The quote in the essay title seems to be an abbreviated version of this longer quote from Tim Crane:
"[I]t seems nomologically possible that many very different token physical entities could all be in the same type of mental state. So the type-identity theory is far too strong to be empirically plausible. But the token identity theory, on the other hand, seems too weak to be satisfactory — for what explains why these mental tokens are identical with these physical tokens? A solution to the mind-body problem is supposed to give an illuminating answer to the question of the relation between the mental and the physical. But it is hard to see how the token identity theory can do this."(1)
Of course it does not matter where the essay's title quote truly came from, since this longer Tim Crane quote accurately captures the point of the statement.
In contemporary philosophy of mind, "physicalism" is the doctrine that all that exists, is just the physical -- the purview of the physical sciences(2). Minds and conscious experience are based on, and can be explained by, physical properties. Physicalism maintains that mental phenomena -- the "how it feels", the qualia, propositional attitudes, and all the conscious experiences -- are the products of physical properties of physical existents. (Physicalism is also called "materialism". But "physicalism" is often preferred because of its relation to "physical sciences", and because the physical sciences often deal with the "immaterial" -- concrete things like forces and fields, and abstract things like numbers and phase-spaces.)
The type-identity theory of mind-brain physicalism (also known as reductive materialism) asserts that mental phenomena (properties, events, processes, states, etc.) can be grouped into types (or kinds) and that these can be identified with types (or kinds) of physical phenomena in the brain.(3) Type-identity theories maintain that any particular kind of mental phenomena is a particular kind of physical phenomena. Where the "is" here is to be understood as strict logical identity. More formally:
For every mental event/property/state M, there exists a physical event/property/state P such for all events/properties/states X, X instantiates M if and only if X instantiates P.
The classic example in the literature is the supposed identity between the mental type of phenomena labelled "pain" with the physical type of brain phenomena labelled "c-fiber firing". According to the type-identity theory, other mental phenomena will turn out, on investigation, to be nothing more than particular kinds of neuron firing patterns in the brain.
The type-identity theory was developed by U.T. Place, Herbert Feigl, J.J.C. Smart, and D.M. Armstrong in the 1950's and 1960's in response to problems perceived with the Behaviorist approach to understanding the mental(4). The classic development of the mind-brain type-identity theory was by D.M. Armstrong in his 1968 book A Materialist of the Mind.
As noted by J.J.C. Smart, one of the incentives driving the development of the type-identity theories was an appeal to Occam's Razor:
"There does seem to be, so far as science is concerned, nothing in the world but increasingly complex arrangements of physical constituents. . . . That everything be explicable in terms of physics . . . except the occurrence of sensations seems to be frankly unbelievable."(5)
Another great advantage of a type-identity theory is that it solves Descartes problem (of mind-brain interaction) by reducing the mental to the physical. It provides a simple way to explain the causal efficacy of the mental. If the mental is the physical, then mental causes just are physical causes. Thus it eliminates the problem or reconciling mental causation with the notion of the causal closure of the physical. And it also allows empirical investigation of mental phenomena through the investigation of the physical substrate.
However, the multiple realizability argument pulled the rug out from underneath type-identity theories. The concept of multiple realizability was introduced by Hilary Putnam(6) and Jerry Fodor(7) in the late 1960's as an objection to the mind-body type-identity theories, and as a consequence of their development of the Functionalist alternative. According to the Multiple Realizability argument,
If "pains" are identical to "c-fiber firings" in human brains, then an octopus, for example could not feel pain because it does not have c-fibers. And a robot, android, or an Alpha Centaurian could not exhibit true consciousness, or have real experiences. It also denies the possibility that "pain" (or some other mental phenomenon, like seeing "red" or believing that it is raining) may not be implemented in me the same way that it is implemented in you.
This conclusion does not seem plausible to most people. It is considered more plausible that mental phenomena can be multiply realized in different kinds of physical entities. Thus the conclusion is that a mind-body type-identity theory is too strong to explicate physicalism because it limits the existence of mental phenomena to brains just like ours. It denies the possibility that other sorts of physical organizations might experience mental phenomena.
In contrast with type-identity physicalism, token-identity physicalism argues that mental phenomena are unlikely to have "steady" physical correlates. In other words, token-identity physicalism argues that while each particular mental phenomenon is identical with some particular physical phenomenon (maintaining the "is" of mind-brain identity), there is no prima facie necessity that any type of mental phenomena has the same type of physical correlate. Token-identity physicalism thus accommodates the multiple realizability intuition.
It is important to realize that token-identity physicalism is strictly logically weaker than type-identity physicalism.(8) Token is to type as member is to set. We can share the same type of haircut, but not the same token haircut. So if type-identity physicalism is true, then token-identity physicalism is also true. But one can consistently deny type-identity and affirm token-identity.
Type-identity theories claim that the identity of particular mental and physical tokens (phenomena, properties, processes, events, states, etc.) depends upon the discovery of lawlike relations between the respective mental and physical types. Token identity claims thus depend upon type-identity in these theories. Empirical evidence for the required type-identity laws is held to be necessary for particular token identity claims. It is necessary that empirical evidence support the claim that "pains" are "c-fiber firing" to support the claim that "this particular pain" is identical to "that particular c-fiber firing". But to accommodate the multiple realizability intuition, token-identity theories must specifically deny the kind of lawlike relations posited by the type-identity theories. The most well known of these token-identity theories is the Anomalous Monism of Donald Davidson.(9) Davidson's theory requires no empirical evidence (it I based on a priori arguments) and depends on there being no lawlike relations between mental and physical types.
Anomalous Monism starts by assuming that some mental phenomena have causal interactions with (cause or are caused by) physical phenomena -- like the physical phenomenon of it raining causing a mental phenomenon of my belief that it is raining. Added to this first premise is a premise to the effect that all cause-effect relations are covered by strict laws (not just ceterus paribus generalities). Davidson then adds the key premise that defines Anomalous Monism -- a premise that there are no strict laws governing cause and effect between mental types of phenomena. Therefore, every causally interacting mental phenomenon must be token-identical to some physical phenomenon. This separates his Anomalous Monism from the problems outlined above adhering to type-identity theories, and makes it a token-identity theory.
But as suggested in the essay's title quote, token identity theories are criticised as being far too weak to be a sound basis on which to found a physicalist view of the mind-body problem -- it is too weak to preserve a physicalist position. Token-identity physicalism can be true even if there is nothing remotely resembling a systematic relationship between the mental and the physical. A systematic relationship between mind and body is fundamental to a robust physicalist position. Here is Jaegwon Kim's analysis of the problem:
". . .token physicalism is a weak doctrine that doesn't say much; essentially, it only says that mental and physical properties are instantiated by the same entities. Any event or occurrence with a mental property has some physical property or other. But the theory says nothing about the relationship between mental properties and physical properties . . . Token physicalism can be true even if there is nothing remotely resembling a systematic relationship between the mental and the physical. . . . As far as token physicalism goes, there could be another world just like [ours] in every physical detail except that mentality and consciousness are totally absent. Token physicalism, therefore, can be true even if mind-body supervenience fails: What mental features a given event has is entirely unconstrained by what biological/physical properties it has, as far as token physicalism goes, and there could be a molecule-for-molecule physical duplicate of you who is wholly lacking in consciousness, that is, a zombie. This means that the theory says nothing about how mental properties of an event might be physically based or explained. Token physicalism, then, is not much of a physicalism. In fact, if we accept mind-body supervenience as defining minimal physicalism, token physicalism falls outside of the scope of physicalism altogether."(10)
This conclusion does not seem plausible to most people. To most physicalists, it is considered more plausible that zombies are logically incoherent, and that the mental supervenes (in some fashion) on the physical. Davidson, for example, explicitly addresses this problem by including within his Anomalous Monism a premise that the mental does in fact supervene on the physical.
Thus the conclusion is that a mind-body token-identity theory without additional premises is too weak to explicate physicalism because it permits no consistent explanatory relationship between the mental and the physical. Token-identity physicalism by itself cries out for a companion theory to fill these holes. (Functionalism is an alternative to Anomalous Monism, but that is a topic for a different essay.)
(1) Crane, Tim; "The Mental Causation Debate" part of "Mental Causation" by Tim Crane and Bill Brewer in Proceedings of the Aristotelian Society, Supplementary Volumes, Vol. 69, (1995), pp. 211-253
(2) Stoljar, Daniel; "Physicalism" in The Stanford Encyclopedia of Philosophy (Fall 2009 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/fall2009/entries/physicalism/>.
(3) Wikipedia contributors; "Type physicalism" in Wikipedia, The Free Encyclopedia. URL=<http://en.wikipedia.org/w/index.php?title=Type_physicalism&oldid=527514709>.
(4) Schneider, Steven; "Identity Theory" in the Internet Encyclopedia of Philosophy, URL=<http://www.iep.utm.edu/identity/>.
(5) Smart, J.J.C; "Sensations and Brain Processes" in The Philosophical Review, Vol 68, No 2 (Apr 1959), 141-156. URL=<http://www.jstor.org/stable/2182164>.
(6) Putman, Hillary; Mind, Language and Reality: Philosophical Papers, Volume 2, Cambridge University Press, New York, New York, 1975.
(7) Fodor, Jerry; "Explanations in Psychology" in Philosophy in America, Max Black (ed.), (Ithaca, NY: Cornell University Press, Ithaca, New York, 1965.
Psychological Explanation: An Introduction to the Philosophy of Psychology, Random House, New York, New York, 1974.
(8) Wetzel, Linda; "Types and Tokens" in The Stanford Encyclopedia of Philosophy (Spring 2011 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/spr2011/entries/types-tokens/>.
(9) Yalowitz, Steven; "Anomalous Monism" in The Stanford Encyclopedia of Philosophy (Winter 2012 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/win2012/entries/anomalous-monism/>.
Davidson, Donald; "Mental Events" in Essays on Actions & Events. Clarendon Press, Oxford, England, 1980.
(10) Kim, Jaegwon; Philosophy of Mind, Third Edition, Westview Press, The Perseus Books Group, Boulder, Colorado, 2010. ISBN 978-0-813-34458-4.
Armstrong, D.M.; A Materialist of the Mind, Revised Edition, Routledge, New York, New York, 1993. ISBN 978-0-415-10031-1. Kindle Edition.
Bickle, John; "Multiple Realizability" in The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/fall2008/entries/multiple-realizability/>.
Kim, Jaegwon; "On the Psycho-Physical Identity Theory," The American Philosophical Quarterly, Vol 3 (1966), Pg 227-235.
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