Identity Theory

A family of views on the relationship between mind and body, Type
Identity theories hold that at least some types (or kinds, or classes)
of mental states are, as a matter of contingent fact, literally
identical with some types (or kinds, or classes) of brain states. The
earliest advocates of Type Identity—U.T. Place, Herbert Feigl, and
J.J.C. Smart, respectively—each proposed their own version of the
theory in the late 1950s to early 60s. But it was not until David
Armstrong made the radical claim that all mental states (including
intentional ones) are identical with physical states, that
philosophers of mind divided themselves into camps over the issue.

Over the years, numerous objections have been levied against Type
Identity, ranging from epistemological complaints to charges of
Leibniz's Law violations to Hilary Putnam's famous pronouncement that
mental states are in fact capable of being "multiply realized."
Defenders of Type Identity have come up with two basic strategies in
response to Putnam's claim: they restrict type identity claims to
particular species or structures, or else they extend such claims to
allow for the possiblity of disjunctive physical kinds. To this day,
debate concerning the validity of these strategies—and the truth of
Mind-Brain Type Identity—rages in the philosophical literature.

1. Early Versions of the Theory

Place accepted the Logical Behaviorists' dispositional analysis of
cognitive and volitional concepts. With respect to those mental
concepts "clustering around the notions of consciousness, experience,
sensation, and mental imagery," however, he held that no behavioristic
account (even in terms of unfulfilled dispositions to behave) would
suffice. Seeking an alternative to the classic dualist position,
according to which mental states possess an ontology distinct from the
physiological states with which they are thought to be correlated,
Place claimed that sensations and the like might very well be
processes in the brain—despite the fact that statements about the
former cannot be logically analyzed into statements about the latter.
Drawing an analogy with such scientifically verifiable (and obviously
contingent) statements as "Lightning is a motion of electric charges,"
Place cited potential explanatory power as the reason for
hypothesizing consciousness-brain state relations in terms of identity
rather than mere correlation. This still left the problem of
explaining introspective reports in terms of brain processes, since
these reports (e.g. of a green after-image) typically make reference
to entities which do not fit with the physicalist picture (there is
nothing green in the brain, for example). To solve this problem, Place
called attention to the "phenomenological fallacy"—the mistaken
assumption that one's introspective observations report "the actual
state of affairs in some mysterious internal environment." All that
the Mind-Brain Identity theorist need do to adequately explain a
subject's introspective observation, according to Place, is show that
the brain process causing the subject to describe his experience in
this particular way is the kind of process which normally occurs when
there is actually something in the environment corresponding to his
description.

At least in the beginning, J.J.C. Smart followed U.T. Place in
applying the Identity Theory only to those mental concepts considered
resistant to behaviorist treatment, notably sensations. Because of the
proposed identification of sensations with states of the central
nervous system, this limited version of Mind-Brain Type Identity also
became known as Central-State Materialism. Smart's main concern was
the analysis of sensation-reports (e.g. "I see a green after-image")
into what he described, following Gilbert Ryle, as "topic-neutral"
language (roughly, "There is something going on which is like what is
going on when I have my eyes open, am awake, and there is something
green illuminated in front of me"). Where Smart diverged from Place
was in the explanation he gave for adopting the thesis that sensations
are processes in the brain. According to Smart (1959), "there is no
conceivable experiment which could decide between materialism and
epiphenomenalism" (where the latter is understood as a species of
dualism); the statement "sensations are brain processes," therefore,
is not a straight-out scientific hypothesis, but should be adopted on
other grounds. Occam's razor is cited in support of the claim that,
even if the brain-process theory and dualism are equally consistent
with the (empirical) facts, the former has an edge in virtue of its
simplicity and explanatory utility.

Occam's razor also plays a role in the version of Mind-Brain Type
Identity developed by Feigl (in fact, Smart claimed to have been
influenced by Feigl as well as by Place). On the epiphenomenalist
picture, in addition to the normal physical laws of cause and effect
there are psychophysical laws positing mental effects which do not by
themselves function as causes for any observable behavior. In Feigl's
view, such "nomological danglers" have no place in a respectable
ontology; thus, epiphenomenalism (again considered as a species of
dualism) should be rejected in favor of an alternative, monistic
theory of mind-body relations. Feigl's suggestion was to interpret the
empirically ascertainable correlations between phenomenal experiences
("raw feels," see Consciousness and Qualia) and neurophysiological
processes in terms of contingent identity: although the terms we use
to identify them have different senses, their referents are one and
the same—namely, the immediately experienced qualities themselves.
Besides eliminating dangling causal laws, Feigl's picture is intended
to simplify our conception of the world: "instead of conceiving of two
realms, we have only one reality which is represented in two different
conceptual systems."

In a number of early papers, and then at length in his 1968 book, A
Materialist of the Mind, Armstrong worked out a version of Mind-Brain
Type Identity which starts from a somewhat different place than the
others. Adopting straight away the scientific view that humans are
nothing more than physico-chemical mechanisms, he declared that the
task for philosophy is to work out an account of the mind which is
compatible with this view. Already the seeds were sown for an Identity
Theory which covers all of our mental concepts, not merely those which
fit but awkwardly on the Behaviorist picture. Armstrong actually gave
credit to the Behaviorists for logically connecting internal mental
states with external behavior; where they went wrong, he argued, was
in identifying the two realms. His own suggestion was that it makes a
lot more sense to define the mental not as behavior, but rather as the
inner causes of behavior. Thus, "we reach the conception of a mental
state as a state of the person apt for producing certain ranges of
behavior." Armstrong's answer to the remaining empirical question—what
in fact is the intrinsic nature of these (mental) causes?—was that
they are physical states of the central nervous system. The fact that
Smart himself now holds that all mental states are brain states (of
course, the reverse need not be true), testifies to the influence of
Armstrong's theory.

Besides the so-called "translation" versions of Mind-Brain Type
Identity advanced by Place, Smart, and Armstrong, according to which
our mental concepts are first supposed to be translated into
topic-neutral language, and the related version put forward by Feigl,
there are also "disappearance" (or "replacement") versions. As
initially outlined by Paul Feyerabend (1963), this kind of Identity
Theory actually favors doing away with our present mental concepts.
The primary motivation for such a radical proposal is as follows:
logically representing the identity relation between mental states and
physical states by means of biconditional "bridge laws" (e.g.,
something is a pain if and only if it's a c-fiber excitation) not only
implies that mental states have physical features; "it also seems to
imply (if read from the right to the left) that some physical
events…have non-physical features." In order to avoid this apparent
dualism of properties, Feyerabend stressed the incompatibility of our
mental concepts with empirical discoveries (including projected ones),
and proposed a redefinition of our existent mental terms. Different
philosophers took this proposal to imply different things. Some
advocated a wholesale scrapping of our ordinary language descriptions
of mental states, such that, down the road, people might develop a
whole new (and vastly more accurate) vocabulary to describe their own
and others' states of mind. This begs the question, of course, what
such a new-and-improved vocabulary would look like. Others took a more
theoretical/conservative line, arguing that our familiar ways of
describing mental states could in principle be replaced by some very
different (and again, vastly more accurate) set of terms and concepts,
but that these new terms and concepts would not—at least not
necessarily—be expected to become part of ordinary language.
Responding to Feyerabend, a number of philosophers expressed concern
about the appropriateness of classifying disappearance versions as
theories of Mind-Brain Type Identity. But Richard Rorty (1965)
answered this concern, arguing that there is nothing wrong with
claiming that "what people now call 'sensations' are (identical with)
certain brain processes." In his Postscript to "The 'Mental' and the
'Physical'," Feigl (1967) confessed an attraction to this version of
the Identity Theory, and over the years Smart has moved in the same
direction.

2. Traditional Objections

A number of objections to Mind-Brain Type Identity, some a great deal
stronger than others, began circulating soon after the publication of
Smart's 1959 article. Perhaps the weakest were those of the
epistemological variety. It has been claimed, for example, that
because people have had (and still do have) knowledge of specific
mental states while remaining ignorant as to the physical states with
which they are correlated, the former could not possibly be identical
with the latter. The obvious response to this type of objection is to
call attention to the contingent nature of the proposed identities—of
course we have different conceptions of mental states and their
correlated brain states, or no conception of the latter at all, but
that is just because (as Feigl made perfectly clear) the language we
use to describe them have different meanings. The contingency of
mind-brain identity relations also serves to answer the objection that
since presently accepted correlations may very well be empirically
invalidated in the future, mental states and brain states should not
be viewed as identical.

A more serious objection to Mind-Brain Type Identity, one that to this
day has not been satisfactorily resolved, concerns various
non-intensional properties of mental states (on the one hand), and
physical states (on the other). After-images, for example, may be
green or purple in color, but nobody could reasonably claim that
states of the brain are green or purple. And conversely, while brain
states may be spatially located with a fair degree of accuracy, it has
traditionally been assumed that mental states are non-spatial. The
problem generated by examples such as these is that they appear to
constitute violations of Leibniz's Law, which states that if A is
identical with B, then A and B must be indiscernible in the sense of
having in common all of their (non-intensional) properties. We have
already seen how Place chose to respond to this type of objection, at
least insofar as it concerns conscious experiences—that is, by
invoking the so-called "phenomenological fallacy." Smart's response
was to reiterate the point that mental terms and physical terms have
different meanings, while adding the somewhat ambiguous remark that
neither do they have the same logic. Lastly, Smart claimed that if his
hypothesis about sensations being brain processes turns out to be
correct, "we may easily adopt a convention…whereby it would make sense
to talk of an experience in terms appropriate to physical processes"
(the similarity to Feyerabend's disappearance version of Mind-Brain
Type Identity should be apparent here). As for apparent discrepancies
going in the other direction (e.g., the spatiality of brain states vs.
the non-spatiality of mental states), Thomas Nagel in 1965 proposed a
means of sidestepping any objections by redefining the candidates for
identity: "if the two sides of the identity are not a sensation and a
brain process but my having a certain sensation or thought and my
body's being in a certain physical state, then they will both be going
on in the same place—namely, wherever I (and my body) happen to be."
Suffice to say, opponents of Mind-Brain Type Identity found Nagel's
suggestion unappealing.

The last traditional objection we shall look at concerns the
phenomenon of "first-person authority"; that is, the apparent
incorrigibility of introspective reports of thoughts and sensations.
If I report the occurrence of a pain in my leg, then (the story goes)
I must have a pain in my leg. Since the same cannot be said for
reports of brain processes, which are always open to question, it
might look like we have here another violation of Leibniz's Law. But
the real import of this discrepancy concerns the purported
correlations between mental states and brain states. What are we to
make of cases in which the report of a brain scientist contradicts the
introspective report, say, of someone claiming to be in pain? Is the
brain scientist always wrong? Smart's initial response to Kurt Baier,
who asked this question in a 1962 article, was to deny the likelihood
that such a state of affairs would ever come about. But he also put
forward another suggestion, namely, that "not even sincere reports of
immediate experience can be absolutely incorrigible." A lot of weight
falls on the word "absolutely" here, for if the incorrigibility of
introspective reports is qualified too strongly, then, as C.V. Borst
noted in 1970, "it is somewhat difficult to see how the required
psycho-physical correlations could ever be set up at all."

3. Type vs. Token Identity

Something here needs to be said about the difference between Type
Identity and Token Identity, as this difference gets manifested in the
ontological commitments implicit in various Mind-Brain Identity
theses. Nagel was one of the first to distinguish between "general"
and "particular" identities in the context of the mind-body problem;
this distinction was picked up by Charles Taylor, who wrote in 1967
that "the failure of [general] correlations…would still allow us to
look for particular identities, holding not between, say, a yellow
after-image and a certain type of brain process in general, but
between a particular occurrence of this yellow after-image and a
particular occurrence of a brain process." In contemporary parlance:
when asking whether mental things are the same as physical things, or
distinct from them, one must be clear as to whether the question
applies to concrete particulars (e.g., individual instances of pain
occurring in particular subjects at particular times) or to the kind
(of state or event) under which such concrete particulars fall.

Token Identity theories hold that every concrete particular falling
under a mental kind can be identified with some physical (perhaps
neurophysiological) happening or other: instances of pain, for
example, are taken to be not only instances of a mental state (e.g.,
pain), but instances of some physical state as well (say, c-fiber
excitation). Token Identity is weaker than Type Identity, which goes
so far as to claim that mental kinds themselves are physical kinds. As
Jerry Fodor pointed out in 1974, Token Identity is entailed by, but
does not entail, Type Identity. The former is entailed by the latter
because if mental kinds themselves are physical kinds, then each
individual instance of a mental kind will also be an individual
instance of a physical kind. The former does not entail the latter,
however, because even if a concrete particular falls under both a
mental kind and a physical kind, this contingent fact "does not
guarantee the identity of the kinds whose instantiation constitutes
the concrete particulars."

So the Identity Theory, taken as a theory of types rather than tokens,
must make some claim to the effect that mental states such as pain
(and not just individual instances of pain) are contingently identical
with—and therefore theoretically reducible to—physical states such as
c-fiber excitation. Depending on the desired strength and scope of
mind-brain identity, however, there are various ways of refining this
claim.

4. Multiple Realizability

In "The Nature of Mental States," (1967) Hilary Putnam introduced what
is widely considered the most damaging objection to theories of
Mind-Brain Type Identity—indeed, the objection which effectively
retired such theories from their privileged position in modern debates
concerning the relationship between mind and body.

Putnam's argument can be paraphrased as follows: (1) according to the
Mind-Brain Type Identity theorist (at least post-Armstrong), for every
mental state there is a unique physical-chemical state of the brain
such that a life-form can be in that mental state if and only if it is
in that physical state. (2) It seems quite plausible to hold, as an
empirical hypothesis, that physically possible life-forms can be in
the same mental state without having brains in the same unique
physical-chemical state. (3) Therefore, it is highly unlikely that the
Mind-Brain Type Identity theorist is correct.

In support of the second premise above—the so-called "multiple
realizability" hypothesis—Putnam raised the following point: we have
good reason to suppose that somewhere in the universe—perhaps on
earth, perhaps only in scientific theory (or fiction)—there is a
physically possible life-form capable of being in mental state X
(e.g., capable of feeling pain) without being in physical-chemical
brain state Y (that is, without being in the same physical-chemical
brain state correlated with pain in mammals). To follow just one line
of thought (advanced by Ned Block and Jerry Fodor in 1972), assuming
that the Darwinian doctrine of evolutionary convergence applies to
psychology as well as behavior, "psychological similarities across
species may often reflect convergent environmental selection rather
than underlying physiological similarities." Other empirically
verifiable phenomena, such as the plasticity of the brain, also lend
support to Putnam's argument against Type Identity. It is important to
note, however, that Token Identity theories are fully consistent with
the multiple realizability of mental states.

5. Attempts at Salvaging Type Identity

Since the publication of Putnam's paper, a number of philosophers have
tried to save Mind-Brain Type Identity from the philosophical
scrapheap by making it fit somehow with the claim that the same mental
states are capable of being realized in a wide variety of life-forms
and physical structures. Two strategies in particular warrant
examination here.

In a 1969 review of "The Nature of Mental States," David Lewis
attacked Putnam for targeting his argument against a straw man.
According to Lewis, "a reasonable brain-state theorist would
anticipate that pain might well be one brain state in the case of men,
and some other brain (or non-brain) state in the case of mollusks. It
might even be one brain state in the case of Putnam, another in the
case of Lewis." But it is not so clear (in fact it is doubtful) that
Lewis' appeal to "tacit relativity to context" will succeed in
rendering Type Identity compatible with the multiple realizability of
mental states. Although Putnam does not consider the possibility of
species-specific multiple realization resulting from such phenomena as
injury compensation, congenital defects, mutation, developmental
plasticity, and, theoretically, prosthetic brain surgery, neither does
he say anything to rule them out. And this is not surprising. As early
as 1960, Identity theorists such as Stephen Pepper were acknowledging
the existence of species (even system)-specific multiple realizability
due to emergencies, accidents, injuries, and the like: "it is
not…necessary that the [psychophysical] correlation should be
restricted to areas of strict localization. One area of the brain
could take over the function of another area of the brain that has
been injured." Admittedly, some of the phenomena listed above tell
against Lewis' objection more than others; nevertheless, prima facie
there seems no good reason to deny the possibility of species-specific
multiple realization.

In a desperate attempt at invalidating the conclusion of Putnam's
argument, the brain-state theorist can undoubtedly come up with
additional restrictions to impose upon the first premise, e.g., with
respect to time. This is the strategy of David Braddon-Mitchell and
Frank Jackson, who wrote in a 1996 book that "there is…a better way to
respond to the multiple realizability point [than to advocate token
identity]. It is to retain a type-type mind-brain identity theory, but
allow that that the identities between mental types and brain types
may—indeed, most likely will—need to be restricted. Identity
statements need to include an explicit temporal restriction." Mental
states such as pain may not be identical with, say, c-fiber excitation
in humans (because of species-specific multiple realization), but—the
story goes—they could very well be identical with c-fiber excitation
in humans at time T. The danger in such an approach, besides its ad
hoc nature, is that the type physicalist basis from which the Identity
Theorist begins starts slipping into something closer to token
physicalism (recall that concrete particulars are individual instances
occurring in particular subjects at particular times). At the very
least, Mind-Brain Type Identity will wind up so weak as to be
inadequate as an account of the nature of mental.

Another popular strategy for preserving Type Identity in the face of
multiple realization is to allow for the existence of disjunctive
physical kinds. By defining types of physical states in terms of
disjunctions of two or more physical "realizers," the correlation of
one such realizer with a particular (type) mental state is sufficient.
The search for species- or system-specific identities is thereby
rendered unnecessary, as mental states such as pain could eventually
be identified with the (potentially infinite) disjunctive physical
state of, say, c-fiber excitation (in humans), d-fiber excitation (in
mollusks), and e-network state (in a robot). In "The Nature of Mental
States," Putnam dismisses the disjunctive strategy out of hand,
without saying why he thinks the physical-chemical brain states to be
posited in identity claims must be uniquely specifiable. Fodor (in
1974) and Jaegwon Kim (1992), both former students of Putnam, tried
coming to his rescue by producing independent arguments which purport
to show that disjunctions of physical realizers cannot themselves be
kinds. Whereas Fodor concluded that "reductionism… flies in the face
of the facts," however, Kim concluded that psychology is open to
sundering "by being multiply locally reduced."

Even if disjunctive physical kinds are allowed, it may be argued that
the strategy in question still cannot save Type Identity from
considerations of multiple realizability. Assume that all of the
possible physical realizers for some mental state M are represented by
the ideal, perhaps infinite, disjunctive physical state P; then it
could never be the case that a physically possible life-form is in M
and not in P. Nevertheless, we have good reason to think that some
physically possible life-form could be in P without being in M—maybe P
in that life-form realizes some other mental state. As Block and Fodor
have argued, "it seems plausible that practically any type of physical
state could realize any type of psychological state in some physical
system or other." The doctrine of "neurological equipotentiality"
advanced by renowned physiological psychologist Karl Lashley,
according to which given neural structures underlie a whole slew of
psychological functions depending upon the character of the activities
engaged in, bears out this hypothesis. The obvious way for the
committed Identity theorist to deal with this problem—by placing
disjunctions of potentially infinite length on either side of a
biconditional sign—would render largely uninformative any so-called
"identity" claim. Just how uninformative depends on the size of the
disjunctions (the more disjuncts, the less informative). Infinitely
long disjunctions would render the identity claim completely
uninformative. The only thing an Identity Theory of this kind could
tell us is that at least one of the mental disjuncts is capable of
being realized by at least one of the physical disjuncts. Physicalism
would survive, but barely, and in a distinctly non-reductive form.

Recently, however, Ronald Endicott has presented compelling
considerations which tell against the above argument. There, physical
states are taken in isolation of their context. But it is only if the
context is varied that Block and Fodor's remark will come out true.
Otherwise, mental states would not be determined by physical states, a
situation which contradicts the widely accepted (in contemporary
philosophy of mind) "supervenience principle": no mental difference
without a physical difference. A defender of disjunctive physical
kinds can thus claim that M is identical with some ideal disjunction
of complex physical properties like "C1 & P1," whose disjuncts are
conjunctions of all the physical states (Ps) plus their contexts (Cs)
which give rise to M. So while "some physically possible life-form
could be in P without being in M," no physically possible life-form
could be in C1 & P1 without being in M. Whether Endicott's
considerations constitute a sufficient defense of the disjunctive
strategy is still open to debate. But one thing is clear—in the face
of numerous and weighty objections, Mind-Brain Type Identity (in one
form or another) remains viable as a theory of mind-body relations.

6. References and Further Reading

* Armstrong, D.M. (1968). A Materialist Theory of the Mind,
London, Routledge.
* Baier, Kurt (1962). Pains. Australasian Journal of Philosophy 40
(May): 1-23.
* Block, Ned & Fodor, Jerry A. (1972). "What psychological states
are not." Philosophical Review 81 (April):159-81
* Borst, Clive V. (ed.) (1970). The Mind/Brain Identity Theory. Macmillan.
* Braddon-Mitchell, D. and Jackson, F. (1996). Philosophy of Mind
and Cognition, Oxford, Blackwell.
* Endicott, Ronald P. (1993). "Species-specific properties and
more narrow reductive strategies." Erkenntnis 38 (3):303-21.
* Feigl, H. (1958). "The 'Mental' and the '"Physical'," in Feigl,
H., Scriven, M. and Maxwell, G. (eds.) Concepts, Theories and the
Mind-Body Problem, Minneapolis, Minnesota Studies in the Philosophy of
Science, Vol. 2, reprinted with a Postscript in Feigl 1967.
* Feigl, H. (1967). The "Mental" and the "Physical," The Essay and
a Postscript, Minneapolis, University of Minnesota Press.
* Feyerabend, Paul K. (1963). "Comment: Mental Events and the
Brain." Journal of Philosophy 60 (11):295-296.
* Fodor, Jerry A. (1974). "Special sciences." Synthese 28:97-115.
* Kim, Jaegwon (1992). "Multiple realization and the metaphysics
of reduction." Philosophy and Phenomenological Research 52 (1):1-26.
* Lewis, D. (1966). "An Argument for the Identity Theory," Journal
of Philosophy, 63, 17-25.
* Lewis, D. (1969). "Review of Art, Mind, and Religion" Journal of
Philosophy 66, 23-35.
* Lewis, D. (1970). "How to Define Theoretical Terms," Journal of
Philosophy, 67, 427-446.
* Lewis, D. (1972). "Psychophysical and Theoretical
Identifications," Australasian Journal of Philosophy, 50, 249-258.
* Nagel, Thomas (1965). "Physicalism." Philosophical Review 74
(July):339-56.
* Place, U.T. (1956). "Is Consciousness a Brain Process?," British
Journal of Psychology, 47, 44-50,
* Place, U.T. (1960). "Materialism as a Scientific Hypothesis,"
Philosophical Review, 69, 101-104.
* Place, U.T. (1967). "Comments on Putnam's "Psychological
Predicates"'. In Capitan, W.H. and Merrill, D.D. (eds) Art, Mind and
Religion, Pittsburgh, Pittsburgh University Press.
* Place, U.T. (1988). "Thirty Years on–Is Consciousness still a
Brain Process?," Australasian Journal of Philosophy, 66, 208-219.
* Putnam, Hilary (1967). "The Nature of Mental States," In W.H.
Capitan & D.D. Merrill (eds.), Art, Mind, and Religion. Pittsburgh
University Press.
* Rorty, Richard (1965). "Mind-body identity, privacy, and
categories," Review of Metaphysics 19 (September): 24-54.
* Ryle, G. (1949). The Concept of Mind, London, Hutchinson.
* Smart, J.J.C. (1959). "Sensations and Brain Processes,"
Philosophical Review, 68, 141-156.
* Taylor, C. (1967). "Mind-body identity, a side issue?"
Philosophical Review 76 (April):201-13.