PHIL 2100: Introduction to Philosophy
Dr. Robert Lane
Lecture Notes: Thursday June 24, 2010



[4.4.] The Identity Theory.


The Identity Theory, a.k.a. the Mind/Brain Identity Theory (df.):  (i) the mind and the brain are one and the same object, and (ii) mental properties are exactly the same thing as physical properties.[1]

·         Philosophers who have defended this view include U.T. Place and J.J.C. Smart.

·         This theory accepts both materialism and mentalism.


materialism (df.): only physical objects and characteristics exist; nothing non-physical exists.


mentalism (df.) mental states are inner causes of outward behavior; beliefs, desires, etc. are inside the subject and cause behavior, which is directly observable.

·         [Remember that the mental states referred to in this definition are not necessarily physical and not necessarily non-physical; the only thing that mentalism is committed to is the idea that they are “inner” causes of behavior. But if you pair this with materialism, as the Identity Theory does, you’ve committed to saying that the inner mental states that cause behavior are physical states.]



[4.4.1.] An A Posteriori Theory.


The identity theory is intended to be an a posteriori theory. According to its defenders, he truth of the claim that “the mind and the brain are one and the same thing” will be discovered by science through observation and experiment.


This makes it a very different kind of theory than either Cartesian dualism or logical behaviorism. Both of those theories were intended to be a priori theories; we were supposed to be able to know that they are true either

·         by introspection and reasoning (dualism), or

·         by analysis of what mentalistic terms mean (logical behaviorism).



[4.4.2.] A Further Success for Materialism.


Throughout the history of science, case after case of supposed non-physical things have been shown to be physical.


The belief that...

...has been replaced by the belief that....


Lightning is Zeus’ thunderbolt.



Lightning is a type of electrical discharge.



Living things differ from non-living things in that they have


élan vital (df.): a special immaterial (non-physical) substance that scientists once believed animates living things with life.


The doctrine that living things differ from non-living things because of élan vital is known as vitalism.



“[L]ife is a physiochemical phenomenon. Organisms are made of the same basic elements (carbon, oxygen, etc.) as nonliving things. Life differs from nonlife because of how these basic physical constituents are arranged. There is no extra nonphysical ingredient that magically makes organisms ‘alive.’” (281)


Defenders of IT believe a further success for materialism, in that it will add another identification will be added to this list:



The mind and the brain are completely different, independent substances (dualism).



The mind is one and the same thing as the brain. (IT)


[4.4.3.] Correlation Experiments: Inconclusive.


How will science show that IT is true?


One possibility is that it could show that IT is true by establishing 100% perfect correlations between mental states and brain states, such as


·         Every time S’s C-fibers fire, S experiences pain (and vice versa)

·         Every time S’s G-fibers fire, S thinks about Lady Gaga (and vice versa)


However, this would not be enough to establish the truth of IT. This is because this sort of 100% correlation is compatible with Cartesian dualism…


compatibility (df.): p and q are compatible if and only if it is possible that p and q both be true at the same time. For example:


“Jim was born in Georgia.”

“Jim was born in Atlanta.”


It is possible for both of these sentences to be true; so they are compatible with each other. On the other hand,


“Jim was born in Alabama.”

“Jim was born in Algeria.”


are not compatible; they cannot both be true.


Notice that the actual truth value of two propositions is irrelevant to whether they are compatible (just as the actual truth value of the premises of an argument is irrelevant to whether the argument is valid)—you don’t need to know where Jim was actually born to know that the sentences in the first set are compatible and that those in the second set are not.


The point about correlation experiments is this:


“There is a perfect correlation between mental states and brain states”

“The mind and the brain are completely different things, capable of existing independently of one another.” [Cartesian dualism]


are compatible; it is possible that they are both true:

·         A dualist could maintain this by saying that every time your C-fibers fire, this (somehow) causes your mind to experience pain.



[4.4.4.] The Parsimony Argument (in Support of IT).


The argument for IT that we will now consider is based on the idea that parsimony is an important characteristic of theories, both in science and philosophy.


parsimony (df.): simplicity or economy of assumptions; the fewer assumptions a theory makes, the more parsimonious it is.


The idea that theories should be parsimonious is captured in the following idea from medieval philosophy:


Ockham’s Razor, a.k.a. the principle of parsimony (df.): do not multiply entities beyond necessity [“entia non sunt multiplicanda praeter”]; in other words, make your ontology (your account of what kinds of thing there are) only as complicated as it needs to be to accommodate the evidence. Do not admit entities (or types of entity) into your ontology unless it is necessary to do so in order to account for the evidence at hand.


This idea derives from William of Ockham (c.1287 - 1347), one of the most important philosophers of the medieval period.[2]


We can understand Ockham’s Razor as a further constraint on abductive reasoning, along with use the Surprise Principle and avoid the Only Game in Town Fallacy. Understood in this way, it says something like this:

·         If H1 and H2 are equally good explanations of a surprising hypothesis O [each of them, if true, would make O unsurprising], and if H1 assumes a simpler ontology than H2, then H1 is to be preferred over H2.

·         For example, suppose I come home to make the surprising observation (O) that all of my belongings are in disarray, some of my valuables are gone, and my back window has been smashed from the outside of the house. Either of these hypotheses would make O unsurprising:

 But we don’t need to postulate the extra 26 people in order to explain O. H1 works just as well as an explanation. Since it assumes a smaller ontology (fewer burglars), Ockham’s Razor says we should prefer it over H1.



The Parsimony Argument begins with Ockham’s Razor…


1.      If two theories, t1 and t2, are equally consistent with the observations, and t1 is more parsimonious than t2, then it is more likely that t1 is true. [Ockham’s Razor]

2.      IT and dualism are equally consistent with the observations of science, in that both predict perfect correlations between brain states and mental states.

3.      IT is more parsimonious than dualism, since it supposes that fewer things exist.

4.      Therefore, it is more likey that IT is true than that dualism is true.


Remember the example of vitalism… Once science has the ability to explain life by appealing to nothing but physical properties, scientists abandoned vitalism and the belief in an élan vital. The mysterious “stuff” was no longer needed to explain life, so scientists gave up their belief in it.


According to IT, if science can ever establish a perfect correlation between types of mental state and types of brain states, then dualism should be given up, since the belief in something non-physical would be superfluous. (This is a more ambitious claim that the conclusion of the Parsimony Argument.)


The Parsimony Argument is definitely valid, so if there is a problem with it, it must be that it has a false premise.


The only questionable premise seems to be (1), Ockham’s Razor itself. Why think that this principle is true? Sober raises just this sort of question on p.283. He doesn’t doubt the truth of the principle—but he asks what kind of evidence there can be for it.



[4.4.5.] The Argument from Introspective Knowledge.


The following are two arguments that have been made against the Identity Theory.[3]


The Argument from Introspective Knowledge

1.      My mental states (e.g., pains, sensations, beliefs, desires) are introspectively known to me as states of my conscious self.

2.      My brain states (e.g., neurons firing in my somatosensory cortex) are not introspectively known to me as states of my conscious self.

3.      If a has some property and b lacks that property, then a and b are not the same thing. [Leibniz’s Law]

4.      Therefore, my mental states are not identical with my brain states [IT is false].



This argument is valid, but it may commit the same mistake as Descartes’ “Indubitable Existence” argument:

·         Once again, we are dealing with propositional attitudes. If you know that you are in pain, this is an attitude that you have toward the proposition I am in pain—you know it. But this does not mean that your pain has a property corresponding to this attitude, e.g., the property of being known by yourself to be in pain.

·         The argument incorrectly assumes that there is a property (“being introspectively known to me as a state of my conscious self”) that my pain has but that my nerve-firing does not. But there is no such property.

·         A correct description of the situation would be as follows:


1.      I know that the proposition “I am in pain” is true.

2.      I do not know the proposition “My C-fibers are firing” is true.


And these premises do not imply that my pain is not identical to the firing of my C-fibers, no more than the fact that Lois Lane wants “I marry Superman” to be true but does not want “I marry Clark Kent” to be true indicates that Superman is not one and the same person as Clark Kent.



[4.4.6.] The Knowledge Argument.


Australian philosopher of mind Frank Jackson (b.1943) came up with the following thought experiment as an argument against IT:


Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’. [...] What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? It seems just obvious that she will learn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false.[4]


Mary has learned everything possible about the physical facts about the human nervous system, including how it responds to various sorts of physical stimuli. And yet there is still something she does not know: what it is like to have a visual sensation of red.


This suggests the following argument:


The Knowledge Argument

1.      It is possible for Mary to know all of the facts about the physical brain without knowing what it is like to have specific qualia.

2.      If this is possible, then having qualia requires something other than or in addition to being in specific brain states.

3.      And if that is true, then IT is false (since IT has to say that qualia are nothing but brain states).

4.      So, IT is false.


This argument has been widely discussed—various philosophers have offered different criticisms of it. Do you think it is sound?[5]


Stopping point for Thursday June 24. For next time read chapter 24 (“Freedom, Determinism, Causality”) and chapter 25 (“A Menu of Positions on Free Will”). An optional reading is the excerpt from Hume’s Enquiry, pp.349-57—we will be talking about some of the ideas in this excerpt tomorrow.




[1] The Identity Theory is also sometimes called reductive materialism.

[2] Although Ockham’s Razor is an idea found in Ockham’s work, the formulation given above occurs nowhere in his writings. For more on Ockham, see Paul Vincent Spade, “William of Ockham,” The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.), URL = < >.


[3] This material is not in your textbook. I take it from Paul Churchland, Matter and Consciousness, revised edition.


[4] Frank Jackson.  “Epiphenomenal Qualia,” Philosophical Quarterly (32) 1982, 127–136.


[5] For more information on this argument, see Martine Nida-Rümelin, “Qualia: The Knowledge Argument,” The Stanford Encyclopedia of Philosophy (Summer 2010 Edition), Edward N. Zalta (ed.), forthcoming URL = <>.


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