important role in supporting antireductionism in philosophy of mind.
The multiple-realizability thesis implies that mental types and
physical types are correlated one-many not one-one. A mental state
such as pain might be correlated with one type of physical state in a
human and another type of physical state in, say, a Martian or
pain-capable robot. This has often been taken to imply that mental
types are not identical to physical types since their identity would
require one type of mental state to be correlated with only one type
of physical state. The principal debate about multiple realizability
in philosophy of mind concerns its compatibility or incompatibility
with reductionism. On the assumption that reduction requires
mental-physical type identities, the apparent multiple realizability
of mental types, such as a pain being both a type of human brain state
and a type of robot state, has been understood to support
antireductionism. More recent work has challenged this understanding.
The antireductionist argument depends on the following premises:
Mental types are multiply realizable;
If mental types are multiply realizable, then they are not identical
to physical types;
If mental types are not identical to physical types, then
psychological discourse (vernacular or scientific) is not reducible to
physical theory.
Among these claims, the most controversial has been Premise 1, the
multiple-realizability thesis. Antireductionists have supported it
both a priori by appeal to conceivability-possibility principles, and
a posteriori by appeal to findings in biology, neuroscience, and
artificial intelligence research. Reductionists have criticized these
arguments, and they have also directly challenged the antireductionist
premises.
Reductionist challenges to Premises 1 and 2 claim that
antireductionists dubiously assume that psychophysical relations must
be reckoned relative to our current mental and physical typologies.
Contrary to this assumption, some reductionists argue that future
scientific investigation will result in the formulation of new mental
and/or physical typologies which fail to support the antireductionist
premises. Typology-based arguments of this sort have been among the
most important and most widely discussed reductionist responses to the
multiple-realizability argument. Responses that target Premise 3 have
been less popular. They argue either that psychophysical reduction can
be carried out without identity statements linking mental and physical
types, or else that ontological issues concerning the identity or
nonidentity of mental and physical types are completely orthogonal to
the issue of reduction.
The multiple-realizability thesis has also played an important role in
recent discussions about nonreductive physicalism. The
antireductionist argument has often been taken to recommend some type
of nonreductive physicalism. Recently, however, Jaegwon Kim has
effectively stood the argument on its head. He argues that
physicalists who endorse multiple realizability are committed either
to denying that mental types are genuine properties, ones that make a
causal difference to their bearers, or else they are committed to
endorsing some type of reductionism which identifies mental types with
physical types.
1. Multiple Realizability and the Antireductionist Argument
Multiple-realizability theses claim that it is possible for the tokens
of a certain type to be realized by tokens of two or more distinct
types. Multiple-realizability theses can be applied to a broad range
of types: chemical, biological, social, mathematical. But what has
been of primary interest in philosophy of mind is the purported
multiple realizability of mental types. In what follows, the
multiple-realizability thesis (MRT) will be understood as the claim
that specifically mental types are multiply realizable.
Roughly, a type φ is multiply realizable if and only if it is possible
for φ-tokens to be realized by tokens of two or more distinct types.
If, for instance, it is possible for tokens of the mental type pain to
be realized by tokens of the types c-fiber firing and q-fiber firing,
where c-fiber firing ≠ q-fiber firing, then pain is a
multiply-realizable mental type. Debate about the MRT in philosophy of
mind has principally concerned its compatibility or incompatibility
with reductionism. The MRT has been widely understood to have
antireductionist implications. It seems to imply that mental types are
not identical to physical types. If psychophysical reduction requires
mental-physical type identities, then the MRT seems to imply that
psychophysical reductionism is false.
The antireductionist argument is roughly as follows: Suppose a certain
type of mental state – pain, say – is multiply realizable. We
discover, for instance, that Alexander's pains are intimately
correlated in a way we label 'realization' with a certain type of
physical occurrence, the firing of his c-fibers. We also discover,
however, that Madeleine's pains are realized not by c-fiber firing but
by a distinct type of physical occurrence, q-fiber firing. Since
c-fiber firing does not in any way involve q-fiber firing, and q-fiber
firing does not in any way involve c-fiber firing, we conclude that
pain can occur without c-fiber firing, and that it can also occur
without q-fiber firing. We conclude, in other words, that neither
c-fiber firing nor q-fiber firing is by itself necessary for the
occurrence of pain. In that case, however, it seems that pain cannot
be identical to either type of physical occurrence since identity
implies necessary coextension. If having a mass of 1 kilogram is
identical to having a mass of 2.2 pounds, then necessarily something
has a mass of 1 kilogram if and only if it has a mass of 2.2 pounds.
Likewise, if pain is identical to c-fiber firing, then necessarily
anything that has pain will also have c-fiber firing; and if pain is
identical to q-fiber firing, then necessarily anything that has pain
will also have q-fiber firing. Madeleine, however, experiences pain
without c-fiber firing, and Alexander experiences pain without q-fiber
firing. Since pain is not correlated with a single physical type, it
seems that pain cannot be identical to a physical type. Moreover,
because the identity of type M and type P implies that necessarily
every M-token is a P-token, we need not actually discover the
correlation of pain with diverse physical types; the bare possibility
of such correlations is sufficient for the argument to succeed. If the
case of Alexander and Madeleine is even possible, it would follow that
pain is not a physical type; and, says the argument, it seems
intuitively certain or at least overwhelmingly probable that this type
of situation is possible not only for pain, but for all mental types.
Since psychophysical reductionism requires that mental types be
identical to physical types, psychophysical reductionism must be
false.
The foregoing line of reasoning has been extremely influential since
1970. It is largely responsible for what has been and continues to be
a widespread, decades-long consensus that psychophysical reductionism
must be false. The argument trades on the following premises:
Mental types are multiply realizable;
If mental types are multiply realizable, then they are not identical
to physical types;
If mental types are not identical to physical types, then
psychological discourse (vernacular or scientific) is not reducible to
physical theory.
These premises will be considered in order.
a. Multiple Realizability and Multiple Correlatability
The term 'multiple realizability' is often used as a label for any
claim to the effect that mental and physical types are correlated
one-many. Properly speaking, however, multiple realizability is tied
to the notion of realization. Since the notion of realization is tied
to a particular account of mental properties and psychological
language it will be helpful to distinguish the multiple-realizability
thesis from a more general multiple-correlatability thesis (MCT), a
claim to the effect that φ-tokens might be correlated with tokens of
more than one type.
The form of a bare multiple-correlatability argument against
psychophysical identification is something like the following:
1. If mental type M = physical type P, then necessarily every M-token
is a P-token and vice versa;
2. It is not necessarily the case that every M-token is a P-token and
vice versa;
Therefore, mental type M ≠ physical type P.
Given reasonable assumptions the first premise follows from Leibniz's
law: type-identity implies necessary token coextension. Premise 2
states the MCT: M-tokens and P-tokens needn't be correlated one-one.
An MCT does not specify whether M- and P-tokens are systematically
related to each other or in what way. It is thus weaker than the MRT
which claims specifically that tokens of one type realize tokens of
the other type.
One important observation here is that the MRT is not the only way of
endorsing an MCT. Bealer (1994), for instance, defends an MCT in a way
that does not appeal to realization at all. Moreover, even Putnam, who
is often credited with having been the first to advance a
multiple-realizability argument against psychophysical identity
theory, appealed to a bare MCT as opposed to an MRT:
Consider what the brain-state theorist has to do to make good his
claims. He has to specify a physical-chemical state such that any
organism… is in pain if and only if (a) it possesses a brain of a
suitable physical-chemical structure; and (b) its brain is in that
physical-chemical state. This means that the physical-chemical state
in question must be a possible state of a mammalian brain, a reptilian
brain, a mollusc's brain… etc. At the same time, it must not be a
possible… state of the brain of any physical possible creature that
cannot feel pain… [I]t is not altogether impossible that such a state
will be found… [I]t is at least possible that parallel evolution, all
over the universe, might always lead to one and the same physical
"correlate" of pain. But this is certainly an ambitious hypothesis
(Putnam 1967a: 436).
Putnam claims it is highly unlikely that pain is correlated with
exactly one physico-chemical state. There is no mention of
realization.
The notion of realization was introduced in connection with
functionalism, the theory Putnam advanced as an alternative to the
identity theory. According to functionalism mental types are not
identical to physical types; they are instead realized by physical
types. Putnam argued that functionalism was more plausible than the
identity theory precisely because it was compatible with mental types
being correlated one-many with physical types. Before discussing this
point, however, it will be helpful to say a word about functionalism
since the term 'functionalism' has been used to refer to theories of
at least two different types: a type originally inspired by a
computational model of psychological discourse and developed in a
series of papers by Putnam (1960, 1964, 1967a, 1967b); and a type of
identity theory endorsed by Lewis (1966, 1970, 1972, 1980) and
independently by Armstrong (1968, 1970). Talk of realization has been
used in connection with both.
b. Identity Theory, Functionalism and the Realization Relation
Early identity theorists claimed that psychological discourse was like
theoretical discourse in the natural sciences. Mental states, they
said, were entities postulated by a theory to explain the behavior of
persons in something analogous to the way atoms, forces, and the like
were entities postulated by a theory to explain motion and change
generally (Sellars 1956: 181-87; 1962: 33-34; Putnam 1963: 330-331,
363; Feigl 1958: 440ff.; Fodor 1968a: 93; Churchland 1989: 2-6). The
entities postulated by psychological discourse – beliefs, desires,
pains, hopes, fears – were to be identified on the basis of empirical
evidence with entities postulated by the natural sciences, most likely
entities postulated by neuroscience. Originally, identity theorists
supposed that theoretical identifications of this sort were a matter
of choice. Empirical data would support correlations between mental
and physical types such as 'Whenever there is pain, there is c-fiber
firing', and scientists would then choose to identify the correlated
types on grounds of parsimony. Identifying pain with c-fiber firing
would yield a more elegant theory than merely correlating the two, and
it would avoid the potentially embarrassing task of having to explain
why pain and c-fiber firing would be correlated one-one if they were
in fact distinct (Smart 1962). Lewis (1966) criticized this model of
theoretical identification, and advanced an alternative which was also
endorsed independently by Armstrong (1968, 1970).
According to the Lewis-Armstrong alternative, theoretical
identifications are not chosen on grounds of parsimony, but are
actually implied by the logic of scientific investigation. In our
ordinary, pre-scientific dealings we often introduce terms to refer to
things which we identify on the basis of their typical environmental
causes and typical behavioral effects. We introduce the term 'pain',
for instance, to refer to the type of occurrence, whatever it happens
to be, that is typically caused by pinpricks, burns, and abrasions,
and that typically causes winces, groans, screams, and similar
behavior. That type of occurrence then becomes a target for further
scientific investigation which aims to discover what it is in fact.
Pain is thus identified by definition with the type of occurrence that
has such-and-such typical causes and effects, and that type of
occurrence is then identified by scientific investigation with c-fiber
firing. Pain is thus identified with c-fiber firing by the
transitivity of identity. Call this sort of view the Lewis-Armstrong
identity theory.
By contrast with the Lewis-Armstrong identity theory, functionalism
claims that psychological states are postulates of abstract
descriptions which deploy categories analogous to those used in
computer science or information-theoretic models of cognitive
functioning. Functionalists agree with identity theorists that
psychological discourse constitutes a theory, but they disagree about
what type of theory it is. Psychological discourse is not like a
natural scientific theory, functionalists claim, but like an abstract
one. The mental states it postulates are analogous to, say, the angles
and lines postulated by Euclidean geometry. We arrive at Euclidean
principles by abstraction, a process in which we focus on a narrow
range of properties and then construct "idealized" descriptions of
them. We focus, for instance, on the spatial properties of the objects
around us. We ignore what they are made of, what colors they have, how
much they weigh, and the like, and focus simply on their dimensions.
We then idealize our descriptions of them: slightly crooked lines, for
instance, we describe as straight; deviant curves we describe as
normal, and so on. According to functionalists, something analogous is
true of psychological discourse. It provides abstract descriptions of
real-world systems, descriptions which ignore the physical details of
those systems (the sorts of details described by the natural
sciences), and focus simply on a narrow profile of their features.
Originally Putnam suggested that those features were analogous to the
features postulated by Turing machines.
A Turing machine is an abstract description which postulates a set of
states related to each other and to various inputs and outputs in
certain determinate ways described by a machine table. A certain
machine table might postulate states, S1,…,Sn, inputs, I1,…, Im, and
outputs O1,…,Op, for instance, which are related in ways expressed by
a set of statements or instructions such as the following:
If the system is in state S13 and receives input I7, then the system
will produce output O32, and enter state S3.
According to Putnam's original proposal, which has come to be called
machine functionalism, psychological descriptions are abstract
descriptions of this sort. They postulate relations among sensory
inputs, motor outputs, and internal mental states. The only
significant difference between Turing machine descriptions and
psychological descriptions, Putnam (1967a) suggested, was that
psychological inputs, outputs, and internal states were related to
each other probabilistically not deterministically. If, for instance,
Eleanor believes there are exactly eight planets in our solar system,
and she receives the auditory input, "Do you believe there are exactly
eight planets in our solar system," then she will produce the verbal
output, "Yes," not with a deterministic probability of 1, but with a
probability between 1 and 0.
Functionalists need not endorse a Turing machine model of
psychological discourse; they could instead understand psychological
discourse by appeal to models in, say, cognitive psychology; but in
general, they make two claims. First, psychological discourse is
abstract discourse which postulates an inventory of objects,
properties, states or the like which are related to each other in ways
expressed by the theory's principles. Second, the behavior of certain
concrete systems maps onto the objects, properties, or states that
psychological discourse postulates. The notion of realization concerns
this second claim.
Let T be a theory describing various relations among its postulates,
S1,…,Sn.The relations among the concrete states of a certain concrete
system might be in some way isomorphic to the relations among S1,…,Sn.
If T says that state S1 results in state S2 with a probability of .73
given state S15, it might turn out that, for instance, Alexander's
brain state B5 results in brain state B67 with a probability of .73
given neural stimulus B4. It might turn out, in other words, that
states B5, B67, and B4 in Alexander's brain provide a model of the
relations among S1, S2, and S15 in T. If this were true for all of
Alexander's brain states, one might say that T described a certain
type of functional organization, an organization which was realized by
Alexander's brain, and one might call Alexander's brain a realization
of T. The states of Alexander's brain are related to each other in
ways that are isomorphic with the ways in which S1,…,Sn, are related
according to T. In fact, concrete systems in general might be said to
realize the states postulated by abstract descriptions. The wooden
table realizes a Euclidean rectangle; the movements of electrons
through the silicon circuitry of a pocket calculator realize a certain
algorithm; the movements of ions through the neural circuitry of
Alexander's brain realize a belief that 2 + 2 = 4, and so forth.
Realization, then, is a relation between certain types of abstract
descriptions, on the one hand, and concrete systems whose states are
in a relevant sense isomorphic with those postulated by abstract
descriptions, on the other. Philosophers of mind have offered several
different accounts of this relation. Putnam (1970: 313-315) suggested
a type of account which has proved very influential. Realization, he
said, can be understood as a relation between higher-order and
lower-order types (he used the term 'properties') or tokens of such
types. Higher-order types are ones whose definitions quantify over
other types. Second-order types, for instance, are types whose
definitions quantify over first-order types, and first-order types are
types whose definitions quantify over no types. Effectively what
Putnam suggested is that having mental states amounted to having some
set of (first-order) internal states related to each other in ways
that collectively satisfied a certain functional description. Being in
pain, for instance, might be defined as being in some concrete
first-order state S1 which results in a concrete first-order state S2
with a probability of .73 given a concrete state S15. In other words,
the various Si postulated by theory T can be understood as variables
ranging over concrete first-order state types such as brain state
types. To say, then, that Alexander's brain is currently realizing a
state of pain is just to say that the triple < B5, B67, B4 > of
concrete first-order states of his brain satisfies the definition of
being in pain, a definition which quantifies over concrete first-order
states of some sort.
The concept of realization is understood slightly differently in
connection with the Lewis-Armstrong identity theory. That difference
reflects the more general difference between the identity theory and
functionalism. Functionalism takes mental states to be states
postulated by an abstract description, whereas the Lewis-Armstrong
identity theory takes mental states to be concrete physical states
which have been described in terms of an abstract vocabulary. To help
illustrate this difference consider a very rough analogy with a
Platonic versus Aristotelian understanding of geometrical objects. The
Platonist claims that 'rectangle' refers to an abstract object
postulated and/or described by Euclidean geometry. The Aristotelian,
by contrast, claims that 'rectangle' is a way of referring to various
concrete objects in terms of their dimensions. There is a roughly
analogous sense in which the functionalist claims that 'pain'
expresses a type of abstract state whereas the Lewis-Armstrong
identity theorist claims that 'pain' expresses a concrete type of
physical state such as c-fiber firing. According to the identity
theorist 'pain' refers to a physical state by appeal to a narrow
profile of that state's properties such as its typical causes and
effects. According to the Lewis-Armstrong identity theory, then, what
a theory such as T provides is not an inventory of abstract states,
but an apparatus for referring to certain physical ones. On the
Lewis-Armstrong theory those physical states, the ones expressed by
the predicates and terms of T, provide a realization of T.
Because the multiple-realizability argument for antireductionism
principally concerns the functionalist notion of realization, the term
'realize' and its cognates should be taken to express that notion in
what follows.
c. Defining Multiple Realizability
Let us consider again the rough definition of multiple realizability
stated earlier: a type φ is multiply realizable if and only if it is
possible for φ-tokens to be realized by tokens of two or more distinct
types. To make this more precise it will be helpful to draw some
distinctions.
First, Shoemaker (1981) distinguishes what he calls a state's core
realizer from what he calls its total realizer. Consider again the
theory T and Alexander's brain. If B5 is the type of brain state which
corresponds to S1 in T, then B5-tokens are core realizers of S1-tokens
in Alexander's brain. The total realizer of an S1-token, on the other
hand, includes tokens of B5 together with tokens of the other types of
states in Alexander's brain whose relations to one another are
collectively isomorphic with the relations among S1,…,Sn, expressed in
T. The MRT has typically been understood to be a claim about core
realizers.
Second, it is helpful to clarify ambiguities in the scope of the modal
operator. The foregoing definition of multiple realizability is
unclear, for instance, about whether or not -tokens must be realized
by tokens of more than one type in the same world, or whether it is
sufficient that -tokens be realized by tokens of more than one type
in different worlds. Similarly, it is unclear about which worlds are
relevant: nomologically possible worlds? metaphysically possible
worlds? The following definition clears up these ambiguities:
[Def] A type M is multiply realizable iffdf. (i) possiblyM, P-tokens
are core realizers of M-tokens, and (ii) possiblyM, Q-tokens are core
realizers of M-tokens, and (iii) P ≠ Q.
Here, 'possiblyM' designates metaphysical possibility. (The subscript
'M' will be used henceforth to indicate that a modal operator covers
metaphysically possible worlds.) Metaphysical possibility is all that
is needed for the multiple-realizability argument to proceed. If M
were identical to P, then it would not be possible for M-tokens to
exist without P-tokens (or vice versa) in any possible world
irrespective of other factors such the laws of nature obtaining at
those worlds.
Consider again the original example concerning pain. According to the
foregoing definition of multiple realizability, pain is multiply
realizable if and only if there is a metaphysically possible world in
which tokens of, say, c-fiber firing are core realizers of
pain-tokens, and there is a metaphysically possible world in which
tokens of a different type – say, q-fiber firing – are core realizers
of pain-tokens. Hence, if token c-fiber firings are core realizers of
Alexander's pain-tokens in world w1, and token q-fiber firings are
core realizers of Madeleine's pain-tokens in world w2, then pain is a
multiply-realizable mental type. Moreover, if w1 and w2 are identical
with the actual world, then we can say not only that pain is multiply
realizable, but that pain is also multiply realized.
d. Multiple Realizability and Mental-Physical Type Identities
As mentioned earlier, the MRT is one way of endorsing an MCT. The
second premise of the antireductionist argument reflects this idea. It
claims that if mental types are multiply realizable, then they are not
identical to physical types. The argument for this premise trades on
the following claim:
P1. Necessarily, for mental type M and physical type P, if M is
multiply realizable, then it is not necessarilyM the case every
M-token is a P-token and vice versa.
The antecedent of this conditional expresses the MRT, and the
consequent expresses an MCT.
Claim P1 is supported by an additional assumption: mental types are
not necessarilyMcorealized. If, for instance, a Q-token realizes an
M-token, then the M-token needn't be realized by some other token in
addition. Hence, to show that M-tokens and P-tokens needn't be
correlated one-one it is sufficient to show that it is possible to
have an M-token without having a P-token. Suppose, then, that in world
w there is a Q-token that realizes an M-token. In order for it to
follow from this that M-tokens couldM occur without P-tokens, we need
to assume that, say, a Q-token doesn't itself require a P-token – that
a Q-token could realize an M-token on its own. We might call this
assumption Corealizer Contingency: mental types don't needM to be
co-realized. Corealizer Contingency implies that it is possibleM for
an M-token to be realized by, say, a Q-token alone, and hence it is
possibleM that there might be an M-token without there being a
P-token. The conclusion that M is not identical to P if M is multiply
realizable now follows from the following premise:
P2. If type M = type P, then necessarilyM every M-token is a P-token
and vice versa.
According to P2 the identity of M- and P-types requires the necessaryM
coextension of M- and P-tokens. By the foregoing argument, however, if
M is multiply realizable it is not necessarilyM the case that there is
an M-token if and only if there is a P-token. Hence, it follows that
if M is multiply realizable, it is not identical to P.
Now for some terminology. For types φ and ψ, call φ one of ψ's
realizing types just in case possiblyM a φ-token realizes a ψ-token.
In that case, one can say that the argument based on P1 and P2
purports to show that if M is multiply realizable, M is not identical
to any of its realizing types.
e. Type Identities and Psychophysical Reductionism
Psychophysical reductionism claims that psychological discourse is
reducible to some type of natural scientific theory such as a
neuroscientific one. Paradigmatically, intertheoretic reduction
reflects a certain type of ontological and epistemological situation.
Domain A is included within Domain B, but for reasons concerning the
way people are outfitted epistemically, they have come to know
A-entities in a way different from the way they have come to know
other B-entities. They have therefore come to describe and explain the
behavior of A-entities using a theoretical framework, TA, which is
different from the theoretical framework they have used to describe
and explain the behavior of other B-entities, the framework TB. The
result is that they do not initially recognize the inclusion of Domain
A in Domain B. People later discover, however, that Domain A is really
part of Domain B; A-entities really just are B-entities of a certain
sort, and hence the behavior of A-entities can be exhaustively
described and explained in B-theoretic terms. This situation is
reflected in a certain relationship between TA and TB. The principles
governing the behavior of A-entities, the principles expressed by the
law statements of TA, are just special applications of the principles
governing the behavior of B-entities in general – the principles
expressed by the law statements of TB. The laws of TA, they say, are
reducible to the laws of TB; and they say that they are able to
provide a reductive description and explanation of A-behavior in
B-theoretic terms. A-statements can be derived from B-statements given
certain assumptions about the conditions that distinguish A-entities
from B-entities of other sorts – so-called boundary conditions. The
descriptive and explanatory roles played by the law statements of TA,
the reduced theory, are thus taken over by the law statements of the
more inclusive reducing theory, TB.
Consider an example. Kepler's laws are thought to have been reduced to
Newton's. Newton's laws imply that massive bodies will behave in
certain ways given the application of certain forces. If those laws
are applied to planetary bodies in particular – if, in other words,
people examine the implications of those laws within the boundaries of
our planetary system – the laws predict that those bodies will behave
in roughly the way Kepler's laws describe. Kepler's laws, the laws of
the reduced theory, are therefore shown to be special applications of
Newton's laws, the laws of the reducing theory. To the extent that
they are accurate, Kepler's laws really just express the application
of Newton's laws to planetary bodies. One upshot of this circumstance
is that people can appeal to Newton's laws to explain why Kepler's
laws obtain: they obtain because Newtonian laws imply that a system
operating within the parameters of our planetary system will behave in
roughly the way Kepler's laws describe.
Intertheoretic reduction is thus marked by the inclusion of one domain
in another, and by the explanation of the laws governing the included
domain by the laws governing the inclusive one. There have been many
attempts to give a precise formulation of the idea of intertheoretic
reduction. Those attempts trade on certain assumptions about the
nature of theories and the nature of explanation. One of the earliest
and most influential attempts was Ernest Nagel's (1961). Nagel
endorsed a syntactic model of theories and a covering-law model of
explanation. Roughly, the syntactic model of theories claimed that
theories were sets of law statements, and the covering-law model of
explanation claimed that explanation was deduction from law statements
(Hempel 1965). According to Nagel's model of reduction, to say that TA
was reducible to TB was to say that the law statements of TA were
deducible from the law statements of TB in conjunction with statements
describing various boundary conditions and bridge principles if
necessary. Bridge principles are empirically-supported premises which
connect the vocabularies of theories which do not share the same stock
of predicates and terms. On the Nagel model of reduction, bridge
principles are necessary for intertheoretic reduction if the reduced
theory's vocabulary has predicates and terms which the vocabulary of
the reducing theory lacks. Suppose, for instance, that LA is a law
statement of TA which is slated for deduction from LB, a law statement
of TB:
LA For any x, if A1(x), then A2(x);
LB For any x, if B1(x), then B2(x).
Since the vocabulary of TB does not include the predicates A1 or A2,
additional premises such as the following are required for the
deduction:
ID1 A1 = B1
ID2 A2 = B2;
Given ID1 and ID2, LA can be derived from LB by the substitution of
equivalent expressions.
The reduction of thermodynamics to statistical mechanics is often
cited as an example of reduction via bridge principles. The term
'heat', which occurs in the law statements of thermodynamics, is not
included in the vocabulary of statistical mechanics. As a result, the
deduction of thermodynamic law statements from mechanical ones
requires the use of additional premises connecting the theories'
respective vocabularies. An example might be the following:
Heat = mean molecular kinetic energy.
Identity statements of this sort are called theoretical
identifications. The theoretical identification of X with Y is
supposed to be marked by two features. First, the identity is supposed
to be discovered empirically. By analogy, members of a certain
linguistic community might use the name 'Hesperus' to refer to a star
that appears in the West in early evening, and they might use the name
'Phosphorus' to refer to a star that appears in the East in early
morning, and yet they might not know but later discover that those
names refer to the same star. Second, however, unlike the
Hesperus–Phosphorus case, in the case of theoretical identifications,
at least one of the predicates or terms, 'X' or 'Y', is supposed to
belong to a theory.
There are numerous episodes of theoretical identification in the
history of science, cases in which we developed descriptive and
explanatory frameworks with different vocabularies the predicates and
terms of which we later discovered to refer to or express the very
same things. The terms 'light' and 'electromagnetic radiation with
wavelengths of 380 – 750nm', for instance, originally belonged to
distinct forms of discourse: one to electromagnetic theory, the other
to a prescientific way of describing things. Those terms were
nevertheless discovered to refer to the very same phenomenon. In the
Nagel model of reduction, theoretical identifications operate as
bridge principles linking the vocabulary of the reduced theory with
vocabulary of the reducing theory. They therefore underwrite the
possibility of intertheoretic reduction.
The Nagel model of reduction has been extensively criticized, and
alternative models of reduction have been based on different
assumptions about the nature of theories and explanation. But the idea
that reduction involves the inclusion of one domain in another implies
that the entities postulated by the reduced theory be identical to
entities postulated by the reducing theory. In claiming to have
reduced Kepler's laws to Newton's, for instance, the assumption is
that planets are massive bodies, not merely objects the behaviors of
which are correlated with the behaviors of massive bodies.
To illustrate the necessity of identity for reduction, imagine that
Domains A and B comprise completely distinct entities whose behaviors
are nevertheless correlated with each other. It turns out, for
instance, that the principles governing the instantiation of A-types
and those governing the instantiation of B-types are isomorphic in the
following sense: for every A-law there is a corresponding B-law, and
vice versa; and in addition, tokens of A-types are correlated one-one
with tokens of B-types. Given this isomorphism, biconditionals such as
the following end up being true:
BC1 Necessarily, for any x, A1(x) if and only if B1(x);
BC2 Necessarily, for any x, A2(x) if and only if B2(x).
Such biconditionals could underwrite the deduction of law statements
such as LA from law statements such as LB. What they could not
underwrite, however, is the claim that TA is reducible to TB. The
reason is that A and B are completely distinct domains which merely
happen to be correlated. This is not a case in which one domain is
discovered to be part of another, more inclusive domain, and hence it
is not a case in which the laws of one domain can be explained by
appeal to the laws of another. Without identity statements such as ID1
and ID2, there is no inclusion of one domain in another, and without
that sort of inclusion, there is no explanation of the reduced
theory's laws in terms of the reducing theory's laws. (See Causey
1977: Chapter 4; Schaffner 1967; Hooker 1981: Part III.)
Sklar (1967) argued that reduction requires bridge principles taking
the form of identity statements by appeal to an example: the
Wiedemann-Franz law. The Wiedemann-Franz law expresses a correlation
between thermal conductivity and electrical conductivity in metals. It
allows for the deduction of law statements about the latter from law
statements about the former. This deducibility, however, has never
been understood to warrant the claim that the theory of electrical
conductivity is reducible to the theory of heat conductivity, or vice
versa. Rather, it points in the direction of a different reduction,
the reduction of the macroscopic theory of matter to the microscopic
theory of matter.
Suppose, then, that we apply the foregoing account of reduction to
psychological discourse. Since that account claims that theoretical
identifications are necessary for intertheoretic reduction, the upshot
is that psychophysical reduction requires mental-physical type
identities. The reduction of psychological discourse to some branch of
natural science would require that mental entities be identified with
entities postulated by the relevant branch of natural science. It
could not involve two distinct yet coordinate domains. This is clear
if we imagine a case involving psychophysical parallelism. Suppose two
completely distinct ontological domains, one comprising bodies, the
other nonphysical Cartesian egos, were governed by principles that
happened to be isomorphic in the sense just described: the laws
governing the behavior of bodies parallel the laws governing the
behavior of the Cartesian egos, and the states of the Cartesian egos
are distinct from but nevertheless correlated one-one with certain
bodily states. In that case, it would be possible to make deductions
about the behavior of Cartesian egos on the basis of the behavior of
bodies, but this deducibility would not warrant the claim that the
behavior of Cartesian egos was reducible to the behavior of bodies.
The behavior of bodies might provide a helpful model or heuristic for
understanding or predicting the behavior of Cartesian egos, but it
would not provide a reducing theory which explained why the laws
governing Cartesian egos obtained. The same point would follow if some
type of neutral monism were true – if, say, mental and physical
phenomena were correlated, but were both reducible to some third
conceptual framework which was neither mental nor physical but
neutral. Mere correlations between mental and physical types, even
ones which are lawlike, are not sufficient to underwrite
psychophysical reduction. Psychophysical reductionism requires the
identity of mental and physical types.
Consider now the putative implications of this claim in conjunction
with the MRT. Psychophysical reduction requires psychophysical type
identities. If mental types are multiply realizable, then they are not
identical to any of their physical realizing types. But if mental
types are not identical to physical types (the tacit assumption being
that the only physical candidates for identification with mental types
are their realizing types), then psychological discourse is not
reducible to physical theory.
2. Arguments for the Multiple-Realizability Thesis
Section 1 discussed the connection between multiple realizability and
antireductionism. Antireductionists argue that if mental types are
multiply realizable, then psychophysical reductionism is false. But
why suppose that mental types are multiply realizable? Why suppose the
MRT is true? The MRT has been supported in at least two ways: by
appeal to conceptual or intuitive considerations, and by appeal to
empirical findings in biology, neuroscience, and artificial
intelligence research. In this section, arguments of both types will
be considered.
a. Conceptual Arguments for the MRT
Conceivability arguments for the MRT claim that conceivability or
intuition is a reliable guide to possibility. If that is the case, and
it is conceivable that mental types might be correlated one-many with
physical types, then it is possible that mental types might be
correlated one-many with physical types. And, say exponents of the
argument, one-many psychophysical correlations are surely conceivable.
Consider the broad range of perfectly intelligible scenarios science
fiction writers are able to imagine – scenarios in which robots and
extraterrestrials with physiologies very different from ours are able
to experience pain, belief, desire, and other mental states without
the benefit of c-fibers, cerebral hemispheres, or other any of the
other physical components that are correlated with mental states in
humans. If these scenarios are conceivable and conceivability is a
more or less reliable guide to possibility, then we can conclude that
these scenarios really are possible. Conceivability arguments for the
MRT, then, trade on the following premises:
CA1 If it is conceivable that mental types are multiply realizable,
then mental types are multiply realizable;
CA2 It is conceivable that mental types are multiply realizable.
Therefore, mental types are multiply realizable.
Conceivability-Possibility Principles (CPs) have been a staple in
philosophy of mind at least since Descartes. He used a CP to argue for
the real distinction of mind and body in Meditation VI:
…because I know that everything I clearly and distinctly conceive can
be made by God as I understand it, it is sufficient that I am able
clearly and distinctly to conceive one thing apart from another to
know with certainty that the one is different from the other – because
they could be separated, at least by God… Consequently, from the fact
that I know that I exist, and I notice at the same time that nothing
else plainly belongs to my nature or essence except only that I am a
thinking thing, I rightly conclude that my essence consists solely in
being a thinking thing… [B]ecause I have on the one hand a clear and
distinct idea of myself, insofar as I am merely a thinking thing and
not extended, and on the other hand, a distinct idea of the body
insofar as it is merely an extended thing and not thinking, it is
certain that I am really distinct from my body, and can exist without
it (AT VII, 78).
Descartes' argument trades on three premises. First, clear and
distinct conceivability is a reliable guide to possibility. In
particular, if it is clearly and distinctly conceivable that x can
exist apart from y, then it is possible for x to exist apart from y.
Second, I can form a clear and distinct conception of myself apart
from my body. Hence, I can exist without it. But third, if x can exist
without y, then clearly x cannot be y. Hence, I cannot be my body. CPs
have become controversial in part because of their association with
arguments of this sort. Jackson's (1982, 1986) knowledge argument and
Searle's (1980) Chinese Room argument as well as a host of other
arguments concerning the possibility of inverted spectra, absent
qualia, and the like trade on CPs.
Unrestricted CPs, ones that do not qualify the notion of
conceivability or limit the scope of the modal operator, have clear
counterexamples. Some of those counterexamples concern the scope of
the operator. DaVinci, for instance, conceived of humans flying with
birdlike wings despite the physical impossibility of such flight.
Similarly, prior to the twentieth century people might have conceived
that it was possible for there to be a solid uranium sphere with a
mass exceeding 1,000 kg – another physical impossibility. Other
counterexamples concern the notion of conceivability. It is unclear,
for instance, whether the conceptions people form of things while
drunk or drugged or in various other circumstances can serve as
reliable guides to possibility.
Because of examples of this sort, exponents of CPs do not endorse
unrestricted versions of them, but versions limited to a particular
type of conceivability, a particular scope for the modal operator, and
a particular subject matter for the claim or scenario being conceived.
Descartes, for instance, spoke of clear and distinct conceivability,
and took the scope of the modal operator to cover metaphysically
possible worlds – or as he puts it, the range of circumstances God
could have brought about. A CP along these lines is immune to
counterexamples such as the uranium sphere and human birdlike flight
since these examples pertain to nomological or physical possibility.
Roughly, p is nomologically possible exactly if p is consistent with
the laws of nature, and p is physically possible exactly if p is
consistent with the laws of physics (physical possibility and
nomological possibility are the same if the laws of physical are the
same as the laws of nature). Since we can know these laws only through
scientific investigation, it seems likely that our conceptions of
nomological and physical possibilities can only be as reliable as our
best scientific knowledge allows them to be. The same can be said of
technological possibility or other kinds of possibility that involve
consistency with conditions that are knowable only a posteriori.
Metaphysical possibility, on the other hand, involves compossibility
with essences – the features things need to exist in any
metaphysically possible world. Knowledge of essences does not
necessarily depend on empirical considerations. Whether or not it does
marks the difference between empirical essentialists and conceptual
essentialists. Roughly, empirical essentialists claim that our
knowledge of essences is analogous to our knowledge of the laws of
physics or of nature: we can learn about them only a posteriori.
Conceptual essentialists disagree: we can come to know essences a
priori.
Descartes is a prototypical conceptual essentialist. He thinks it is
possible to discover something's essence by means of a certain kind of
conceptual analysis. Consider, for instance, his argument in
Meditation II that his essence consists in thinking alone:
Can I not affirm that I have at least a minimum of all those things
which I have just said pertain to the nature of body? I attend to
them… [N]othing comes to mind… Being nourished or moving? Since now I
do not have a body, these surely are nothing but figments. Sensing?
Surely this too does not happen without a body… Thinking? Here I
discover it: It is thought; this alone cannot be separated from me… I
am therefore precisely only a thinking thing… (AT VII, 26-27).
The procedure Descartes follows for forming a clear and distinct
conception of something's essence is roughly as follows. First, he
reckons that the object in question has certain properties. He then
considers whether it can exist without these properties by "removing"
them from the object one-by-one in his thought or imagination. If he
can conceive of the object existing without a certain property, he can
conclude that that property does not belong to the object's nature or
essence. He thus takes himself to arrive by turns at a clearer, more
distinct conception of what the object essentially is. When he applies
this procedure to himself, he initially reckons that he has various
bodily attributes such as having a face, hands, and arms, and being
capable of eating, walking, perceiving, and thinking. He then
considers whether he could still exist without these features by
"removing" them from himself conceptually. He concludes that he could
exist without all of them except the property of thinking. He can form
no conception of himself without it, he says, whereas he can form a
clear and distinct conception of himself without any bodily
attributes. He concludes, therefore, that he can form a clear and
distinct conception of himself as a thinking thing alone apart from
his body or any other.
Conceptual essentialism was en vogue for a long time in modern
philosophy, but empirical essentialism experienced a revival in the
late twentieth century due to the work of Kripke (1972) and Putnam
(1975b). According to empirical essentialists, discerning something's
essence is not a task that can be accomplished from an armchair. It
requires actual scientific investigation since the conceptions we
initially form of things may not correspond to their essential
properties. We might have learned to identify water, for instance, by
a certain characteristic look or smell or taste, but if we brought a
bottle of water to a distant planet with a strange atmosphere that
affected our senses in unusual ways, the contents of the bottle might
no longer look, smell, or taste to us the same way. This would not
mean that the substance in the bottle was no longer water; it would
still be the same substance; it would simply be affecting our senses
differently on account of the planet's strange atmosphere. It would
still be water, in other words, despite the fact that it did not have
the characteristics we originally associated with water. The essential
features of water would remain the same even if its "accidental"
features underwent a change. According to empirical essentialists, the
essential features of something, the features that enable us to claim
that, for instance, the contents of the bottle are essentially the
same on Earth and on the distant planet, are features it is up to
science to discern — features which might not correspond to our
intuitive, prescientific conception of water.
Empirical essentialists tend to be inhospitable to
conceivability-possibility arguments of the sort represented by CA1
and CA2. They can attack the argument in the following ways. First,
against CA1, they can argue that the conceivability of multiple
realizability is a guide to possibility which is only as reliable as
our best scientific knowledge of mental phenomena and their realizers,
and that in its current incomplete state, our scientific knowledge
does not provide us with the resources sufficient to act as a reliable
guide to possibility in this matter. Against CA2, on the other hand,
they can argue that in our current state of scientific knowledge we
cannot conceive of mental types being multiply realizable for either
of two reasons: (a) we don't know enough about mental types and their
realizers to form any clear conception of whether or not they are
multiply realizable, or (b) we do know enough about mental types and
their realizers to form a clear conception that they are not multiply
realizable.
b. Empirical Arguments for the MRT
Empirical arguments for the MRT largely avoid the aforementioned
worries concerning CPs. They generalize from findings in particular
scientific disciplines. Various scientific disciplines, they claim,
provide inductive grounds that support the possibility of mental types
being realized by diverse physical types. Those disciplines include
evolutionary biology, neuroscience, and cognitive science – artificial
intelligence research in particular.
The argument Putnam (1967a) originally advanced against the identity
theory is an example of an appeal to evolutionary biology. According
to Putnam, what we know about evolution suggests that in all
likelihood it is possible for a given mental type to be correlated
with multiple diverse physical types. Block and Fodor (1972: 238) and
Fodor (1968a; 1974) have advanced similar arguments.
We can formulate the appeal to biology in roughly the following way.
The phenomenon of convergent evolution gives us good reason to suppose
there are beings in the universe that are mentally similar to humans.
One reason for this is that the possession of psychological capacities
would seem to be (at least under certain circumstances) selectively
advantageous. The ability to experience pain, for instance, would seem
to increase my chances of survival if, say, I am in danger of being
burned alive. The pain I experience would contribute to behavior aimed
at removing the threat. Likewise, if I am in danger of being eaten by
a large carnivore, my chances of survival will be enhanced if I am
able to feel fear and to respond to the threat appropriately.
Similarly, it is plausible to suppose that in many circumstances my
chances of surviving and successfully reproducing will be improved by
having more or less accurate beliefs about the environment – knowing
or believing that fires and large carnivores are dangerous, for
instance. There are, in short, many reasons for thinking that
possessing mental states of the sort humans possess would be
selectively advantageous for beings of other kinds. This gives us some
reason to suppose that there might be beings in the universe that are
very similar to us mentally. On the other hand, there are analogous
reasons to suppose that those beings are probably very different from
us physically. The last forty years of biological research have shown
us that life can evolve in a broad range of very different
environments. Environments once thought incapable of supporting life
such as deep sea volcanic vents have been discovered to support rich
and diverse ecosystems. It seems very likely, then, that living
systems will be capable of evolving in a broad range of environments
very different from those on Earth. In that case, however, it seems
very unlikely that mentally-endowed creatures evolving in those
environments will be physically just like humans. Our current state of
biological knowledge suggests, then, that there are most likely beings
in the universe who are like us mentally but who are unlike us
physically. Evolutionary biology thus gives us some reason to suppose
the MRT is true.
A second kind of argument appeals not to evolutionary biology but to
neuroscience. One such argument, for instance, appeals to the
phenomenon of brain plasticity (Block and Fodor 1972: 238; Fodor 1974:
104-106; Endicott 1993). Brain plasticity is the ability of various
parts of the brain or nervous system to realize cognitive or motor
abilities. (See Kolb and Whishaw 2003: 621-641 for a description of
brain plasticity and research related to it.) If the section of motor
cortex that controls, say, thumb movement is damaged, cells in the
adjacent sections of cortex are able to take over the functions
previously performed by the damaged ones. What this seems to suggest
is that different neural components are capable of realizing the same
type of cognitive operation. And this gives us some reason to suspect
it is possible for tokens of one mental type to be realized by tokens
of more than one physical type.
Finally, a third type of empirical argument appeals to work in
artificial intelligence (AI) (Block and Fodor, op cit.; Fodor, op
cit). Some AI researchers are in the business of constructing
computer-based models of cognitive functioning. They construct
computational systems that aim at mimicking various forms of human
behavior such as linguistic understanding. Incremental success in this
type of endeavor would lend further support to the idea that mental
types could be realized by diverse physical types: not just by human
brains but by silicon circuitry.
One criticism of empirical arguments for the MRT is that they are
merely inductive in character (Zangwill 1992: 218-219): the denial of
multiple realizability is still consistent with their premises. In
addition, Shapiro (2004) argues against the appeal to biology on the
grounds that a view which denies the MRT is just as probable given
convergent evolution as a view which endorses it. Against the appeal
to neuroscience, moreover, Bechtel and Mundale (1999) argue that the
argument's principle of brain state individuation is unrealistically
narrow. Real neuroscientific practice individuates brain states more
broadly. In addition, the neuroscientific data is compatible with
there being a single determinable physical type which simply takes on
multiple determinate forms (Hill 1991). Finally, the appeal to AI
would seem to be little more than a promissory note. That work hasn't
produced anything approaching a being with psychological capacities
like our own. The argument is thus little different from a conceptual
argument for the MRT. Moreover, there are arguments purporting to show
that silicon-based minds are impossible. Searle's (1980) Chinese Room
argument is an example.
3. Responses to the Antireductionist Argument
Reductionists have several ways of responding to the
multiple-realizability argument. It will be helpful to divide them
into two groups. Typology-based responses target Premises 1 and 2 of
the antireductionist argument: the MRT and the claim that the MRT is
incompatible with mental-physical type identities. Reduction-based
responses, on the other hand, target Premise 3 of the antireductionist
argument, the claim that mental-physical type identities are necessary
for reduction. These responses will be discussed in order.
a. Typology-Based Responses
Typology-based responses to the multiple-realizability argument take
the definition of 'multiple realizability' to include a condition
relating types to specific typologies. A condition of this sort was
left implicit in the definition of multiple realizability given in
Section 1-c. An explicit statement of such a condition would take
something like the following form:
[Def*] A type M is multiply realizable relative to typologies T and T*
iff df. (i) M is a type postulated by T; (ii) P and Q are types
postulated by T*; (iii) possiblyM, P-tokens are core realizers of
M-tokens; (iv) possiblyM, Q-tokens are core realizers of M-tokens, and
(v) P ≠ Q.
According to typology-based responses, the multiple-realizability
argument trades on the unwarranted and highly dubious assumption that
psychophysical relations must be reckoned only relative to our current
mental and physical typologies. In all likelihood, they claim, future
scientific investigation will result in the formulation of new mental
and/or physical typologies which will no longer support the MRT or the
claim that it implies the non-identity of mental and physical types.
Kim (1972), it seems, was the first to appreciate the range of
typology-based strategies available to opponents of the
multiple-realizability argument. They include the postulation of a new
mental typology, the postulation of a new physical typology, and the
postulation of both a new mental and a new physical typology. The
first strategy includes the local reduction move. The second strategy
includes the postulation of overarching physical commonalities, the
postulation of broad physical types, and the disjunctive move.
Finally, the third strategy includes the coordinated typology
strategy, the idea that mental and physical typologies will develop in
a coordinated way that yields one-one mental-physical type
correlations. These options are represented in Figure 1.
Figure 1: Typology-based Responses
Relative to our current mental and physical typologies, the MRT
implies that a mental type, M, is correlated with multiple physical
types P1,…,Pn as in Column I. Psychophysical identification requires,
however, that each mental type line up with a single physical type.
Reductionists can respond to the argument either by "breaking up" M
into a number of "narrower" mental types M1,…,Mn each of which
corresponds to a single physical type as in Column II. This is the
strategy represented by the local reduction move. Reductionists can
also respond, however, by "gathering" the diverse physical types
together under a single overarching physical type, P, which
corresponds to M as in Column III. This is the strategy represented by
the postulation of overarching physical commonalities, the postulation
of broad physical types, and the disjunctive move. Finally,
reductionists can respond by claiming that mental and physical
typologies will both be altered in various ways that eventually yield
one-one correlations between mental and physical types as in Column
IV.
Typology-based responses can be understood to target either Premise 1
or Premise 2 of the antireductionist argument. Which they are
understood to target depends on whether any of the types in question
are defined relative to our current typologies. Consider an example.
Someone who claims that the mental types postulated by our current
typology will be retained in a new typology alongside more "narrow"
mental types which are correlated one-one with physical types will
claim that that Premise 2 is false: the MRT is compatible with
mental-physical type identities. By contrast, someone who claims that
the mental types postulated by our current typology will not be
retained in a new typology will claim instead that the MRT is false:
all mental types are really of a narrow variety; each corresponds to a
single physical type.
i. New Mental Typologies: the Local Reduction Move
The local reduction move (LRM) has also been called an appeal to
'narrow mental types', or to 'species-specific' or 'structure-' or
'domain-specific reductions'. Its exponents include Kim (1972: 235;
1989; 1992), Lewis (1969, 1980), Enc (1983: 289-90), P.M. Churchland
(1988: 40-41), P.S. Churchland (1986: 356-358), Causey (1977:
147-149), and Bickle (1998). According to the LRM, a mental predicate
or term such as 'pain', which seems to express a single mental type,
really expresses multiple diverse mental types. The case of 'pain' is
analogous to the case of 'jade'. The latter was originally taken to
refer to a single mineralogical type. Scientific investigation
revealed, however, that 'jade' really corresponds to two distinct
mineralogical types: jadeite and nephrite. Exponents of the LRM claim
that mental predicates and terms are the same way. 'Pain' doesn't
express a single overarching mental type found in humans, in Martians,
and in robots; 'pain' is instead an imprecise term which corresponds
to multiple diverse mental types including pain-in-humans,
pain-in-Martians, and pain-in-robots. As a result, we shouldn't be
seeking to identify physical types with "broad" mental types such as
pain; we should instead be seeking to identify them with "narrower"
mental types such as pain-in-humans, pain-in-Martians, and
pain-in-robots.
In support of the LRM, Enc (ibid.) has drawn an analogy with
thermodynamics (cf. Churchland 1986 and Churchland 1988). Heat, he
argues, is multiply realized at the level of microphysical
interactions. Temperature-in-gases is different from
temperature-in-solids, which is different from temperature-in-plasmas
and temperature-in-a-vacuum. The multiple realizability of heat,
however, does not imply that thermodynamics has not been reduced to
statistical mechanics; it merely implies that the reduction proceeds
piecemeal. Temperature-in-gases is identified with one type of
mechanical property; temperature-in-plasmas, with a different
mechanical property, and so on. Thermodynamics is thus reduced to
statistical mechanics one lower-level domain at a time through the
mediation of restricted domain-specific thermodynamic types:
temperature-in-gases, temperature-in-solids, and the like. Something
similar could be true of psychophysical reduction. Psychology could
reduce to physical theory by way of various domain-specific mental
types such as pain-in-humans and pain-in-Martians.
Several criticisms of the LRM have appeared in the literature.
Zangwill (1992: 215), for instance, argues that the thermodynamic
example is irrelevant to the philosophy of mind. Another criticism
claims that narrower mental types would be too narrow for the
explanatory purposes psychological discourse aims to satisfy (cf.
Putnam 1975c: 295-298; Fodor 1974: 114; Pylyshyn 1984: Chapter 1;
Endicott 1993: 311-312). Science seeks the broadest, most
comprehensive generalizations it can get, the argument claims, but the
LRM seems to violate this methodological canon since the narrow mental
types it postulates would prevent us from formulating broad
cross-species generalizations. Sober (1999) attacks the argument's
major premise: science doesn't always work by seeking the broadest,
most comprehensive generalizations. Moreover, even if narrow mental
types didn't allow for the formulation of the most comprehensive
generalizations, we might still be better off with local reductions
for a variety of reasons including ontological parsimony and the value
of grounding higher-level explanations in mental-physical type
identities. (Endicott 1993: 311). (Bickle 1998: 150ff. criticizes this
objection to the LRM in other ways as well.)
A third criticism claims that the LRM would fail to explain what all
the phenomena called 'pain' have in common (Block 1980b: 178-9).
Against this, Kim (1992) has argued that diverse types such as
pain-in-humans and pain-in-Martians would still have in common their
satisfaction of a certain functional description or causal role, and
this commonality would be sufficient to explain the commonalities
among diverse instances of pain.
A final criticism of the LRM claims that there are no mental types
narrow enough to line up with physical types in a way that would
support reduction. Endicott (1993: 314-318) argues that if we
postulate mental types narrow enough to avoid multiple realizability
we risk postulating types that are so narrow it no longer makes sense
to speak of a reduction of types as opposed to a mere identification
of tokens. The burden for exponents of the LRM, then, is to postulate
types with the right sort of grain: narrow enough to avoid the
implications of the multiple-realizability argument, but not so narrow
that the notion of reduction drops out of the picture. (Endicott
(1993) criticizes the LRM in other ways as well.)
ii. New Physical Typologies I
Reductionists can also respond to the multiple-realizability argument
by positing new physical typologies. Kim states the idea in the
following terms:
…the mere fact that the physical bases of two nervous systems are
different in material composition or physical organization with
respect to a certain scheme of classification does not entail that
they cannot be in the same physical state with respect to a different
scheme (Kim 1972: 235).
At least three suggestions have been advanced in the literature to
this effect. The first claims that we might discover something had in
common by all of the apparently diverse realizers of a mental type. We
could discover, for instance, that c-fiber firing in humans and
q-fiber firing in Martians actually have something interesting in
common – that they are in fact instances of a broader physical type
which is correlated one-one with pain. According to this strategy, the
diverse realizers of a mental type are analogous to electricity,
magnetism, and light – types of phenomena which initially seemed
diverse but which were later discovered to belong to a single
overarching type.
Hill (1991: 105) suggests something like the postulation of
overarching physical commonalities in the following terms:
[I]t is not enough to appeal to a case in which a single qualitative
characteristic is associated with two or more distinct
neurophysiological state-types. One must go on to provide an
exhaustive characterization of the distinct levels of description and
explanation that belong to neuroscience, and show that no such level
harbors a kind under which all of the states in question may be
subsumed (Hill 1991: 105).
Shapiro (2000, 2004) has a similar idea. Although aluminum and steel
count as diverse types relative to one scheme of classification, he
argues, they don't count as diverse realizations of corkscrews because
they have too much in common relative to the performance of the
activities that qualify something as a corkscrew. (Gillett 2003
criticizes Shapiro's argument.) Similarly, Bechtel and Mundale (1999)
cite examples from cognitive neuroscience which suggest that there are
lower-level properties which are nevertheless the same in a more
general functional respect.
The discovery of overarching commonalities is not the only way of
developing a new physical typology. Reductionists might decide to
individuate realizing types in a way that comprises a broad swath of
environmental factors. Antony and Levine (1997), for instance, argue
that we should understand realization in terms of the total realizers
of mental types instead of their core realizers (see Section 1-c). If
realizers are individuated this broadly, however, mental types will no
longer be multiply realizable.
Finally, reductionists could develop a new physical typology on the
basis of disjunctive physical types. If reductionists are willing to
countenance the existence of disjunctive properties, they could
identify a mental type with the disjunction of its realizing types.
This particular response to the multiple-realizability argument has
generated an extensive literature, and deserves separate treatment.
iii. New Physical Typologies II:the Disjunctive Move
The possibility of identifying mental types with disjunctive physical
types has repeatedly asserted itself in the literature on multiple
realizability. Given an inventory of basic physical predicates P1,…,Pn
the idea is to use Boolean operations to construct disjunctive
predicates which express disjunctive types (e.g. P1vP3, P7vP15vP39).
Putnam (1967) dismissed the disjunctive move out of hand, but it has
since been taken very seriously. Kim (1978), Clapp (2001), and Antony
(1998, 2003), for instance, have all defended it in one way or
another.
Criticisms of the disjunctive move have been thoroughly discussed in
the literature (Antony 1999, 2003; Antony and Levine 1997; Block
1980b, 1997; Block and Fodor 1972; Clapp 2001; Endicott 1991, 1993;
Fodor 1974, 1997; Jaworski 2002; Kim 1972, 1978, 1984, 1992, 1998;
Macdonald 1989; Melnyk 2003; Owens 1989; Pereboom 2002; Pereboom and
Kornblith 1991; Putnam 1967a; Seager 1991; Teller 1983). The
criticisms discussed in what follows fall into two broad categories:
law-based criticisms and metaphysical criticisms. In discussing them,
it will be helpful to introduce the following terms: if P1,…,Pn are
the types that realize mental type M, call P1,…,Pn an R-disjunction,
and call a generalization featuring an R-disjunction as its antecedent
an R-disjunctive generalization.
1) Law-Based Criticisms
Law-based criticisms of the disjunctive move focus on the nature of
scientific laws. They claim that predicates such as 'believes',
'desires', and 'is in pain' express genuine properties. If mental
types are genuine properties, and mental types are identical to
R-disjunctive types, then it follows by the indiscernibility of
identicals that R-disjunctive types must be genuine properties as
well. Fodor (1974) suggested, however, that genuine properties were
expressed by the predicates of law statements – a plausible idea if
genuine properties make a causal or explanatory difference to their
bearers, and causal/explanatory regularities are expressed by law
statements. Law-based criticisms of the disjunctive move argue that
R-disjunctive generalizations are not genuine law statements, and
because they are not genuine law statements, R-disjunctive predicates
do not express genuine properties.
Methodological criticisms of the disjunctive move such as Fodor's
(1997: 157-9) claim that the postulation of R-disjunctive types
violates standard canons of scientific method. Standard inductive
practice aims at formulating the strongest generalizations warranted
by the limited available evidence, and closed law statements, as Fodor
calls them, are stronger than open ones. Closed law statements are law
statements that do not feature open-ended disjunctive predicates such
as a psychological generalization with the form 'Necessarily, for any
x, if Mx, then M*x'. Open law statements are law statements that do
feature open-ended disjunctive predicates. An example would be an
R-disjunctive generalization with the form 'Necessarily, for any x, if
P1x v P2x v… then M*x'. Given reasonable assumptions, the MRT implies
that a given mental type will be correlated with an indefinitely large
number of realizing types. Consequently, the MRT will most likely
imply the existence of open generalizations of the latter sort as
opposed to closed generalizations of the former one. Because
scientific practice aims at formulating the strongest generalizations,
and closed generalizations are stronger than open ones, standard
scientific method dictates a preference for closed generalizations
over open generalizations such as those featuring R-disjunctions.
There are good methodological reasons, then, for supposing that
R-disjunctive generalizations are not genuine law statements and that
their predicates do not express genuine properties. The problem with
this argument is that its point is merely methodological. It does not
rule out the possibility of there being R-disjunctive types or
R-disjunctive laws (a point Fodor recognizes). It thus falls short of
refuting the disjunctive move.
Other law-based criticisms correspond to two different features of law
statements: their ability to ground explanations, and their
projectibility – their ability to be confirmed by their positive
instances. Explanation-based criticisms of the disjunctive move claim
that R-disjunctive generalizations cannot express laws because they do
not function explanatorily the way law statements do. One such
criticism claims, for instance, that explanations must be relevant to
our explanatory interests, and appeals to R-disjunctive
generalizations are clearly irrelevant to the interests we have in
explaining human behavior (Pereboom and Kornblith 1991; Putnam 1975c,
1981). If, for instance, we want to know why Caesar ordered his troops
to cross the Rubicon, it doesn't satisfying our interests to respond,
"Because he was either in neural state N1 or in neural state N2 or…"
One criticism of this argument is that the notion of relevance is
highly context dependent. Although there are good reasons to suppose
appeals to R-disjunctive generalizations are irrelevant in
"pedestrian" contexts such as the context involving Caesar's actions,
there are also good reasons to suppose that appeals to R-disjunctive
generalizations might be relevant in scientific contexts in which
reduction is at stake (Jaworski 2002).
Confirmation-based criticisms, on the other hand, claim that
R-disjunctive generalizations cannot express laws because they are not
confirmed in the way law statements are. In particular, they are not
projectible; they are not confirmed by their positive instances.
Exponents of confirmation-based criticisms include Owens (1989) and
Seager (1990), but Kim's (1992) version of this criticism is both the
best developed and most widely discussed representative of this
approach.
Kim's argument trades on two premises. First, if some evidence e
confirms p and p entails q, then e also confirms q. Second, no
generalization can be confirmed without the observation of some of its
positive instances. Given these premises, the argument purports to
show that generalizations with disjunctive antecedents cannot express
laws. If they did express laws, they would be confirmed by their
positive instances the way all law statements are. But clearly they
are not, the argument claims. To show this, assume for the sake of
argument that generalizations with disjunctive antecedents are
confirmed by their positive instances – call this the Disjunctive
Confirmation Hypothesis. Consider now an example: every piece of jade,
says Kim, is a piece of either jadeite or nephrite, and vice versa.
Suppose, then, that a certain number of jadeite samples confirm the
following:
(1) All jadeite is green.
Since each piece of jadeite is also a piece of jade (that is a piece
of jadeite or nephrite) each piece of green jadeite is also a positive
instance of (2):
(2) All jade is green (i.e. all jadeite or nephrite is green).
So if (1) is confirmed by the samples of jadeite, then by the
Disjunctive Confirmation Hypothesis, so is (2). But '∀x((Jx v Nx) →
Gx)' implies '∀x(Nx → Gx)' in the predicate calculus, so if (2) is
confirmed by the samples, then by Kim's first premise, so is (3):
(3) All nephrite is green.
The problem, however, is that none of the samples are samples of
nephrite. Because no generalization can be confirmed without the
observation of some positive instances (Kim's second premise), we must
reject the assumption which sanctioned this confirmation procedure,
namely the Disjunctive Confirmation Hypothesis. (A parallel example:
suppose a sexually active adult is a sexually active man or woman, and
that a certain number of sexually active men confirm 'No sexually
active man becomes pregnant'. Parity of reasoning yields the
conclusion that those men confirm 'No sexually active adult becomes
pregnant', and hence 'No sexually active woman becomes pregnant'!) If
the Disjunctive Confirmation Hypothesis is rejected, however, it
follows that R-disjunctive generalizations fail to be confirmed in a
lawlike manner and hence fail to express laws.
The principal shortcoming of this argument is that many disjunctive
predicates are capable of occurring in law statements. Suppose, for
instance, that 'All emeralds are green' expresses a law statement.
Consider a term that is necessarily coextensive with 'emeralds' such
as 'emeralds in the northern hemisphere or elsewhere'. Since this term
expresses the same class as 'emeralds' it seems that 'All emeralds in
the northern hemisphere or elsewhere are green' will be confirmed by
its positive instances if 'All emeralds are green' is. But if these
are both law statements, then there will have to be some way of
distinguishing legitimate disjunctive predicates such as 'is a
northern or a non-northern emerald' from illegitimate disjunctive
predicates such as 'is jadeite or nephrite', and it seems the only way
of doing that is to consider the objects to which these predicates
apply. Hence, says Kim, "There is nothing wrong with disjunctive
predicates as such; the trouble arises when the kinds denoted by the
disjoined predicates are heterogeneous… so that instances falling
under them do not show the kind of 'similarity', or unity, that we
expect of instances falling under a single kind" (Kim 1992: 321). A
confirmation-based criticism seems to depend, therefore, on some type
of metaphysical criticism.
2) Metaphysical Criticisms
Metaphysical criticisms of the disjunctive move claim the idea of a
disjunctive property is somehow metaphysically suspect. There are at
least two arguments of this sort.
Armstrong (1978: II, 20) argues that accepting disjunctive properties
would violate the principle that the same property is present in its
diverse instances. Objects a and b, for instance, might both have the
disjunctive property PvQ despite the fact that a has it by virtue of
having property P instead of Q, and b has it by virtue of having Q
instead of P. Clapp (2001) criticizes this argument on the grounds
that determinables and their corresponding determinates seem to
provide counterexamples. For example, being red, being blue, being
yellow, and so forth, are determinates of the determinable being
colored. Since everything that is colored must be a determinate shade,
anything that satisfies the predicate 'is blue, or is red, or is
yellow,…' will also satisfy the predicate 'is colored'. Consequently,
if a is red and b is blue, they will have in common the property being
colored.
A second metaphysical criticism argues that mental types cannot be
identical to R-disjunctive types because R-disjunctions do not express
natural kinds. One basic assumption of the multiple-realizability
debate is that mental types are natural kinds. Consequently, if mental
types are identical to R-disjunctive types, the latter must be natural
kinds as well. But R-disjunctive types are not natural kinds, the
argument claims. The reason is that natural kindhood is based on
similarity, and instances of R-disjunctions are not similar to each
other in the right sort of way (Fodor 1974: 109ff.; 1997: 156, Block
1978: 266, Macdonald 1989: 36-7, Armstrong 1978: Vol. II, 20, Kim
1992, Antony and Levine 1997: 87ff.).
Individual instances or members of a natural kind are similar in
important ways that have a bearing on, for instance, the
projectibility of law statements. The generalization 'All Ks are F' is
projectible only if Ks remain similar across actual and counterfactual
circumstances in ways that have a bearing on their F-ness. Only if Ks
are similar to each other in these ways can the observation of any K
provide evidence about the F-ness of any other K. Inductive projection
about Ks requires, then, that Ks be similar to each other in stable
ways. One version of this similarity-based argument understands the
relevant similarity in terms of causality (Kim 1992). Kim labels this
the "Principle of Causal Individuation of Kinds": "Kinds in a science
are individuated on the basis of causal powers; that is, objects and
events fall under a kind, or share in a property, insofar as they have
similar causal powers" (Kim 1992: 326). The argument, then, is that
R-disjunctive types can qualify as natural kinds only if they are
causally similar – only if, for instance, R-disjunctive tokens have
similar effects. But, the argument claims, R-disjunctive tokens are
not causally similar. If they were causally similar; if, for instance,
c-fiber firing and q-fiber firing produced the same effects, they
probably wouldn't qualify as diverse realizers of pain. The causal
diversity of R-disjunctive tokens seems to be an implication of the
MRT. Consequently, R-disjunctive types are not natural kinds.
Criticisms of this argument have sometimes appealed to the
considerations that support physical commonalities among R-disjuncts
(See Section 3-a-iii). Block (1997), Antony and Levine (1997), Shapiro
(2000), and others have argued, for instance, that diverse physical
realizers must have something interesting in common in order to
satisfy the functional descriptions associated with mental states. If
being in pain amounts to being in some lower-order physical state with
such-and-such typical effects, then c-fiber firing and q-fiber firing
must each be able to produce those effects to qualify as instances of
pain. They must therefore be causally similar to that extent at least.
Importantly, critics of this argument have typically not sought to
defend the disjunctive move per se, but rather implications the
argument has for nonreductive physicalism (see Section 4 below.)
iv. Coordinate Typologies
Another typology-based response to the antireductionist argument
claims that mental and physical typologies are to some extent
interdependent, and as a result they will eventually converge in a way
that yields one-one correlations between mental and physical types.
Something like this idea is suggested by Kim:
The less the physical basis of the nervous system of some organisms
resembles ours, the less temptation there will be for ascribing them
sensations or other phenomenal events (Kim 1972: 235).
Similarly, Enc argues (1983: 290) that our mental typology will
eventually be altered to reflect our lower-level scientific
investigations. Couch (2004) makes a similar point: if scientists find
physical differences among the parts of a system, they are likely to
seek higher-level functional differences as well. (Cf. Hill 1991:
Chapter 3.)
One argument in favor of coordinate typologies is suggested by Kim
(1992), Bickle (1998: Chapter 4), and Bechtel and Mundale (1999). The
idea is roughly that there can be higher-level regularities only if
they are grounded in lower-level ones. Consequently, if we discuss
higher-level regularities such as those expressed by familiar
psychological generalizations, we have good reason to think these are
underwritten by regularities at lower levels. This dependence of
higher-level regularities on lower-level regularities gives us some
reason to suspect that mental and physical typologies will tend to
converge. (Sungsu Kim (2002) criticizes Bechtel and Mundale's
argument. Couch (2004) defends it.)
b. Reduction-Based Responses
Reduction-based responses to the multiple-realizability argument
attack the claim that reduction requires bridge principles taking the
form of identity statements. Robert Richardson (1979), for instance,
argues that a Nagelian account of intertheoretic reduction can be
underwritten by one-way conditionals. Consider again the theories TA
and TB discussed in Section 1e. Imagine that TA is slated for
reduction to TB, and that LA is a law statement of TA which is
supposed to be derived from LB, a law statement of TB:
LA For any x, if A1(x), then A2(x);
LB For any x, if B1(x), then B2(x).
Since the vocabulary of TB does not include the predicates A1 or A2,
additional premises linking the vocabularies of the two theories are
required. Earlier, in Section 1-e, we said that the derivation of LA
from LB required bridge principles taking the form of identity
statements:
ID1A1 = B1
ID2A2 = B2;
It seems, however, that LA might be derived from LB on the basis of
bridge principles along the following lines instead:
C1 Necessarily, for any x, if B1(x), then A1 (x);
C2 Necessarily, for any x, if B2 (x), then A2(x).
If one-way conditionals of this sort are sufficient for reductive
derivations, then the non-identity of mental and physical types is not
incompatible with reductionism after all. Reductive derivations might
proceed via bridge principles such as C1 and C2 even if identity
statements along the lines of ID1 and ID2 are false.
The problem with this understanding of reduction, one indicated by
Patricia Kitcher (1980) in her criticism of Richardson, is that a
derivation via one-way conditionals does not result in ontological
simplification (cf. Bickle 1998: 119-120). It doesn't show that what
we originally took to be two kinds of entities are really only one.
Ontological simplification of this sort is taken to be a central
feature of reduction – the upshot of showing that A-entities are
really just B-entities.
Reduction-based responses to the multiple-realizability argument have
not been as popular as typology-based responses on account of
widespread commitment to the idea that reduction involves ontological
simplification (Sklar 1967; Schaffner 1967; Causey 1972; 1977: Chapter
4; Hooker 1981: Part III; Churchland 1986). Yet Bickle (2003) has
recently suggested another type of reduction-based response. It claims
not that bridge principles along the lines of C1 and C2 are sufficient
for reduction, but that ontological issues concerning the identity or
non-identity of properties are completely orthogonal to the issue of
reduction. If that is the case, then issues concerning psychophysical
reduction could be addressed independently of issues concerning the
identity or non-identity of mental and physical types.
4. Multiple Realizability and Nonreductive Physicalism
Multiple realizability has recently played an important role in the
attempt to articulate an acceptable form of nonreductive physicalism
(NRP). NRP can be characterized by a commitment to three claims,
roughly:
Physicalism: Everything is physical – all objects, properties, and
events are the sort that can be exhaustively described and/or
explained by the natural sciences.
Mental Realism: Some mental types are genuine properties.
Antireductionism: Mental and physical types are not identical.
Jaegwon Kim has articulated a well-known difficulty for a particular
type of NRP: realization physicalism. Realization physicalism claims
that properties postulated by nonphysical frameworks are higher-order
properties that are realized by lower-order properties or their
instances in the sense described in Section 1-b. Having a mental
property amounts to having some lower-order property that satisfies a
certain associated description or condition. Having pain, for
instance, might be defined as having some lower-order property that is
typically caused by pinpricks, abrasions, burns, and the like, and
that typically causes wincing, groaning, and escape-directed
movements. Here '…is typically caused by pinpricks, abrasions, burns…
and typically causes wincing, groaning, escape-directed movements'
expresses the condition associated with being in pain. Any properties
whose instances satisfy this causal profile count as instances of
pain, and the lower-order properties (or property instances) that
satisfy that condition are said to realize pain.
Kim argues that realization physicalism is an unstable theory: either
its commitment to Mental Realism and Antireductionism imply a
rejection of Physicalism, or else its commitment to Physicalism and
Mental Realism imply a rejection of Antireductionism. His argument
trades on two assumptions.
First, Kim assumes that genuine properties are ones that make a causal
difference to their bearers. We can distinguish between two senses of
'property'. Properties in a broad or latitudinarian sense are roughly
the ontological correlates of predicates. Properties in a narrow,
causal sense, on the other hand, are properties in the broad sense
that make a causal difference to their bearers. Hence, weighing 1 kg
and weighing 2.2 pounds are different properties in the broad sense
since they correspond to different predicates, but they are not
different properties in the causal sense since they are necessarily
coextensive and influence the causal relations into which their
bearers enter in exactly the same ways. One might even do well to
eliminate talk of broad properties altogether, says Kim (1998: Chapter
4), and speak instead simply of properties in the causal sense which
are expressible by different predicates. Hence, there is a single
(causal) property expressed by the predicates 'weighs 1 kg' and
'weighs 2.2 pounds'.
Second, Kim assumes that if physicalism is true, the only genuine
(i.e. causal) properties that exist are physical properties. Denying
this, he says, would be tantamount to denying physicalism; it would be
to accept the existence of "emergent causal powers: causal powers that
magically emerge at a higher level" (1992: 326).
Given these assumptions, Kim poses the following difficulty for
realization physicalists. According to Antireductionism, mental types
are not identical to physical types. In that case, however, it is
unclear how mental types could manage to be genuine properties. If
Physicalism is true, then all causal properties are physical. This
seems to imply a principle along the following lines (it is stated
here without the qualifications Kim adds):
If a higher-order property M is realized by a lower-order property P,
then the causal powers of this instance of M are identical to the
causal powers of P.
Kim (1992: 326) calls this the 'Causal Inheritance Principle'. This
principle would appear to present realization physicalists with an
uncomfortable choice. They could (a) deny the causal status of mental
types; that is, they could reject Mental Realism and deny that mental
types are genuine properties. Alternatively, they could (b) reject
Physicalism; that is, they could endorse the causal status of mental
types, but deny their causal status derives from the causal status of
their physical realizers. Or finally, they could (c) endorse Mental
Realism and Physicalism, and reject Antireductionism. Given the
assumption that mental types are genuine properties, a commitment to
Physicalism would imply that mental types are identical to physical
types. This is the option Kim favors. Kim is nevertheless sympathetic
with the idea that the mental types postulated by our current mental
typology are multiply realizable relative to the physical types
postulated by our current physical typologies. He argues, moreover,
that R-disjunctive types cannot be natural kinds for reasons discussed
in Section 3-a-iii-3. If those types are not natural kinds, however,
then we have good reason to suppose that the mental types postulated
by our current mental typology are not natural kinds either. Each of
those mental types is necessarily coextensive with an R-disjunction,
and no mental type can have causal powers beyond those of the
individual disjuncts. If those disjuncts are causally dissimilar, then
instances of the corresponding mental type must be causally dissimilar
as well. Suppose, however, that causal similarity is necessary for
natural kind status. In that case, it follows that the mental types
postulated by our current mental typology cannot be natural kinds.
Consequently, Kim favors the local reduction move discussed in Section
3-a-i. We need a new mental typology that postulates new narrow mental
types that are correlated one-one with physical types.
5. References and Further Reading
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Antony, Louise M. 2003. "Who's Afraid of Disjunctive Properties?"
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Armstrong, D.M. 1978. A Theory of Universals: Universals and
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Bickle, John. 1998. Psychoneural Reduction: The New Wave. Cambridge,
MA: MIT Press.
Bickle, John. 2003. Philosophy and Neuroscience: A Ruthlessly
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Block, Ned. 1978. Troubles With Functionalism. In Perception and
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