## 1. Multiple Realizability Arguments


### 1.1 What is Multiple Realizability?



The multiple realizability contention about the mental is that a given
psychological kind (like pain) can be realized by many distinct
physical kinds: by different brain states in earthly biological
pain-bearers, by electronic states in properly programmed digital
computers, by green slime states in imagined extraterrestrials, and so
on. Correctly characterizing the realization relation remains a
contentious matter in analytic metaphysics (Gillett 2003; Polger 2004)
and this issue quickly reaches detailed technical depths. But whatever
the correct account of realization turns out to be, about whatever
kinds turn out to be related by realization, the multiple
realizability contention about the mental holds that a given
psychological kind (like pain) can stand in that relationship to many
distinct physical kinds. Further discussion of this issue with
numerous references will arise in
 section 3
 below.


### 1.2 Initial Multiple Realizability Arguments Against "Reductive" Mind-Brain Identity Theories



Hilary Putnam introduced multiple realizability into the philosophy of
mind. Challenging the "brain state theorists", who held
that every mental kind is identical to some yet-to-be-discovered
neural kind, Putnam (1967) notes the wide variety of terrestrial
creatures seemingly capable of experiencing pain. Humans, other
primates, other mammals, birds, reptiles, amphibians, and even
mollusks (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 of
pain-bearing species, and correlated exactly with each occurrence of
the mental kind. (This is a necessary condition of the hypothesized
type-identity.) Biological considerations speak intuitively against
this requirement. Neuroanatomy differs across terrestrial
brain-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 organisms
not closely related due to their having to adapt to similar
environments or ecological niches. Such realizers of the same function
will highly likely be distinct "physical-chemical"
kinds.



In addition, Putnam (1967) points out that early mind-brain identity
theorists insisted that these identities, while contingent, hold by
virtue of natural (scientific) law. So then any physically possible
pain-bearer must also be capable of possessing that physical-chemical
kind. This point marks the introduction of the crucial distinction
between multiple realiz*ation* and multiple
realiz*ability* into the discussion, as well as the well-known
philosophers' imagined metaphysical possibilities. Silicon-based
androids, artificially intelligent electronic robots, and Martians
with green slime pulsating within their bodies all seem to be possible
pain realizers. But they lack "brain states" comparable to
ours at any level of physical description. Further still, these
mind-brain identity theories were supposed to be completely general.
*Every* mental kind was held to be identical to some neural
kind. So the critic needs to find only one mental kind, shared across
these structure-types yet realized differently at the
physical-chemical level. Putnam (1967) acknowledges that the early
identity theories were being offered an empirical hypothesis. But this
consequence was "certainly ambitious", and almost
certainly false.



One quick word on this distinction between multiple
realiz*ability* and realiz*ation*, for it marks a point
where treatment of this topic in metaphysics and cognitive science
diverged. Metaphysicians, many of whom came quickly to reject the
contingent identity claims of the early mind-brain identity theorists
in favor of the necessity of identity claims, focused on multiple
realiz*ability*, since the possibility of distinct physical
realizers of the same psychological kind was sufficient to block any
such identity between realized kind and any one of its possible
realizers. Philosophers who took their cue from the cognitive and
brain sciences focused on proposed instances of multiple
realiz*ation*, of actual instances of the relation among
existing cognizers. The emphasis in this entry will be on the latter
issue, although some discussion of the metaphysical issues will arise
in
 section 2
 and
 section 3
 below. 



Stated in canonical form, Putnam's original multiple
realizability argument draws an anti-identity theory conclusion from
two premises:


1. (the multiple realizability contention) (At least) some mental
kinds are multiply realizable by distinct physical kinds.
2. If a given mental kind is multiply realizable by distinct physical
kinds, then it cannot be identical to any one (of those) specific
physical kind.
3. (the anti-identity thesis conclusion) (At least) some mental kinds
are 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 arguing
that reductionism imposes too strong a constraint on acceptable
theories in special sciences like psychology and economics. According
to Fodor, reductionism is the conjunction of "token
physicalism" with the claim that there are natural kind
predicates in an ideally completed physics corresponding to each
natural kind term in any ideally completed special science. He
characterizes "token physicalism" as the claim that all
events that science talks about are physical events. Token physicalism
is a logically weaker thesis than reductionism or type-type
physicalism. To illustrate why, consider the following string of
numerals:



1 1 2



This string contains two types of numerals (1 and 2), but three tokens
of the two types (two tokens of the numeral type 1 and one token of
the numeral type 2). Mental states are similarly ambiguous. When you
and I both entertain the belief that Fodor advocated a Language of
Thought, one type of mental state is entertained, but two tokens of
that type occur (your belief state and my belief state). Type-type
physicalism insists that each mental state type is identical to some
physical state type, i.e., some specific brain state type on the
mind-brain identity theory. This view runs afoul of multiple
realizability. But token physicalism only insists that each token
occurrence of each type of mental state is identical to some token
occurrence of some physical state type--not necessarily a token
occurrence of the same physical state type on each occasion, e.g.,
when you and I, with our brains differentiated by our separate life
histories, entertain the same belief about Fodor's favorite
philosophy of mind.



Fodor gave reductionists the best-known account of reduction at that
time, the one at work in Paul Oppenheim and Hilary Putnam's
(1958; Fodor subtitles his 1974 as "The Disunity of Science as a
Working 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 reducing
theories' vocabularies by way of "bridge laws" (not
Nagel's term!) and claims a reduction when the laws of the
reduced theory are derived from the laws of the reducing theory plus
the bridge laws. According to Fodor (1974), if reductionism is to
establish physicalism, these cross-theoretic bridge laws must assert
(contingent) identities of reduced and reducing kinds. But given
multiple realizability, the only way this can obtain is if the
physical science constituent of a psychophysical bridge law is a
disjunction of all the terms denoting possible physical realizations
of the mental kind. Given the extent and variety of actual (not to say
possible) physical realizations, Fodor insists that it is
overwhelmingly likely that the disjunctive component will not be a
kind-predicate of any specific physical science. It is also
overwhelmingly likely that the disjunctive component will not appear
in any genuine law of some specific physical science. Multiple
realizability thus demonstrates that the additional requirement
imposed by reductionism, beyond that of token physicalism, is
empirically untenable. (For a recent argument that multiple
realization is consistent with Oppenheim and Putnam's unity of
science via reductions, see Shapiro 2018)


### 1.3 Early Multiple Realizability Arguments for Functionalism



The multiple realizability premise was also part of early arguments
for functionalism. Functionalism in the philosophy of mind
individuates 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 as
crying out, nursing the damaged area, and seeking out pain relieving
drugs. Any internal state that mediates a similar pattern of causes
and effects is of the mental type pain, regardless of the specific
physical mechanisms that realize that pattern of causes and effects in
any given case. Ned Block and Jerry Fodor (1972) note that the
multiple realizability of mental types on physical types shows that
any physicalist type-identity hypothesis will fail to be sufficiently
abstract. Functionalism, on the other hand, seems to be at the correct
level of abstraction up from explanations of behavior based on
physical mechanisms. In particular, it seems sufficiently abstract to
handle multiple realizability. Block and Fodor (1972) also note that
multiple realizability at the level of physical description is a
common characteristic of ordinary functional kinds, like mousetraps
and valve lifters. So it appears to be a reasonable empirical
hypothesis that characterizing mental kinds as functional kinds is at
the correct level of abstraction.



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


### 1.4 The Multiple Realizability Argument from Psychological Explanations



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



We won't discover that generalization, however, if we focus on
the person's neurophysiology and resulting muscular
contractions. That level of explanation logically is too weak, for it
cannot tell us that the prior sequence of neural events and muscular
contractions will lead to the subsequent action of dialing another 1.
A physiological explanation for any given instance will only link one
way of learning the emergency phone number to one way of coming to
know that an emergency occurred to one sequence of neural events and
resulting muscular contractions producing the behavior
(nonintentionally described). However, the number of physical events
constituting each of these cognitive classes--the learning, the
coming to know, and the action of dialing--is potentially
unlimited, with the constituents of each class often unrelated to each
other at the physiological level of description. (This is
Pylyshyn's appeal to multiple realizability.) So if there is a
generalization at the higher level of description available for
capturing--and in the example sketched above, surely there
is--an exclusively reductionist approach to psychological
explanation will miss it. Thus because of multiple realizability,
reductionism violates a tenet of scientific methodology: seek to
capture all capturable generalizations. Such methodological arguments
did not end with Pylyshyn's. Fodor (1975: Chapter 5) and Terence
Horgan (1993) raise related methodological caveats about reductionism
based on multiple realizability. Bickle (1998: Chapter 4) responds
directly to these.


### 1.5 Appeals to Multiple Realizability in a Token System Over Time



Anti-reductionists quickly came to stress the more radical form of
multiple realizability mentioned in
 section 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 kinds
required to render them identifiable with specific physical (i.e.,
neural) kinds due to the more radical type of multiple realizability
would render psychology incapable of capturing most all
generalizations across realizers. Ronald Endicott (1993) gives
Block's point empirical backing by noting detailed facts about
plasticity in individual human brains. The capacity for distinct
neural structures and processes to subserve a given psychological
function owing to trauma, damage, changing task demands, development,
and other factors is extensive. These facts count further against any
proposed reduction of, or identities between psychological and
physical kinds. Horgan (1993) clearly appeals to this radical sense of
multiple realizability when he writes:



> 
> 
> Multiple realizability might well begin at home. For all we now know
> (and I emphasize that we really do *not* now know), the
> intentional mental states we attribute to one another might turn out
> to be radically multiply realizable at the neurobiological level of
> description, *even in humans*; indeed, even in
> *individual* humans; indeed, even in an individual human
> *given the structure of his central nervous system at a single
> moment of his life*. (1993: 308; author's emphases)
> 
> 
> 



This radical sense of multiple realizability was widely adopted among
nonreductive physicalists, whose solution to the mind-body problem
probably remains the most popular position in Anglo-American
philosophy of mind. Putnam's original multiple realizability
argument against the mind-brain identity (see
 SS1.2
 above) remains central to this view, with the second premise now
replaced with:



(2')
 If some mental kinds are multiply realizable in this radical
single-token-system-over-time sense, then psychology cannot be reduced
to 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 multiple
realizability challenge to reductionism (see
 SS1.2
 above) rests on a misunderstanding of Ernest Nagel's
"classic" account of intertheoretic reduction. Although
Nagel's (1961) detailed examples of historical cases of
intertheoretic reduction in science all involve biconditional
cross-theory "conditions of connectivity" (what Fodor and
others called "bridge laws"), one-way conditional
connections expressing sufficient conditions at the reducing level are
all that his "principle of derivability" requires.
Richardson cites passages from Nagel (1961) indicating that Nagel
himself saw the force of this point. Multiple realizability only
challenges necessity (and nondisjunctive) reducing conditions, and so
is not a challenge to a projected, distinctively Nagelian reduction of
psychology to some physical science. However, this response seems not
to dispose of the original argument's force against mind-brain
identity theory.


### 2.2 Domain-Specific Identities and Reductions



David Lewis (1969) argues that the inconsistency between the identity
theorist's thesis and multiple realizability evaporates when we
notice a tacit relativity of the former to contexts. A common sense
example illustrates his point. The following three claims appear
logically 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 one
physical-chemical realization of pain.
(2') 
 The physical-chemical realization of pain is C-fiber firing.
(3') 
 The physical-chemical realization of pain is ... (something
else entirely).


(2') and (3') reflect the multiple realizability
contention. 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 "per
nervous system-type" to (1'), "in human nervous
systems" to (2'), and "in mollusk nervous
systems" to (3'). Inconsistencies evaporate. Lewis's
point is that identity claims are always specific to a domain. One
might wonder whether Lewis's point generalizes to capture the
science fiction examples Putnam hinted at in passing (see
 SS1.2
 above). Lewis was certainly aware of Putnam's (1967)
argument's reach (Lewis offers this argument as part of a review
of the book in which Putnam's piece first appeared). So we can
assume Lewis thought domain-specific reductions generalize to
these metaphysically possible cases as well. (See Lewis's own
elaboration of this argument in his 1983.)



Many reductionist philosophers subsequently elaborated on
Lewis's point with the help of scientific examples. Patricia
Churchland (1986: Chapter 7), Clifford Hooker (1981), Berent Enc
(1983), and other philosophers of science have described historical
intertheoretic reductions where a given reduced concept is multiply
realized at the reducing level. A common example is the concept of
temperature from classical equilibrium thermodynamics. Temperature in
a gas is identical to mean molecular kinetic energy. Temperature in a
solid, however, is identical to mean maximal molecular kinetic energy,
since the molecules of a solid are bound in lattice structures and
hence restricted to a range of vibratory motions. Temperature in a
plasma is something else entirely, since the molecular constituents of
a plasma have been ripped apart. Even a vacuum can have a
("blackbody") temperature, though it contains no molecular
constituents. "Temperature" of classical thermodynamics is
thus multiply realized microphysically in a variety of distinct
physical states. Yet this is a "textbook" intertheoretic
reduction in the history of science. It is just that the reduction,
and the subsequent ontological identifications across theories, are
specific to the domain of physical state.



Lewis's original response also underlies Jaegwon Kim's
(1989, 1992) appeals to structure-specific "local
reductions". Kim agrees that multiple realizability rules out a
general reduction of (structure-independent) psychology to some
physical science. But it permits, and even sanctions, a local
reduction of psychology to a theory of the physical mechanisms of
given structure-types. (Kim admits that the relevant structure-types
here will probably be narrower than biological species.) Local
reductions involve "structure-specific bridge laws" where
the mental-physical biconditional occurs as the consequent of a
conditional whose antecedent denotes a specific structure-type (e.g.,
"if *X* is a member of structure type *S*, then
*X* is in mental state *M* if and only if *X* is in
physical state *P*"). Conditionals whose antecedents denote
different structure types will typically have biconditionals as
consequents whose mental term-constituents are co-referential but
whose physical term-constituents denote different physical events.
Multiple realizability forces this much revision to the bridge laws of
classical reductionism. But according to Kim, domain-specific local
reductions are the rule rather than the exception in science
generally, and are sufficient for any reasonable scientific or
philosophical purpose.


### 2.3 Appeals to Neuroscience's Successes



Jaegwon Kim (1992) suggests, and John Bickle (1998: Chapter 4)
emphasizes that guiding methodological principles in contemporary
neuroscience assume continuity of underlying neural mechanisms across
the nervous systems of different species. This assumption informs many
neuroscience experimental techniques and conclusions drawn from
experimental results. Continuity is assumed both within and across
species. If radical multiple realizability of the sort championed by
anti-reductionist philosophers actually obtained among existent
species, contemporary neuroscientific experimental techniques and
practices built upon this continuity assumption should bear little
scientific fruit. Why experiment on the macaque visual system to
investigate human visual processing, if we can't safely assume
some fairly strong continuity across these species? Why should
positron emission tomography (PET) and functional magnetic resonance
imaging (fMRI) reveal common areas of high metabolic activity during
psychological task performance, both across and within individual
humans--now down to less than a millimeter of spatial resolution
in fMRI--if multiple realizability were as prevalent or
widespread as many nonreductive philosophers assume? (see
 SS1.5
 above). Standard neuroscientific experimental procedures, and even
clinical diagnostic tools widely used by practicing neurologists and
neuropsychologists, would seem hopelessly naive in the face of common
philosophical assertions about the extent of multiple realizability.
But these procedures and tools do work, and are not hopelessly
naive.



So Kim and Bickle draw the modus tollens conclusion, and insist that
these neuroscientific successes are evidence that psychological kinds
are not as widely or as radically multiply realized in brains as many
functionalists and nonreductive physicalists assume. It turns out that
even celebrated neural plasticity is systematic within and across
nervous systems. It has a regular progression following damage to a
principal structure; there are underlying neural mechanisms that
subserve it, evolutionarily conserved across species. Furthermore, the
newly realized function following brain damage is typically seriously
degraded compared to initial performance. Persons can still talk,
manipulate spatial representations, or move their extremities, but
their performances are typically qualitatively and quantitatively
worse than normals. This fact gives rise to tricky questions about
individuation of psychological function. Are these alternative neural
structures realizing *the same* psychological function, *the
same* mental kind, as before, as the philosophical uses of
multiple realizability require? (This last response has been further
developed more recently; see
 SS2.4
 and
 SS3.1
 below.)



William Bechtel and Jennifer Mundale (1999) provide extensive
empirical details about hypothesized or assumed brain type-identities
across species in neuroscientific practice. Their explicit target is a
methodological consequence sometimes drawn from the multiple
realizability premise: if psychological states are multiply realized
across biological species, then neuroscience will be of little use
toward understanding cognition. But as details of the neuroscience of
vision demonstrate, neuroscientists have successfully used
understanding of the brain to decompose visual function. The
neuroscientific goal has been to



> 
> 
> show how functional considerations get built into developing the
> structural taxonomy and how that taxonomy in turn can be a heuristic
> guide in developing information-processing models. This project has
> not been impaired by multiple realization of psychological states;
> rather, it relies on the assumption that there is a common realization
> of mechanisms for processing visual information across species. (1999:
> 201)
> 
> 
> 



It is difficult to argue against contemporary neuroscience's
empirical successes. So even if one accepts multiple realizability in
its original type-type form advocated by Putnam (see
 SS1.2
 above), Bechtel and Mundale's argument cautions against drawing
strong consequences from it about psychology's
*methodological* autonomy from neuroscience.


### 2.4 Challenging the Individuation of Mental Kinds



Over the past two decades philosophers have been more willing than
previously to challenge the truth of the multiple realizability
premise itself (see
 SS3.2
 below.) One approach of this kind challenges the way proponents of
multiple realizability individuate mental kinds.



An early version of this argument is from Nick Zangwill (1992), who
concludes that multiple realizability across biological species has
"never been proven". The multiple realizability contention
assumes a type-identity of mental kinds across species. According to
Zangwill this assumption is problematic, given that the obvious
sensory and motor differences across species by themselves yield
different cause-and-effect patterns at all but the grossest level of
description. If successful, Zangwill's challenge undercuts the
multiple realizability argument by denying that the same mental kinds
obtain across species, to be realized by different physical
mechanisms.



Lawrence Shapiro (2000) also contends that philosophers are too quick
to claim that a single given kind is multiply realized. Some
properties of the realizers are relevant to the purposes, activities,
or capacities that define a given functional kind, but other
properties are not. Consider corkscrews. That functional kind can be
"multiply realized" in two tokens that differ only in
their color. That physical difference does not make them genuinely
different realizations of corkscrew, however, because that one makes
no relevant difference, that is, no difference to their performances
as corkscrews. Similarly for two corkscrews that differ only in that
one is made of aluminum and the other of steel. Although that
compositional difference might matter for some functional kinds, it
doesn't matter for corkscrews. As Shapiro notes, 



> 
> 
> steel and aluminum are *not* different realizations of a
> waiter's corkscrew because, relative to the properties that make
> them suitable for removing corks, they are identical. (2000: 644) 
> 
> 
> 



Establishing relevant multiple realizability requires pointing to
property differences in the realizers that make for a functional
difference for that kind of thing.



Shapiro then points out that this requirement sets up a dilemma for
proponents of multiple realizability. Consider what appears to be a
genuine case of multiple realizability, that is, two objects that
"do the same thing" but in very different ways. Either the
realizing kinds genuinely differ in their causally relevant properties
or they do not. If they do not, then we don't really have a case
of multiple realizability (like the corkscrews that differ only in
color or metal composition). If they do so differ, then they genuinely
are different kinds. But then functionally they are not the same kind,
and again we don't have an instance of multiple
realizability--of a *single* functional kind with distinct
realizations.



The usual justification for grouping distinct realizers under a single
functional kind is that such a classification reveals interesting
similarities, of the sort we expect to be captured by laws or
generalizations of some higher level science. But according to
Shapiro, when the realizing kinds differ significantly in their
causally relevant properties for the function at issue, any shared
laws or generalizations are "numbingly dull" (2000: 649),
e.g., all realizers of mouse traps can be used to catch mice, or both
camera eyes and compound eyes have the function of enabling the
organism to see. Shapiro remarks: 



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



The first horn acknowledges a single functional kind but denies that
it is multiply realized. The second horn undercuts the principal
reason for grouping genuinely different physical kinds under a single
functional kind. Shapiro concludes that taken together these two horns
blunt any claim of *multiple* realizations of *the same*
functional type. His argument has been central to more recent disputes
as concerns about multiple realization shifted to the metaphysics of
science (see
 SS3).



Shapiro's dilemma is in the spirit of an earlier "causal
powers" argument from Jaegwon Kim (1992) Kim insists that
scientific kinds are individuated by their causal powers, and the
causal powers of each instance of some realized kind are identical to
those of its realizer. From these principles it follows that instances
of a mental kind with different physical realizations are distinct
kinds because of the distinct causal powers of their realizers. A
structure-independent mental kind is not a causal kind, and hence is
disqualified as proper scientific kinds. Multiple realizability yields
the failure of structure-independent mental kinds to meet a standard
of what makes something a scientific kind.



Mark Couch (2004) presses a related dilemma. Defending a claimed
multiple 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 to
case. Successfully challenging either step blocks any multiple
realizability argument appealing to that kind. As we saw in the
previous section, Bechtel and Mundale (1999) describe cases in which
cognitive neuroscientists treat the physical realizers (brain states)
as type-identical across species, thus attacking step (i). In other
cases--Couch's example is primate versus octopus
eyes--one can appeal to easily-found differences in functional
properties, attacking step (ii). The two types of eyes have different
visual pigments in their photoreceptors, different retinas, and
different ways of focusing light. These physical differences lead to
straightforward input-output (functional) differences: in the optic
stimuli the two eyes respond to, in reaction times, and more. Their
functions may be similar, but similarity isn't identity and the
multiple realization argument since Putnam's original version
(see
 SS1.2
 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 stressed
by both Couch and Shapiro). In actual scientific practice, discovered
physical (neural) differences typically incline psychologists to
search for functional differences. Couch's point is that the
individuation of psychological states, like the individuation of brain
states, is an empirical issue; Shapiro and Couch both suggest that
claims about multiple realizability often rely heavily on
"folk" psychological intuitions about individuating mental
kinds.



Bechtel and Mundale (1999) note that proponents of multiple
realizability often appeal to different amounts of
"granularity" in how they individuate mental versus
neurobiological kinds. Proponents often analyze psychological states
at a coarse-grained level, in which only the loosest input-output
similarities across species are deemed sufficient for mental kind
identities. Yet they insist on very fine-grained individuation for
brain states, in which small differences across species are sufficient
for neural type-differences. But psychological ascriptions admit of
finer grains and neural ascriptions admit of coarser grains. Bechtel
and 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 multiple
realizability assume a "Dimensioned" view of realization
that allows realizer/realized properties to be instantiated in the
*distinct* individuals that bear part-whole relations to one
another. Shapiro instead assumes a "Flat" view of
realization, which demands that realizer/realized properties be
instantiated in the *same* individual. Gillett argues that
Shapiro's arguments fail under the Dimensioned view of
realization, and that he never defends the Flat view over the
Dimensioned view. Gillett concludes that failing to directly address
the nature of the realization relation more generally vitiates
Shapiro's critique, leaving it as simply begging the question
against Fodor's and others original defenses of the multiple
realization premise. These more general accounts later emerged to
dominate more recent discussions (see
 SS3
 below). 


### 2.5 Reduction and Identity Theory Reconceived



The more radical type of multiple realizability seems to force
increasingly narrower domains for reductions to be relativized; at the
extreme, to individuals at times. This much "local
reduction" seems inconsistent with the assumed generality of
science. To avoid this problem, some philosophers of science have
suggested significant changes to the "classical"
(Nagelian) account of (intertheoretic) reduction to enable the account
to handle multiple realizability of reduced kinds, a feature of a
number of "textbook" cases of scientific reductions.



Following suggestions by Clifford Hooker (1981) and Berent Enc
(1983), John Bickle (1998: Chapter 4) argues that even the radical
type of multiple realizability (in the same token system over time
sense) is a feature of some widely-accepted historical scientific
reductions. For example, it occurs in the reduction of classical
equilibrium thermodynamics to statistical mechanics and microphysics.
For any token aggregate of gas molecules, there is an indefinite
number of realizations of a given temperature: a given *mean*
molecular kinetic energy. Microphysically, the most fine-grained
theoretical specification of a gas is its microcanonical ensemble, in
which the momentum and location (and thus the kinetic energy) of each
molecule is specified. Indefinitely many distinct microcanonical
ensembles of a token volume of gas molecules can yield the same
*mean* molecular kinetic energy. Thus at the lowest level of
microphysical description, a given temperature is vastly multiply
realizable in the same token system over times. Nevertheless, the case
of temperature is a textbook case of scientific reduction. So even
this radical type of token-token multiple realizability is no barrier
in principle to reducibility. An actual case in science exemplifies
it.



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 this
supplement builds the possibility of handling multiple realizability
directly into his revised definition of the reduction relation. Let
*S* be the predicate, "satisfies functional theory
*F*", *T* be the class of systems to which the token
system in question belongs, *S'* be an appropriate
predicate in some lower level theory of T-system causal mechanisms,
and *T\** be the class of systems to which *S'*
applies. Then, according to Hooker, 



> 
> 
> systems of type *S* of class *T* are contingently
> token/token identical with systems of type *S'* in class
> *T\** =*df* every instance (token) of a type
> *S* system externally classified as in class *T* is
> contingently identical with some instance (token) of a type
> *S'* system externally classified as in class
> *T\**. (1981: 504) 
> 
> 
> 



By "externally classified", Hooker refers to the sort of
cross-classification that holds across different
determinable/determinate hierarchies. To address some self-assessed
shortcomings of Hooker's own formulation of his general account
of reduction, Bickle (1998) reformulates Hooker's insights,
including his token-token reduction supplement, within a
set-theoretically characterized "semantic" account of
theory structure and relations. Bickle's account draws directly
on formal work from "structuralist" philosophy of science.
More recently Klein (2013) advocated for a semantic "sets of
models" account of theory structure as a strategy for
accommodating multiple realized reduced kinds.



Other reconceptions of both reduction and the mind-brain identity
theory have been proposed explicitly to handle multiple realizability.
Elliott Sober (1999) insists that a reductionist thesis actually
follows from the multiple realizability premise. He begins by
attacking Putnam's (1967) "objective" account of
superior explanation, namely that one explanation is superior to
another if the former is more general. According to Putnam, superior
explanations "bring out the relevant laws". But Sober
reminds us that explanatory generalizations at lower levels bring out
more details. Science "aims for depth as well as breadth"
and there is no "objective rule" concerning which endeavor
is "better" (1999: 550). Both reductionists and
anti-reductionists err in privileging one aim at the expense of the
other. Sober then notes that multiple *realizability*
presupposes some form of asymmetric determination: the lower level
physical properties that are present at a given time determine the
higher level properties that are present. But this assumption commits
its proponents to the causal completeness of physics, the thesis that
physical effects have only physical causes (a doctrine that Sober
sketches toward the end of his 1999). If one is also concerned with
causal explanation--that is, if one holds that individual
occurrences are explained by citing their causes--then the causal
completeness of physics in turn commits multiple realizability
proponents to physics' possessing an important kind of
explanatory 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 it
explicitly against multiple realizability. HIT insists that identity
claims in science typically are hypotheses adopted in the course of
empirical investigations, which serve to guide subsequent research.
They are not conclusions reached after empirical research has been
conducted. Concerning the multiple realizability of psychological on
brain (physical) states, cognitive neuroscience's heuristic
identity claims assert type-commonalities in comparative studies
across species, not type-differences. Bechtel and McCauley illustrate
their hypothesis with case studies: Brodmann's early twentieth
century work mapping the brain into functionally relevant areas;
Ferrier's late-nineteenth century work employing electrical
stimulation to cortex; and more recent detailed maps of visual
processing regions in the primate brain. All of these landmark
functional anatomical studies used multiple species. As Bechtel and
McCauley remind us,



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



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



When reduction or identity theory gets reconceived in ways built to
accommodate multiple realizability, are reductionists/identity
theorists and proponents of multiple realizability then simply talking
past one another? Perhaps not. It is worth reminding ourselves that
many nonreductive physicalists have employed multiple realizability to
argue against *all forms* of psychophysical reductionism. If
better general accounts of scientific reduction or identity theory
make room for multiple realizability, these demonstrations count
against this broader challenge. And if "nonreductive"
physicalists were to reformulate their position to oppose only a
specific brand of psychophysical reductionism or identity theory, that
would weaken their position significantly, perhaps to the point of
showing that it targets a straw person, but also making it compatible
with forms of "reductive" physicalism. In fact, this
reconception move on the part of reductionists and identity theorists
traces back to Fodor's (1974) essay. While Fodor's
arguments there explicitly targeted a reductionism built on the
classical Nagelian account, he suggests in footnote 2 that 



> 
> 
> what I shall be attacking is what many people have in mind when they
> refer to the unity of science, and I suspect (though I shan't
> try to prove it) that many of the liberalized versions of reductionism
> suffer from the same basic defect as what I shall take to be the
> classical form of the doctrine. 
> 
> 
> 



The problem was, even back then, new work on (intertheoretic)
reduction in the philosophy of science was attempting explicitly to
make room for multiple realizability of reduced on reducing kinds.
Fodor's "shan't trying to prove" the
generality of his attack against even the earliest of those
reconceptions begged explicitly one of the questions they were
addressing.


### 2.7 Lower Level Reductive Unity?



In searching for reductive unity underlying the variety of cognitive
systems, Paul Churchland (1982) once recommended descending
"below" neurobiology and even biochemistry, to the level
of nonequilibrium thermodynamics. He insisted that finding reductive
unity there was more than a bare logical possibility because of some
parallels between biological processes, whose multiply realized kinds
find reductive unity there, and cognitive activity, especially
learning.



Concerning Pylyshyn's (1984) attack on reductionist methodology
(see
 SS1.5
 above), Patricia Churchland (1986: Chapter 9) suggests that
functional theories are constructed in lower level sciences. New
levels of theory thus get inserted between those describing the
structure of the lower level kinds and those of purely functional
kinds: between, for example, the physiology of individual neurons and
cognitive psychology. We might find a common neurofunctional property
for a given type of psychological state across a wide variety of
distinct brains (or even the same brain over time). And if the scope
of the macro-theory doesn't extend beyond that of its
microfunctional counterpart, then reduction will be achieved despite
vast multiple realizability at the microstructural level.
Neurocomputational approaches that have blossomed since the early
1990s give real empirical credence to Churchland's
suggestion.



Bickle (2003) claims that if we leave our neuroscientific
understanding at the systems level, psychoneural multiple
realizability seems obvious. Neural systems differ significantly
across species. But neuroscience does not stop its inquiries at the
systems level. As it moves further down, into cellular physiology and
increasingly into the molecular biology of nervous tissue, identities
of mechanisms across species have been found. Many molecular
mechanisms of neural conductance, transmission, and plasticity are the
same, from invertebrates through mammals. This matters for psychology
because mechanisms of cognition and consciousness are increasingly
being found at these levels. Bickle's key example is memory
consolidation, the conversion of labile, easily disrupted short-term
memories into more durable, stable long-term form. Work with fruit
flies, sea slugs, and mice has revealed the role of the cyclic
adenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMP
responsive-element binding protein (CREB) signaling pathway in key
forms of experience-driven synaptic plasticity. Across these very
distinct taxa, this molecular intracellular signaling pathways has
been implicated experimentally in memory consolidation. By altering a
single protein in this cascade (using biotechnology and molecular
genetics), experimenters have built mutant organisms whose short-term
memory remains intact (as does their sensory, motor, and motivational
capacities), but which cannot consolidate these short-term memories
into long-term form. Bickle quotes with approval statements like the
following, from insect biologists Josh Dubnau and Tom Tully:



> 
> 
> In all systems studied, the cAMP signaling cascade has been identified
> as one of the major biochemical pathways involved in modulating both
> neuronal and behavioral plasticity. ... More recently,
> elucidation of the role of *CREB*-mediated transcription in
> long-term memory in flies, LTP and long-term memory in vertebrates,
> and long-term facilitation in *A. californica* [a sea slug]
> suggest that CREB may constitute a universally conserved molecular
> switch for long term memory. (1998: 438)
> 
> 
> 



Memory consolidation is just one psychological phenomenon, and so its
"ruthless" reduction to molecular events doesn't
establish a general claim about unitary mechanisms across widely
divergent taxa for other shared cognitive kinds. For that argument,
Bickle turns to principles of molecular evolution. The first principle
holds that evolution at the molecular level--changes to the amino
acid sequence of a given protein--is much slower in functionally
important ("constrained") domains than in functionally
less important ones. The second principle is that molecular evolution
is much slower in all domains of "housekeeping" proteins,
especially in ones that participate in cell-metabolic processes in
many tissue types. These two principles imply that these molecules,
their domains, and the intracellular processes they participate in
will remain constant across existing biological species that share the
common ancestor that first possessed them. (This is what Dubnau and
Tully refer to above as a "universally conserved"
molecular switch.) In the end, any psychological kind that affects an
organism's behavior must engage the cell-metabolic machinery in
individual neurons. In the brain, causally speaking, that's
where the rubber meets the road. But that's the machinery
conserved across existing biological species--changes to it,
especially its functionally constrained domains, have (almost)
inevitably been detrimental to an organism's survival. So we
should expect that the molecular mechanisms for any causally
efficacious cognitive kind be "universally conserved". The
discovery of these shared mechanisms of memory consolidation is not
some isolated case, but follows from the core principles of molecular
evolution. As "molecular and cellular cognition" proceeds,
we should expect more examples of unitary realizers--ruthless
reductions--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, nonreductive
physicalism retained its status as the most popular position in
Anglo-American philosophy of mind. Its proponents continued to appeal
to the standard multiple realizability arguments (see
 SS1
 above) to challenge all versions of psychophysical reductionism and
identity theory. However, the challenges to multiple realizability and
the arguments using it, scouted in
 section 2
 above, began to attract wider notice. New versions of type-identity
theory and reductive physicalism began to be formulated more widely
(Gozzano and Hill, 2012). The nonidentity of mental content properties
with any physical properties no longer seemed to be the
"practically received wisdom", as Ernest LePore and Barry
Loewer (1989) called it thirty years ago, although it has clearly
remained an entrenched view.



Some of these arguments continued lines introduced in
 section 2
 above. But they warrant separate treatment, not just to keep my
presentation historically accurate, but also because more general
considerations from the metaphysics of science increasingly came to
motivate specifics of the new discussions, although some of these more
general features had been scouted previously in works focused on
philosophy of mind (see especially William Lycan 1987).


### 3.1 The Empire Strikes Back



Expectedly, proponents of multiple realizability quickly responded to
the myriad challenges outlined in
 section 2
 above. Carl Gillett and Ken Aizawa were perhaps the most vocal
defenders of multiple realizability against a number of them. Gillett
(2003) develops a precise framework for understanding compositionality
relations in science generally, and uses this framework to define
property realization and multiple realization, and to distinguish
further "flat" from "dimensioned" realization.
Recall from
 section 2.4
 above that "flat" realization involves both realized and
realizing properties inhering in a single object.
"Dimensioned" realization involves realized and realizing
properties inhering in distinct individuals that stand in a
compositional relationship to one another. For Gillett (2002, 2003),
this distinction is important for two reasons. First, scientific
explanations employ dimensioned realizations, because inter-level
mechanistic explanations relate distinct individuals. Second, Fodor
and other proponents of the standard multiple realizability arguments
were assuming a dimensioned account, while the arguments of Shapiro
and other recent critics (see
 SS2
 above) challenge the existence of multiple realizability only by
assuming a flat realization. According to Gillett, no recent proponent
of multiple realizability has assumed or defended flat realization as
the correct account involved in any of the scientific cases at
issue.



Explicitly applying Gillett's precise framework for realization
and related notions, Ken Aizawa and Gillett defend the existence of
multiple realization in a variety of sciences (2009a). They also argue
for the "massive multiple realization" of properties at
every level of organization, from the structure and function of
proteins in neurons to those of human cognition to those of social
interactions (2009b). Their detailed scientific example in the latter
essay is visual processing. They contend that neuroscientists, unlike
philosophers, are unfazed by massive multiple realization. Multiple
realization has been so contentious in philosophy of mind, they
insist, because philosophers tacitly assume flawed or scientifically
unfounded accounts of realization, like the flat view. According to
Aizawa and Gillett (2009b), philosophers uncritically accept a
narrative that links multiple realization to the strict methodological
autonomy of psychology from neuroscience. They also contend that the
empirical details of vision research show that a co-evolutionary
research methodology is not just consistent with, but explicitly
motivated by massive multiple realization. So this uncritically
accepted philosophical narrative not only blinds philosophers to facts
that scientists recognize as unproblematic; it is also empirically
false.



Soon afterwards, Aizawa and Gillett (2011) distinguish two strategies
scientists might adopt to deal with putative cases of multiple
realization. One strategy is simply to take multiple realization at
face value and live with multiply realized kinds. The other is to
split the higher-level multiply realized kind into a variety of
sub-kinds, one for each of its distinct lower level realizers, and
then eliminate the original higher-level kind, at least for the
purposes of further scientific investigation. Do scientists always
favor the second strategy, as recent philosophical critics of multiple
realizability would seem to recommend? With its well-known
distinctions between different types or systems, memory research would
seem to be a clear instance of using such an
"eliminate-and-split" strategy. Yet, Aizawa and Gillett
argue, such an assessment oversimplifies the actual scientific details
in even this much-discussed case. Here too they sense an important
general methodological lesson: psychology took account of neuroscience
discoveries, so even when taking multiple realization at face value
scientists do not advocate strict methodological autonomy. But the
actual details of how psychology takes neuroscientific discoveries
into account depends both on the nature of the psychological kinds in
question and the needs of theorizing specific to psychology.



Aizawa has also replied to a number of the challenges to the standard
multiple realizability argument scouted in
 section 2
 above. After separating three distinct arguments in Bechtel and
Mundale (1999;
 SS2.3
 above), Aizawa (2009) sets his critical sights on their Central
Argument, which argues against multiple realization from the existence
and continued success of brain mapping studies. He argues that Bechtel
and Mundale misrepresent the actual nature of these studies, and more
generally of the methods employed in functional localization studies.
Working with exactly the scientific examples Bechtel and Mundale
discuss (mostly from the functional neuroanatomy of vision), Aizawa
argues that claims about psychological functions do not play the
specific role in these studies that Bechtel and Mundale insist. So the
success of these studies does not imply the falsity of multiple
realization. Later in that paper Aizawa challenges two of Bechtel and
Mundale's key assertions. He denies that if psychological
properties were multiply realized, then functional taxonomy of the
brain would have to be carried out independently of psychological
function. And he denies that multiple realization rules out
comparisons of brains across different species. Hence all the premises
of Bechtel and Mundale's Central Argument are false. Aizawa
(2007) criticizes Bickle's (2003) argument that a unitary
realization of memory consolidation across species has been found at
the level of molecular mechanisms, despite widespread neural
differences in these brains at higher levels of neuroscientific
description
 (SS2.3
 above). According to Aizawa, the protein components of these
evolutionarily conserved molecular mechanisms, and the NNA sequences
coding for them, are themselves multiply realized across species.
Aizawa (2013) presented numerous scientific examples of multiple
realization by "compensatory differences". In such cases,
changes to one or more properties that jointly realize a realized
property *G* are compensated for by changes in others of the
jointly realizing properties. Although his overall goal in this paper
is to bring this form of multiple realization to wider recognition and
study by philosophers of science, he uses the broader
"Gillett-Aizawa framework" to argue that highly specific
determinate properties, not just generic determinable properties, are
multiply realized in this specific fashion. Such multiply realized
determinate properties are indeed exactly similar across distinct
realizations, and so answer the dilemma posed by Shapiro and others
 (SS2.4
 above).



Carrie Figdor (2010) challenges criticisms of multiple realization
that appeal to neuroscience, citing explicitly the works discussed in
 section 2.3
 and
 section 2.7
 above. She undertakes to demonstrate not only the empirical
plausibility of mind-brain multiple realization, but also to clarify
the terms of the empirically-focused debate. Her scientific focus is
on "localizationism", especially the "brain mapping
projects" using state-of-the-art functional neuroimaging (2010:
420). Concerning the terms of the empirically-focused debate, Figdor
argues that none of the multiple realization hypotheses common to the
philosophical literature is appropriate to this scientific endeavor.
Instead she appeals to "degeneracy", a concept from
general biology meaning "the ability of elements that are
structurally different to perform the same function or yield the same
output", a characteristic of complexity ranging from the genetic
code to immune systems to populations (e.g., Edelman and Gally 2001).
This is the notion, Figdor claims, that has been co-opted by cognitive
neuroscientists to mean 



> 
> 
> for a given cognitive function *F*, there is more than one
> nonisomorphic (nonidentical) structural element that can subserve
> *F*, 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 in
this fashion is the frequency of anomalous results in functional
neuroimaging studies that show activations of brain areas that differ
across subjects, or even in a single subject across performances of
the same experimental task,. Figdor discusses numerous examples from
recent cognitive neuroscience, and carefully separates cases of
degeneracy that count as multiple realizations from others that do
not. She concludes that her "sketch of degeneracy in cognitive
neuroscience suffices to show that [multiple realizability] is a
viable empirical hypothesis (2010: 237). And she concludes by
addressing specifically Bechtel and Mundale's (1999) empirical
arguments challenging multiple realizability from functional imaging
studies (see
 SS2.3
 above). Bechtel and Mundale's claim that neuroscience has been
useful for psychology is true, Figdor asserts, but does not challenge
a degeneracy-based empirical case for multiple realizability. And
Bechtel and Mundale's stronger claim, that neuroscientists
implicitly assume the falsity of multiple realization as reflected in
their accepted practices, is itself shown to be false by the use of
degeneracy in cognitive neuroscience.


### 3.2 New Attacks on Multiple Realizability



Critics of multiple realizability and the standard arguments it has
figured into also extended their reach early in the twenty-first
century. Lawrence Shapiro (2008) raises some methodological
difficulties involved in testing whether a given psychological kind
actually is multiply realized. (For a related argument see Thomas
Polger 2009.) Shapiro reminds us of the crucial role that auxiliary
assumptions play in hypothesis testing generally (within a broadly
hypothetico-deductive model), and considers a collection of explicit
auxiliary assumptions that might be implicitly used to establish a
multiple realization hypothesis. He presents a recent ferret
brain-rewiring experiment as a scientific example, in which axonal
inputs from the primary visual tract were redirected in ferret embryos
to project to primary auditory cortex (see Sharma, Angelucci, &
Sur 2000 for the scientific details). An auxiliary hypothesis
requiring multiply realized higher-level (in this case, psychological)
kinds to be "exactly similar"--identical--across
distinct realizers won't help the proponent of the standard
multiple realizability argument with this purported case. It is easy
to measure better visual performance in the normally-wired control
ferrets compared to the re-wired experimental animals. While the
experimentally re-wired animals have some visual function, it is
diminished significantly compared to controls. "Exact
similarity" (identity) of visual function is thus not present
across these groups. On the other hand, one might argue for the
multiple realizability premise in this ferret re-wiring case using an
auxiliary hypothesis that only requires similarity in multiply
realized higher level properties, yet still requires that differences
across the realizers should not be limited only to the differences
that cause differences in the realized (in this case, visual)
properties. Shapiro speculates that this auxiliary assumption seems
best to capture the sense of multiple realization stressed by
proponents of the standard argument. But if we adopt it, again the
ferret re-wiring case seems not to provide an empirical instance of
multiple realization. Shapiro remarks: "the differences in
ferret brains explain nothing more than differences in ferret visual
properties" (2008: 523). Shapiro also argues that his detailed
discussion of hypothesis testing difficulties for any multiple
realization hypothesis reveals a flaw in Bechtel and Mundale's
(1999) influential criticism (discussed in
 SS2.3
 above). Bechtel and Mundale's examples, drawn from the
comparative functional neuroanatomy of vision, only compared
homologous brain structures. But these only have differences that make
a difference in their visual properties, nothing else. Instead of
these examples, Shapiro insists, "one should be looking at
different brains that reveal similar visual properties despite their
differences" (2008: 524)--exactly the kinds of evidence
that Bechtel and Mundale's emphasis on homologies doesn't
consider.



Shapiro and Polger (2012) build upon Shapiro's account of the
complexity of actually testing for scientifically-justified multiple
realization. They insist that these actual details render the
significance of multiple realization far more dubious than
philosophers of mind typically suppose. They introduce explicit
criteria to capture the common assumption that multiple realization
requires not merely differences between realizing kinds, but
"differently the same": the features of entities *A*
and *B* that lead them to be classified differently by the
realizing science *S*2 "must be among those that
lead them to be commonly classified" by the realized science
*S*1 (2012: 282, criterion iii). This explicit
criterion rules out the popular assertion that camera eyes versus
compound eyes, with different photoreceptive chemicals in their
retinal cones, is a genuine (empirical) instance of multiple
realization. Considered coarsely, such eyes are doing the same thing
in the same way, so they're not "differently the
same". Considered finely, the two kinds of eyes are sensitive to
different 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 quantitative
differences. The relevant variation between entities *A* and
*B* in realizing science *S*2 "must be
greater than" the individual differences between *A* and
*B* recognized by the realized science *S*1
(2012: 282, criterion iv). The variation recognized by the realizing
science must not merely map onto individual differences between
*A* and *B* recognized by the realized science. The demands
in actually establishing multiple realization are thus quite strict.
Not any old variation will do. According to Shapiro and Polger, these
strict demands show both that multiple realization in the sense
required to fund the standard argument is "a relatively rare
phenomenon"--despite the vast variability everywhere in the
world--and that a "relatively modest" mind-brain
identity theory has little to worry from it (2012: 284).



Similar in some ways to Couch's arguments (discussed in
 SS2.4
 above), Colin Klein raises a challenge to the scientific
contributions made by multiply realized kinds. Noting the variety of
things that materials science classifies as "brittle,"
Klein (2008) notes that few to none of the many scientific discoveries
about realization-restricted brittle things--for example, about
brittle steel--generalize to other realization-restricted
types--such as brittle glass. Klein insists that generalizations
about genuine scientific kinds should be projectable across instances
of those kinds, so this requirement seems not to be met by a
significant class of multiple realized kinds, namely, the
realization-restricted ones. Applying this point to psychological
kinds, instead of supporting a scientifically-backed nonreductive
physicalism, it appears rather that special sciences should abandon
multiply realized kinds. Klein notes that proponents of
scientifically-based multiple realizability can find terms in special
sciences that figure in legitimate explanations, and so appear to
refer to projectable multiply realize kinds. But close investigation
of some paradigmatic examples reveals these to be idealizations of
actual kinds. Special-science kind-terms are thus typically ambiguous.
Sometimes a given term refers to an actual but realization-restricted
kind. Other times it refers to features of explanatory but non-actual
idealized models. (Klein 2008 illustrates this ambiguity with his
detailed example from materials science.) Neither suffices to provide
a kind of actual multiple realization that the standard argument
requires. However, he insists that his argument isn't entirely
negative for non-reductive physicalism. Idealizations can function in
explanations that are autonomous in an important sense from lower
level sciences. And furthermore, Klein argues, Jaegwon Kim's
(1992) assumption that
all explanatory work in science must appeal to realization-restricted
kinds and properties (discussed in
 SS2
 above), is simply incorrect. Still, Klein insists, there appear to be
no actual and projectable--hence genuinely
scientific--multiple realized kinds.



Bickle (2010) questions whether the "second-wave" of
criticisms of the standard argument, those that challenge the multiple
realization premise itself (discussed in
 SS2.4
 and this section above), give aid and comfort to psychoneural
reductionists. Since psychoneural reductionism was one of the explicit
targets of the standard multiple realization argument, one might
plausibly assume that they do. Yet none of those who offer these
"second-wave" challenges (Shapiro, Polger, Couch) identify
themselves as "reductionists" (with the possible exception
of Shapiro, and more recently Bechtel 2009, although his response to
the multiple realization argument figures nowhere in his plumb for
"mechanistic reduction"). Some of these
"second-wave" critics, such as Polger 2004, are explicitly
anti-reductionist. (Though Polger 2004 is also explicitly
anti-anti-reduction. He argues that multiple realization has little if
anything to do with reduction.) This fact alone should give a
psychoneural reductionist pause. Second, the direction that the
second-wave debates have developed, starting with Gillett's
(2003) criticisms--deeply into the nature of the realization
relation, and so deeply into the metaphysics of science rather than
into science itself--should prompt the psychoneural reductionist
with a metascientific bent to simply tell the second-wavers, thanks
for nothing! Does that leave psychoneural reductionism back on its
heels, in light of the standard multiple realization argument? Not at
all, Bickle (2010) insists. For the "first-wave"
actual-scientific-history challenge to the first premise of the
standard argument, and the initial critical discussions (see
 SS2.1,
 SS2.2, and
 SS2.5
 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 rejoining
arguments of metaphysicans like Kim (1992), they thereby dismissed the
entire first-wave of challenges. Many of the challenges discussed in
 section 2
 above shows that this is not the case. There are numerous examples of
multiply realized kinds that are components of scientific theories
widely acknowledged to having been reduced to other theories. So
multiple realization alone is no barrier to actual scientific
reduction. The detailed scientific cases that illuminate many of those
"first-wave" challenge to the standard multiple
realizability argument remain unanswered to this day.



So well into the second decade of the new millennium, a renewed
critical interest in multiple realizability continued to generate new
discussions and arguments on both sides, pro and can. The assumption,
unfortunately still too common in philosophy of mind, that multiple
realizability "seals the deal" against reductive
physicalism and mind-brain identity theory, was mistaken back in the
1970s and 1980s (see
 SS2
 above), and grew even more discredited by the full scope of published
literature on the topic that has followed since. But equally
unfortunately, the literature on multiple realizability had taken off
in numerous varied directions, and following out any one of these
leads one quickly into detailed complicated and technical discussions,
in both philosophy and science, and often at quite a distance from
those who follow out arguments in other directions. And the fate of
one of the most influential arguments in late-twentieth century
Anglo-American philosophy hangs in the balance, in all of these
debates. By the second decade of the new millennium, concerns about
multiple realizability arguments had grown beyond their philosophy of
mind origins, into more general questions about relationships
obtaining between sciences addressing higher and lower levels of the
world's organization. What the issue needed now was a single
work that captured both the full scope of writings on this topic, and
with a focus unifying all of these diverging literatures. Fortunately,
such a work appeared, and it and its critical reception is the focus
on the next section.


## 4. *The Multiple Realization Book* and Its Initial Critical Reception



Thomas Polger's and Lawrence Shapiro's *The Multiple
Realization Book* (2016) provided what the increasingly
fragmenting literature on multiple realizability needed. It is the
first book-length treatment of the topic, including many of the
broader arguments the topic had become part of. The book pulls
together much of the history of work on the topic, plus the more
recent work by both proponents and opponents, and usefully organizes
all of this around numerous key themes that had come to frame the
myriad debates. Polger's and Shapiro's goal is not simply
to review or clarify the various positions, however. They have a
position to push. They end up developing and defending a mind-brain
identity theory that explicitly recognizes a sense of the autonomy of
psychology from neuroscience. And as one might expect, critics of
their view quickly responded. The unifying focus that this book
offers, and its initial critical reception, make a detailed discussion
of it a useful focus for where the broader issues about multiple
realizability stand now, at the end of the second decade of the
twentieth century.



Polger and Shapiro begin by focusing on the key contention that
functionalists and "realization physicalists" have
emphasized to support their views over identity theories. That thesis
is "Generality", namely that 



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



And the key premise offered in support of this contention, going back
to Putnam's work (presented in
 SS1
 above) is multiple realizability. This contention sets the agenda for
the book: Polger and Shapiro will argue that there is not the sort of
widespread multiple realization of mental states and processes that
would be needed to support functionalism or realization physicalism
over identity theory. Importantly, Polger and Shapiro do not deny that
cases of multiple realization exist. Rather, they insist that the
debate between functionalists and mind-brain identity theorists should
be guided primarily by the question, 



> 
> 
> does the best overall model of psychological and neuroscientific
> processes 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 has
of late come to be fought in the metaphysics of science literature. In
keeping, they begin by offering an account of the ontological
realization relation. And following the lead of their (Shapiro &
Polger 2012) paper (discussed in
 SS3.2
 above) they use this account to scout the types of evidence and
concerns relevant to deciding between realization physicalisms and
identity theories. They restrict the scope of their arguments to
actual proposed cases of realization, eschewing at least initially
concern with philosophers' imagined cases; they are solely
concerned at the outset with multiple realization as an empirical
hypothesis. They are also careful to point out that multiple
realization is logically narrower than mere variation. Working
directly with Putnam's (1967) discussion, Polger and Shapiro
articulate the "Basic Recipe" for finding occurrences of
multiple realization: "Multiple realization occurs if and only
if two (or more) systems perform the same function in different
ways" (2016: 45). While this characterization requires further
refinement, 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 same
behavior in the same circumstances, Polger and Shapiro insist, does
not establish sameness of underlying psychological function generating
that behavior. 



Further refinements introduce "relevance" into Polger and
Shapiro's account. The very function shared by the two kinds
that classifies them as the same in the realized science must be what
is realized by different mechanisms that classify them as different in
the realizing science. Further, the relevant differences in the
realizers contribute to the sameness of function; the kinds are
"not just differently different, they are differently the
same" (2016: 75). Polger and Shapiro show that simple (camera)
and compound eyes meet their fully revised "Official
Recipe" 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 not
> just to the differences in function. (2016: 77) 
> 
> 
> 



Armed with their Official Recipe, Polger and Shapiro next seek to
dismiss the evidence commonly put forward by proponents of multiple
realization as arguments for functionalism or realization-physicalism
over mind-brain identity theory. They first consider "direct
evidence", empirical cases presented as genuine instances of
multiple 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 in
a specific cortical region take on a new function following damage to
nerves or axons providing their normal input. A more
"radical" variety is exemplified by the well-known work in
developing ferrets, in which retinal projections were redirected to
the medial, rather than the lateral geniculate nucleus in the
thalamus, and hence retinal input was projected to auditory, rather
than visual cortex (see the discussion of Shapiro 2008 in
 SS3.2
 above). Polger and Shapiro argue that both of these varieties fail to
meet at least one condition of their Official Recipe, so neither
counts as a genuine instance of mental-to-physical multiple
realization. The "common" case involves change in
function--in the specific example, neuronal activity in the
reorganized region of primate somatosensory cortex now represents
stimulation to a different region of the monkey's hand. In the
"radical" ferret rewiring case, the visual capacities of
the experimental ferrets' auditory cortex, compared to those of
intact control ferrets, differ significantly, e.g., the experimental
ferrets show significantly deficient performances on visual
discrimination tasks compared with controls' performances.



A second kind of direct evidence that Polger and Shapiro consider is
kind splitting in scientific practice. Again, their Official Recipe
figures into their response. They reformulate Aizawa and
Gillett's (2011) key example, the variations in cone opsins in
human color vision (see discussion in
 SS3.1
 above) into Official Recipe format, and deny that it is a case of
genuine multiple realization. The cited differences among the opsins
are not distinct from individual differences among those possessing
human trichromatic color vision, violating one condition of the
Official Recipe.



Polger and Shapiro then extend their discussion of evidence to
consider two "indirect" kinds for mental-to-physical
multiple realiz*ability*. This evidence seeks to show that, in
light of various observations, the multiple realiz*ability*
hypothesis is more likely to be true than the mind-brain identity
hypothesis. They begin by offering a canonical form for indirect
evidence arguments, which compares the likelihood of multiple
realizability versus the unlikelihood of mind-brain identities in
light of the evidence. They then recast Block and Fodor's (1972)
appeal to neural plasticity (see discussion in
 SS1.3
 above), an appeal to evolutionary convergence of psychological states
and processes, and an appeal to the "conceptual
possibility" of artificial minds into this canonical form, and
argue that all three arguments fail. Finally they consider an appeal
to the prevalence of the "computational theory of mind"
within current cognitive science and psychology as an indirect
argument for multiple realizability. In agreement with some recent
mechanists (e.g., Kaplan and Craver 2011), Polger and Shapiro insist
that many "computational explanations" are better
understood "as precise descriptions of the task or behavior to
be explained" rather than as explanations (2016: 154). And while
they admit that the "representationalist" variety of
computationalism (of which Fodor's 1975 Language of Thought is a
paradigmatic example) does rule out the possibility of mind-brain
identities, they point out that this kind of computationalism is
"not the only game in town" anymore (2016: 158). Numerous
viable competing alternatives to it exist--Polger and Shapiro
mention connectionism, strong and "ruthless" reductionism,
and mechanism explicitly in this context--and all those make room
for mind-brain identities.



Polger and Shapiro wrap up the book by absolving their identity
theory, which rests on their treatment of multiple realizability, of
some mistaken charges. They argue that their account is not
ontologically eliminativist about psychological kinds, because none of
their three types of arguments against multiple
realization--"unificationist" arguments which find
relevant commonalities in the realizing kinds, "heuristic,
abstraction, and idealization" arguments that characterize
practices in contemporary cognitive science, and their plump for
taxonomic "kind splitting"--leads to any kind of
troubling eliminativist conclusion. And by adopting James
Woodward's (2003) "interventionist
difference-making" account of causal explanation, Polger and
Shapiro deflect the charge that their identity theory undermines the
legitimacy of psychological explanations, and thus rules out any
autonomy for psychology vis-a-vis neuroscience. Such a charge, they
insist, rests on an overly stringent and misguided understanding of
scientific explanation. Identity theorists can be and should be
pluralists about scientific explanation. Neuroscientists can appeal to
neural causes, psychologists and cognitive scientists to mental
causes. Since both can cite causal invariances at their respective
levels, both offer genuine explanations. Despite the mind-brain
identities, psychology remains a methodologically autonomous science.
(Their defense of psychology's autonomy resembles
Woodward's recent more general defense of a general kind of
explanatory 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 for
philosophical discussions of multiple realization, both historical and
recent. Similarly, initial criticisms of the book ranged from ones
squarely in philosophy of mind, to others aimed at more broadly
metaphysics of science considerations. Not surprisingly, given that
the book defends a version of mind-brain identity theory, which has
been considered indefensible, or at best fringe, for some time, it met
with rapid criticism from philosophers of mind. Ronald Endicott (2017)
takes Polger and Shapiro to task for offering few (if any) examples of
explicit mind-brain identities. Endicott's criticism is fair
enough, in one sense. Polger and Shapiro nowhere attempt to catalogue
or 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, and
neuroscience has progressed rapidly over that time. Polger and Shapiro
do criticize numerous claims to have found multiple realization of
mind on brain, so perhaps those discussions can serve implicitly as
mind-brain identity claims. But circa 2016, one reasonably could have
hoped for some examples of specific mind-brain identities, and a
defense of the evidence that supports them.



Umut Baysan (2018) raises two challenges. His first is squarely within
philosophy of mind. He contends that Polger and Shapiro change
"the rules of the game" over which functionalists/realization physicalists and identity theorists are disputing. Polger
and 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 existential
claim, i.e., that there are some instances of mind-brain identities.
Second, and more generally, Baysan takes Polger and Shapiro to task
for limiting their concern to the empirical hypothesis, multiple
realization, and refusing to speak to concerns about metaphysically
possible realizers of mental kinds, or multiple realizability. Baysan
insists that some philosophers are still concerned with the latter,
and a work that purports to be a book-length treatment of the general
topic will leave these philosophers justifiably disappointed. On the
first point, Baysan is right that Polger and Shapiro do seek to
reorient this fifty-year-old debate, and they characterize
functionalism and realization physicalism as quite general hypotheses.
But Baysan's account of Polger and Shapiro's weakened
identity theory seems to miss the point of their explicit
reformulation of the multiple realization problem, into first and
foremost the question, 



> 
> 
> does the best overall model of psychological and neuroscientific
> processes make substantial and important use of identities? (Polger
> and Shapiro 2016: 34) 
> 
> 
> 



That reformulated focus requires a much stronger defense than does
Baysan's existential hypothesis interpretation of their identity
theory. By it Polger and Shapiro are committed to arguing that
identity theory provides "the best overall model" of
psychology's relationship to neuroscience. (Whether they succeed
in making this argument is a separate issue.) And as far as a need to
discuss metaphysical possibilities, and hence multiple realizability
in the sense that some philosophers understand it, that depends on who
one takes one's audience to be. If one is concerned strictly
about multiple realization as a part of the philosophy of the special
sciences--as some discussants of multiple realization
are--metaphysical possibilities and realizations of the mental in
"metaphysically possible worlds" can strike one as
philosophical jibber-jabber, the kind of discussions that give
philosophy a bad name within broader twenty-first-century intellectual
contexts. Curiously, however, Polger and Shapiro seem not to want to
take that extreme a position. They locate their work squarely in the
*metaphysics* of science, not strictly in philosophy of science
or metascience. Does that leave them open to the charge of needing to
consider at least some metaphysical possibilities, and so some kinds
of multiple realiz*ability*? We will turn to this problem later
in this section, in light of Polger and Shapiro's replies to
other criticisms.



Danny Booth (2018) is sympathetic to Polger and Shapiro's
broader project, and to their conclusion about the importance of
mind-brain identities in contemporary cognitive science. But he
worries about their deference to scientists concerning how mental
kinds are individuated, and thus identified across distinct physical
realizations, as their Official Recipe for multiple realization
proposes. He points out that many scientists view questions about
mental kind individuation as "philosophical", and so punt
this concern back to philosophers. Scientists also sometimes
individuate mental kinds on quite narrow grounds; Booth here cites
Jacqueline Sullivan's recent discussions of the difficulty of
identifying mental kinds across neuroscience labs that employ
different protocols (see discussion of Sullivan's work in the
"Philosophy of Neuroscience" entry, this encyclopedia).
Sometimes they do so purely on behavioral grounds which operationalize
mental kinds for laboratory experimentation. The latter approach flies
directly in the face of Polger and Shapiro's explicit rejection
of mental kind identity in their Official Recipe for multiple
realization. Booth also worries about Polger and Shapiro's more
general appeal to an interventionist account of causation to salvage
"actual" autonomy for psychology. Nothing stops
functionalists or realization physicalists from adopting a similar
approach to saving psychology's autonomy, and in fact many avail
themselves of this approach. Booth worries that 



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



That Polger and Shapiro's view shares more points in common with
traditional functionalism than with traditionally
"reductionist" versions of identity theory leaves Booth
wondering whether Polger and Shapiro's account itself faces some
of the same challenges they direct at functionalism. 



Mark Couch (2018) worries about Polger and Shapiro's general
characterization of multiple realization in terms of the taxonomies
offered by different sciences, as opposed to more typical accounts in
terms of properties or kinds in the world. About this shift, Couch
first points out that scientists in any given field aren't
always in agreement about how to taxonomize the phenomena they
address. So appealing to "scientists' taxonomies"
might not be sufficient, and philosophers might be forced back to
considering "independently existing kinds" (2018: 421).
Polger and Shapiro's refocused account also might not address
what has interested philosophers about multiple realization, namely,
concerns about metaphysical relations holding between independently
existing properties and kinds; independent, that is, of any taxonomic
practices. This leads Couch to worry about Polger and Shapiro's
account of "relevant differences" in their "Official
Recipe", especially their account of how we decide when this
condition has been met. He senses an internal tension lurking here. On
the one hand, 



> 
> 
> the sciences that deal with the putative realizers tell us which
> differences in them amount to different ways of producing their
> effects 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 "relevant
differences": "a matter of whether a property has an
explanatory role with respect to a function" (Couch 2018: 424).
This "unofficial" understanding, Couch insists, sees
relevance as an explanatory relation between properties or kinds in
nature, and thus not merely a matter of what scientists "tell
us". These two readings of "relevant differences"
strike Couch as making it unclear how the Official Recipe will apply
to Polger's and Shapiro's own examples of multiple
realized kinds.



In contrast with Booth and Couch, who challenges Polger and Shapiro
for deferring too much to scientists and their taxonomies, Mazviita
Chirimuuta chides them for not paying enough attention to actual
"functional thinking" in the life sciences, and thus
relying too heavily on "toy examples" like corkscrews.
Chirimuuta agrees with Polger and Shapiro that the old
philosophers' notion of multiple realization,
"MR1.0", needs replacing, but suggests looking directly
into three areas of the life sciences for its replacement,
"MR2.0". One area is evolutionary approaches, where
Chirimuuta mentions explicitly Ernst Mayr's proximate/ultimate
explanation distinction, the latter referring to the evolutionary
function 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 a
function in the nervous system and then works back to see how the
physical components perform it. The third area is the study of
robustness in biological systems, whereby the function of a system is
maintained despite alterations in lower-level mechanisms. Chirimuuta
notes that Criterion IV (the "differently the same"
condition mentioned above) of Polger and Shapiro's
"Official Recipe" for genuine multiple realization would
probably preclude many cases of biological robustness, in the same
fashion Polger and Shapiro used it to rule out the
"radical" ferret rewiring example (discussed above). But
for Chirimuuta, this consequence suggests that Polger and
Shapiro's "Official Recipe" may be "missing
something interesting about the organization of living systems"
(2018: 408); and so may not be sensitive enough to real scientific
practice. She notes that for all their talk about relying on
scientists' judgments and taxonomies, Polger and Shapiro's
favorite "toy example", the corkscrew, comes up a lot when
the conditions on genuine multiple realization are being articulated
and defended. Her point is that a closer look at some real sciences
that engage in functional thinking, at the point when Polger and
Shapiro are developing and defending their basic analysis of multiple
realization, might have generated a different account of what multiple
realization is ("MR2.0"). As opposed to what Polger and
Shapiro actually do with real scientific examples, namely bring them
up to debunk claims about successful multiple realization based on
their already existing detailed analysis. Such an approach seemingly
would have produced an account tied much more closely to actual
scientific practice.



In their (2018) Polger and Shapiro respond to these criticisms. They
begin by clarifying what they call a common misconception of their
view (and not just among these critics), that they don't take
seriously multiple realiz*ability*. They claim they do, but
they insist that even these metaphysical concerns "are not
immune from evidential demands" (2018: 448). The best evidence
for multiple realiz*ability* would be widespread instances of
multiple realiz*ation*; but this, as they argued extensively in
their (2016), is exactly what is lacking. Other kinds of evidence for
multiple realizability include equipotentiality, evolutionary
convergence, and computational explanations in the relevant sciences,
but Polger and Shapiro insist that they have argued in their book that
these 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 directions
concerning their dependence on science. Booth and Couch both insisted
that Polger and Shapiro rely too heavily on taxonomies postulated by
scientists to characterize genuine instances of multiple realization.
Polger and Shapiro take both Booth's and Couch's worry to
be that their approach thereby shackles their account to "the
whim of scientists" (2018: 449). They remind us that by
"taxonomy" they refer to "whatever a particular
science views as belonging within its domain" (2018: 449). The
sciences themselves provide those; the best evidence we can have for
any science's "ontological commitments" are
"the taxonomic representations and models that it uses"
(2018: 450). Both Polger and Shapiro, and their multiple realization
proponents, are assuming that the relevant sciences track real
distinctions in the world. Polger and Shapiro agree that sciences can
and do mischaracterize these, but "that problem has to do with
the fallibility of evidence and not with our account of multiple
realization" (2018: 450). Polger and Shapiro also admit that
their reference to the notion of taxonomies has not always been clear;
but nevertheless appealing to scientific taxonomies as a form of
evidence for multiple realization is useful. Their focus on scientific
taxonomies is thus not incompatible with more traditional
philosophical discussions about multiple realization.



They then turn to Chirimuuta's contrary criticism, that in
developing 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 when
> description is restricted to more fine-grained vocabulary (be it
> physical, chemical, or neuroanatomical), 
> 
> 
> 



with these classifications thus averting us to the "limitations
of purely reductive research agendas in biology and
neuroscience" (Chirimuuta 2018: 409). If Chirimuuta is here
advocating pluralism about scientific explanation, Polger and Shapiro
are in full agreement; they are avowed methodological pluralists. But
if Chirimuuta is here advocating some kind of "Essential
Autonomy Thesis", that higher-level regularities are
"somehow invisible" to would-be reducing explanations, or
"can only be captured in the proprietary vocabulary of a special
science", then Chirimuuta's MR2.0 sounds suspiciously like
MR1.0, with the latter being a view she herself insists needs to be
rejected. They then insist that Chirimuuta's own discussion of
the role of functional thinking in biology and neuroscience stresses
its *methodological* importance. Again, Polger and Shapiro have
no complaint about that. Their critical focus is on an inference from
this pluralist methodology and the ability to model a system in this
fashion to the conclusion that the system is *ipso facto*
multiply realizable" (2018: 454-455). Being a fellow
rejecter of MR1.0 and its accompanying Essential Autonomy Thesis,
Polger and Shapiro insist that Chirimuuta should also reject any such
inference.



In closing this section, it should be clear that the extension of
philosophical discussions about multiple realization, which began with
the "second-wave" of criticisms around the year 2000,
continues apace. The specific philosophical arena in which these
discussions take place continues to expand, from the topic's
origins in mid-twentieth century philosophy of mind through its
turn-of-the millennium path into the philosophy of the cognitive
sciences and neuroscience, and lately more squarely in the general
metaphysics of science. These shifts can make the details of the
evolving discussions difficult to track. Polger and Shapiro's
(2016) book, and the critical scrutiny across all of these fields that
it spurred, provides a useful framework for organizing these many
directions, despite what one judges about its skepticism about the
extent of multiple realization and its defense of a mind-brain
identity theory. The literature over the last twenty years should put
to rest the still-too-prominent view in the philosophy of mind that
multiple realization spells doom for reductive materialism and
mind-brain identity theory once and for all, as well as the
still-too-prominent view in some quarters in philosophy of science
that multiple realization, or at least its anti-reductionist
consequences, has been vanquished for good. Both views look uninformed
and badly outdated in light of the detailed history of multiple
realization, a history which now spans more than a half-century of
Anglo-American philosophy.