping ferrets, in which retinal projections were redirected tothe medial, rather than the lateral geniculate nucleus in thethalamus, and hence retinal input was projected to auditory, ratherthan visual cortex (see the discussion of Shapiro 2008 in§3.2 above). Polger and Shapiro argue that both of these varieties fail tomeet at least one condition of their Official Recipe, so neithercounts as a genuine instance of mental-to-physical multiplerealization. The “common” case involves change infunction—in the specific example, neuronal activity in thereorganized region of primate somatosensory cortex now representsstimulation to a different region of the monkey’s hand. In the“radical” ferret rewiring case, the visual capacities ofthe experimental ferrets’ auditory cortex, compared to those ofintact control ferrets, differ significantly, e.g., the experimentalferrets show significantly deficient performances on visualdiscrimination tasks compared with controls’ performances.

A second kind of direct evidence that Polger and Shapiro consider iskind splitting in scientific practice. Again, their Official Recipefigures into their response. They reformulate Aizawa andGillett’s (2011) key example, the variations in cone opsins inhuman color vision (see discussion in§3.1 above) into Official Recipe format, and deny that it is a case ofgenuine multiple realization. The cited differences among the opsinsare not distinct from individual differences among those possessinghuman trichromatic color vision, violating one condition of theOfficial Recipe.

Polger and Shapiro then extend their discussion of evidence toconsider two “indirect” kinds for mental-to-physicalmultiple realizability. This evidence seeks to show that, inlight of various observations, the multiple realizabilityhypothesis is more likely to be true than the mind-brain identityhypothesis. They begin by offering a canonical form for indirectevidence arguments, which compares the likelihood of multiplerealizability versus the unlikelihood of mind-brain identities inlight of the evidence. They then recast Block and Fodor’s (1972)appeal to neural plasticity (see discussion in§1.3 above), an appeal to evolutionary convergence of psychological statesand processes, and an appeal to the “conceptualpossibility” of artificial minds into this canonical form, andargue that all three arguments fail. Finally they consider an appealto the prevalence of the “computational theory of mind”within current cognitive science and psychology as an indirectargument for multiple realizability. In agreement with some recentmechanists (e.g., Kaplan and Craver 2011), Polger and Shapiro insistthat many “computational explanations” are betterunderstood “as precise descriptions of the task or behavior tobe explained” rather than as explanations (2016: 154). And whilethey admit that the “representationalist” variety ofcomputationalism (of which Fodor’s 1975 Language of Thought is aparadigmatic example) does rule out the possibility of mind-brainidentities, they point out that this kind of computationalism is“not the only game in town” anymore (2016: 158). Numerousviable competing alternatives to it exist—Polger and Shapiromention connectionism, strong and “ruthless” reductionism,and mechanism explicitly in this context—and all those make roomfor mind-brain identities.

Polger and Shapiro wrap up the book by absolving their identitytheory, which rests on their treatment of multiple realizability, ofsome mistaken charges. They argue that their account is notontologically eliminativist about psychological kinds, because none oftheir three types of arguments against multiplerealization—“unificationist” arguments which findrelevant commonalities in the realizing kinds, “heuristic,abstraction, and idealization” arguments that characterizepractices in contemporary cognitive science, and their plump fortaxonomic “kind splitting”—leads to any kind oftroubling eliminativist conclusion. And by adopting JamesWoodward’s (2003) “interventionistdifference-making” account of causal explanation, Polger andShapiro deflect the charge that their identity theory undermines thelegitimacy of psychological explanations, and thus rules out anyautonomy for psychology vis-à-vis neuroscience. Such a charge, theyinsist, rests on an overly stringent and misguided understanding ofscientific explanation. Identity theorists can be and should bepluralists about scientific explanation. Neuroscientists can appeal toneural causes, psychologists and cognitive scientists to mentalcauses. Since both can cite causal invariances at their respectivelevels, both offer genuine explanations. Despite the mind-brainidentities, psychology remains a methodologically autonomous science.(Their defense of psychology’s autonomy resemblesWoodward’s recent more general defense of a general kind ofexplanatory autonomy, see Woodward forthcoming.)

We have dwelled in some detail on Polger and Shapiro’s (2016)book because of its scope and the unifying focus it offers forphilosophical discussions of multiple realization, both historical andrecent. Similarly, initial criticisms of the book ranged from onessquarely in philosophy of mind, to others aimed at more broadlymetaphysics of science considerations. Not surprisingly, given thatthe book defends a version of mind-brain identity theory, which hasbeen considered indefensible, or at best fringe, for some time, it metwith rapid criticism from philosophers of mind. Ronald Endicott (2017)takes Polger and Shapiro to task for offering few (if any) examples ofexplicit mind-brain identities. Endicott’s criticism is fairenough, in one sense. Polger and Shapiro nowhere attempt to catalogueor discuss specific identity claims. Neither, of course, did U.T.Place or J.J.C. Smart in their seminal works first defending the view;but those works were published sixty or more years ago, andneuroscience has progressed rapidly over that time. Polger and Shapirodo criticize numerous claims to have found multiple realization ofmind on brain, so perhaps those discussions can serve implicitly asmind-brain identity claims. But circa 2016, one reasonably could havehoped for some examples of specific mind-brain identities, and adefense of the evidence that supports them.

Umut Baysan (2018) raises two challenges. His first is squarely withinphilosophy of mind. He contends that Polger and Shapiro change“the rules of the game” over which functionalists/realization physicalists and identity theorists are disputing. Polgerand Shapiro saddle the former with defending a general hypothesis,i.e., all psychological kinds are functional kinds or realized kinds;while identity theorists seek only to defend a far weaker existentialclaim, i.e., that there are some instances of mind-brain identities.Second, and more generally, Baysan takes Polger and Shapiro to taskfor limiting their concern to the empirical hypothesis, multiplerealization, and refusing to speak to concerns about metaphysicallypossible realizers of mental kinds, or multiple realizability. Baysaninsists that some philosophers are still concerned with the latter,and a work that purports to be a book-length treatment of the generaltopic will leave these philosophers justifiably disappointed. On thefirst point, Baysan is right that Polger and Shapiro do seek toreorient this fifty-year-old debate, and they characterizefunctionalism and realization physicalism as quite general hypotheses.But Baysan’s account of Polger and Shapiro’s weakenedidentity theory seems to miss the point of their explicitreformulation of the multiple realization problem, into first andforemost the question,

does the best overall model of psychological and neuroscientificprocesses make substantial and important use of identities? (Polgerand Shapiro 2016: 34)

That reformulated focus requires a much stronger defense than doesBaysan’s existential hypothesis interpretation of their identitytheory. By it Polger and Shapiro are committed to arguing thatidentity theory provides “the best overall model” ofpsychology’s relationship to neuroscience. (Whether they succeedin making this argument is a separate issue.) And as far as a need todiscuss metaphysical possibilities, and hence multiple realizabilityin the sense that some philosophers understand it, that depends on whoone takes one’s audience to be. If one is concerned strictlyabout multiple realization as a part of the philosophy of the specialsciences—as some discussants of multiple realizationare—metaphysical possibilities and realizations of the mental in“metaphysically possible worlds” can strike one asphilosophical jibber-jabber, the kind of discussions that givephilosophy a bad name within broader twenty-first-century intellectualcontexts. Curiously, however, Polger and Shapiro seem not to want totake that extreme a position. They locate their work squarely in themetaphysics of science, not strictly in philosophy of scienceor metascience. Does that leave them open to the charge of needing toconsider at least some metaphysical possibilities, and so some kindsof multiple realizability? We will turn to this problem laterin this section, in light of Polger and Shapiro’s replies toother criticisms.

Danny Booth (2018) is sympathetic to Polger and Shapiro’sbroader project, and to their conclusion about the importance ofmind-brain identities in contemporary cognitive science. But heworries about their deference to scientists concerning how mentalkinds are individuated, and thus identified across distinct physicalrealizations, as their Official Recipe for multiple realizationproposes. He points out that many scientists view questions aboutmental kind individuation as “philosophical”, and so puntthis concern back to philosophers. Scientists also sometimesindividuate mental kinds on quite narrow grounds; Booth here citesJacqueline Sullivan’s recent discussions of the difficulty ofidentifying mental kinds across neuroscience labs that employdifferent protocols (see discussion of Sullivan’s work in the“Philosophy of Neuroscience” entry, this encyclopedia).Sometimes they do so purely on behavioral grounds which operationalizemental kinds for laboratory experimentation. The latter approach fliesdirectly in the face of Polger and Shapiro’s explicit rejectionof mental kind identity in their Official Recipe for multiplerealization. Booth also worries about Polger and Shapiro’s moregeneral appeal to an interventionist account of causation to salvage“actual” autonomy for psychology. Nothing stopsfunctionalists or realization physicalists from adopting a similarapproach to saving psychology’s autonomy, and in fact many availthemselves of this approach. Booth worries that

this leaves very little that distinguishes Modest Identity Theory frommore traditional versions of functionalism. (2018: 440)

That Polger and Shapiro’s view shares more points in common withtraditional functionalism than with traditionally“reductionist” versions of identity theory leaves Boothwondering whether Polger and Shapiro’s account itself faces someof the same challenges they direct at functionalism.

Mark Couch (2018) worries about Polger and Shapiro’s generalcharacterization of multiple realization in terms of the taxonomiesoffered by different sciences, as opposed to more typical accounts interms of properties or kinds in the world. About this shift, Couchfirst points out that scientists in any given field aren’talways in agreement about how to taxonomize the phenomena theyaddress. So appealing to “scientists’ taxonomies”might not be sufficient, and philosophers might be forced back toconsidering “independently existing kinds” (2018: 421).Polger and Shapiro’s refocused account also might not addresswhat has interested philosophers about multiple realization, namely,concerns about metaphysical relations holding between independentlyexisting properties and kinds; independent, that is, of any taxonomicpractices. This leads Couch to worry about Polger and Shapiro’saccount of “relevant differences” in their “OfficialRecipe”, especially their account of how we decide when thiscondition has been met. He senses an internal tension lurking here. Onthe one hand,

the sciences that deal with the putative realizers tell us whichdifferences in them amount to different ways of producing theireffects and which do not. (Polger and Shapiro 2016: 62)

But this “official view” seems at odds with a second“unofficial” way Polger and Shapiro cash “relevantdifferences”: “a matter of whether a property has anexplanatory role with respect to a function” (Couch 2018: 424).This “unofficial” understanding, Couch insists, seesrelevance as an explanatory relation between properties or kinds innature, and thus not merely a matter of what scientists “tellus”. These two readings of “relevant differences”strike Couch as making it unclear how the Official Recipe will applyto Polger’s and Shapiro’s own examples of multiplerealized kinds.

In contrast with Booth and Couch, who challenges Polger and Shapirofor deferring too much to scientists and their taxonomies, MazviitaChirimuuta chides them for not paying enough attention to actual“functional thinking” in the life sciences, and thusrelying too heavily on “toy examples” like corkscrews.Chirimuuta agrees with Polger and Shapiro that the oldphilosophers’ notion of multiple realization,“MR1.0”, needs replacing, but suggests looking directlyinto three areas of the life sciences for its replacement,“MR2.0”. One area is evolutionary approaches, whereChirimuuta mentions explicitly Ernst Mayr’s proximate/ultimateexplanation distinction, the latter referring to the evolutionaryfunction or “purpose” of a behavior, and so serves as a“necessary, non-replaceable completement” of the former(2018: 407). The second area is reverse engineering, which posits afunction in the nervous system and then works back to see how thephysical components perform it. The third area is the study ofrobustness in biological systems, whereby the function of a system ismaintained despite alterations in lower-level mechanisms. Chirimuutanotes that Criterion IV (the “differently the same”condition mentioned above) of Polger and Shapiro’s“Official Recipe” for genuine multiple realization wouldprobably preclude many cases of biological robustness, in the samefashion Polger and Shapiro used it to rule out the“radical” ferret rewiring example (discussed above). Butfor Chirimuuta, this consequence suggests that Polger andShapiro’s “Official Recipe” may be “missingsomething interesting about the organization of living systems”(2018: 408); and so may not be sensitive enough to real scientificpractice. She notes that for all their talk about relying onscientists’ judgments and taxonomies, Polger and Shapiro’sfavorite “toy example”, the corkscrew, comes up a lot whenthe conditions on genuine multiple realization are being articulatedand defended. Her point is that a closer look at some real sciencesthat engage in functional thinking, at the point when Polger andShapiro are developing and defending their basic analysis of multiplerealization, might have generated a different account of what multiplerealization is (“MR2.0”). As opposed to what Polger andShapiro actually do with real scientific examples, namely bring themup to debunk claims about successful multiple realization based ontheir already existing detailed analysis. Such an approach seeminglywould have produced an account tied much more closely to actualscientific practice.

In their (2018) Polger and Shapiro respond to these criticisms. Theybegin by clarifying what they call a common misconception of theirview (and not just among these critics), that they don’t takeseriously multiple realizability. They claim they do, butthey insist that even these metaphysical concerns “are notimmune from evidential demands” (2018: 448). The best evidencefor multiple realizability would be widespread instances ofmultiple realization; but this, as they argued extensively intheir (2016), is exactly what is lacking. Other kinds of evidence formultiple realizability include equipotentiality, evolutionaryconvergence, and computational explanations in the relevant sciences,but Polger and Shapiro insist that they have argued in their book thatthese kinds of evidence are lacking, too.

They then take up the two criticisms, one from both Booth and Couch,the other from Chirimuuta, which pull in opposite directionsconcerning their dependence on science. Booth and Couch both insistedthat Polger and Shapiro rely too heavily on taxonomies postulated byscientists to characterize genuine instances of multiple realization.Polger and Shapiro take both Booth’s and Couch’s worry tobe that their approach thereby shackles their account to “thewhim of scientists” (2018: 449). They remind us that by“taxonomy” they refer to “whatever a particularscience views as belonging within its domain” (2018: 449). Thesciences themselves provide those; the best evidence we can have forany science’s “ontological commitments” are“the taxonomic representations and models that it uses”(2018: 450). Both Polger and Shapiro, and their multiple realizationproponents, are assuming that the relevant sciences track realdistinctions in the world. Polger and Shapiro agree that sciences canand do mischaracterize these, but “that problem has to do withthe fallibility of evidence and not with our account of multiplerealization” (2018: 450). Polger and Shapiro also admit thattheir reference to the notion of taxonomies has not always been clear;but nevertheless appealing to scientific taxonomies as a form ofevidence for multiple realization is useful. Their focus on scientifictaxonomies is thus not incompatible with more traditionalphilosophical discussions about multiple realization.

They then turn to Chirimuuta’s contrary criticism, that indeveloping their account of multiple realization they ignore“functional thinking” in real biology and neuroscience.They begin by focusing on one of Chirimuuta’s claims, that

functional classifications capture regularities not apparent whendescription is restricted to more fine-grained vocabulary (be itphysical, chemical, or neuroanatomical),

with these classifications thus averting us to the “limitationsof purely reductive research agendas in biology andneuroscience” (Chirimuuta 2018: 409). If Chirimuuta is hereadvocating pluralism about scientific explanation, Polger and Shapiroare in full agreement; they are avowed methodological pluralists. Butif Chirimuuta is here advocating some kind of “EssentialAutonomy Thesis”, that higher-level regularities are“somehow invisible” to would-be reducing explanations, or“can only be captured in the proprietary vocabulary of a specialscience”, then Chirimuuta’s MR2.0 sounds suspiciously likeMR1.0, with the latter being a view she herself insists needs to berejected. They then insist that Chirimuuta’s own discussion ofthe role of functional thinking in biology and neuroscience stressesitsmethodological importance. Again, Polger and Shapiro haveno complaint about that. Their critical focus is on an inference fromthis pluralist methodology and the ability to model a system in thisfashion to the conclusion that the system isipso factomultiply realizable” (2018: 454–455). Being a fellowrejecter of MR1.0 and its accompanying Essential Autonomy Thesis,Polger and Shapiro insist that Chirimuuta should also reject any suchinference.

In closing this section, it should be clear that the extension ofphilosophical discussions about multiple realization, which began withthe “second-wave” of criticisms around the year 2000,continues apace. The specific philosophical arena in which thesediscussions take place continues to expand, from the topic’sorigins in mid-twentieth century philosophy of mind through itsturn-of-the millennium path into the philosophy of the cognitivesciences and neuroscience, and lately more squarely in the generalmetaphysics of science. These shifts can make the details of theevolving discussions difficult to track. Polger and Shapiro’s(2016) book, and the critical scrutiny across all of these fields thatit spurred, provides a useful framework for organizing these manydirections, despite what one judges about its skepticism about theextent of multiple realization and its defense of a mind-brainidentity theory. The literature over the last twenty years should putto rest the still-too-prominent view in the philosophy of mind thatmultiple realization spells doom for reductive materialism andmind-brain identity theory once and for all, as well as thestill-too-prominent view in some quarters in philosophy of sciencethat multiple realization, or at least its anti-reductionistconsequences, has been vanquished for good. Both views look uninformedand badly outdated in light of the detailed history of multiplerealization, a history which now spans more than a half-century ofAnglo-American philosophy.

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Acknowledgments

Mara McGuire assisted greatly with the research forsection 4. Thanks also to Mark Couch, Carrie Figdor, Mahi Hardelupas, ThomasPolger, and Lawrence Shapiro for helpful comments on an earlier draftof this latest revision/update. Thanks to an anonymous reviewer forsuggestions for improvement and for reminding me about some literatureI had overlooked.