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 50s-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.
Table of Contents (Clicking on the links below will take you to that part of this article)
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, Smart followed 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 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, "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") 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 in 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 in a 1965 paper, Richard Rorty answered this concern, arguing that there is nothing wrong with claiming that "what people now call 'sensations' are (identical with) certain brain processes." Two years later, in his Postscript to "The 'Mental' and the 'Physical'," Feigl confessed an attraction to this version of the Identity Theory, and over the years Smart has moved in the same direction.
II. 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."
III. 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.
IV. Multiple Realizability
In his 1967 paper, "The Nature of Mental States," 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.
V. 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 (eighteen
years later), 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.
Steven Schneider
Author Information:
Department of Philosophy
Harvard University
HomePage: http://members.bellatlantic.net/~sschneid/