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Article Summary: “Psychophysical and Theoretical Identifications” by David Lewis


Lewis supplements his original argument for the identity theory of mind (presented in “An Argument for the Identity Theory,” also summarized on this website) by positing a theory of the meaning of mental state terms. In conjunction with the idea that folk psychology is just a term-introducing theory, Lewis’s theory of the meaning of mental state terms implies that these meanings can be used to reduce mental states to physical states.

Lewis begins with an account of theorizing. The account involves an investigator trying to figure out who committed a crime. The investigator introduces terms X,Y, and Z for people who were responsible, in various ways, for the crime in question. The terms are not worked on any further but are treated as if everyone understands their meaning. These terms are T-terms, or theoretical terms, whereas the other terms used to describe the crime are O-terms, or old terms. Lewis points out that this method of introducing terms seems just like treating the terms as existentially bound variables (a point that is relevant later).

Suppose we find out that Plum, Peacock, and Mustard committed the crime in the fashion described, thus making the theory true. We would say of this triplet that they realize the theory. Further, this is the only triplet which realized the theory: there cannot be some other set of persons which also realized it, thus committing the same crime as well. Thus we say that the triplet uniquely realizes the theory. Lewis extrapolates from this example to the meaning of theoretical terms more generally: theoretical terms are introduced with an implicit functional definition. Further, we did nothing else to imbue the terms with meaning, and therefore the meaning of the terms just are the implicit functional definitions. So, X,Y, and Z in the crime case are just names for people who occupied causal roles R1, R2, and R3. These are the sole meanings of the theoretical terms.

If a term ends up being realized, then it is a definite description of its realizer. Meanwhile, if it is unrealized, then it is an improper description. Lewis says as well that just because a term ends up being an improper description, it is not meaningless. For the description obtains in some possible world or other, just not ours. Further, if the description of a theoretical term is almost met by an entity in the world, where a minor revision to the description would make the entity a realization of the term, the entity is a near realization of the term. In these cases, Lewis thinks that near realizations should be treated as realizations. It is only when a description is totally off the mark that we should say a term is non-referential.

Lewis then presents a formalization of the above understanding of the meaning of mental state terms. This method is a modified version of the Ramsey method for identifying theoretical terms. The method has a number of definitive steps, all explicated in formal logic:

1. We begin by noticing that a theory can be described as a long conjunction of sentences where theoretical terms appear. Call this the postulate of theory T.


2. Then we replace the t-terms with existentially bound variables (Lewis breaks this step into two, replacing the terms with free variables and then existentially quantifying them in step 3. There is literally no difference in rolling them up into one step):

(∃x) T[x]

3. Lewis wants to avoid the possibility of multiple realization (in his example of the crime investigator he states that only one triplet can realize the theory, and so too he thinks of all theoretical terms) so he adds notation stating that there is ‘exactly one’ realization of T:

(∃!x) T[x]

4. Now we formulate what is known as the Carnap sentence of a theory, which is a conditional with the Ramsey sentence as antecedent and the postulate as the consequent:

(∃x) T[x]→T[t]

All this means is that if there is a realization of T, then the t-terms in T name components of its realization.

5. Now we do something similar to the Carnap sentence that we did to the Ramsey sentence: we add a condition that there be just one, unique realization of T:

(∃!x) T[x]T[t]

6. Then, the remaining cases where T is not realized can be described with another conditional:

~(∃!x) T[x]→ t = *

You can figure out that the antecedent just means ‘there is no realization of T’; the consequent meanwhile means that all t-terms of T are denotationless. Such is the formal method for specifying the meaning of mental terms (now called the Ramsey-Lewis method).

The method leads to the reduction of theoretical terms to their referents via two potential avenues. The first is where it is discovered that theoretical posits of some other theory ‘r’ are found to realize T. Let T[r] be the sentence expressing this discovery. This sentence, in conjunction with the postulate of T, implies that t =r. Lewis calls this conjoined sentence a ‘weak reduction premise.’

Another potentiality is that a set of theoretical terms from other theory are found to uniquely realize T. If this is so, then taken with T[r], the set of theoretical terms will be reducible to the theoretical terms of T. We will not need to use the postulate of T to reduce its terms. Lewis calls this alternative method a ‘strong reduction premise.’

Lewis concludes by returning to the case of mental state terms. He thinks that mental state terms are best treated as theoretical posits of folk psychology (although he recognizes that this is not actually true) because believing as much best explains the analyticity of folk psychological platitudes and the plausibility of behaviorism. If we accept this, then the meaning of mental state terms becomes, for any given term, ‘occupant of causal role R‘ where folk psychological platitudes specify the relations inherent to R. And this paves the way for psychoneural reduction: if mental state term M just means ‘occupant of R,’ then an empirical investigation will in all probability find a neural state N which occupies R. This will mean that M = N, and the identity theory will be redeemed.

Logical Outline

Argument One: The Meaning of Theoretical Terms

1. A theoretical term is introduced by introducing an occupant of a causal role.

2. After a theoretical term is introduced, it has meaning.

3. Yet, nothing is done to imbue a term with meaning besides introducing it.

4. Therefore, the meaning of a theoretical term is just a statement along the lines of ‘occupant of causal role R‘ (i.e., a functional definition). [1-3]

Argument Two: Folk Psychology As a Term-introducing Theory

1. We should treat folk psychology as a term-introducing theory if it best explains the meaning of mental state terms.

2. Treating f-psychology as a term-introducing theory explains the apparent analyticity of f-psychology platitudes.

3. Treating f-psychology as a term-introducing theory explains the plausibility of behaviorism.

4. Therefore, treating f-psychology as a term-introducing theory best explains the meaning of mental state terms. [2,3]

5. Therefore, we should treat folk psychology as a term-introducing theory. [4,1]

How these conclusions supplement Lewis’s original argument:

Argument Outline – Mental States Equal Physical States

1. For any given mental state M, M fills causal role R

S1. Folk psychology posits mental states as theoretical terms (from Argument Two)

S2. Theoretical terms just mean ‘occupant of causal role R.’ (from Argument One)

S3. Therefore, a mental state term M just means ‘occupant of causal role R.’ [1,2]

2. Because of the explanatory adequacy of physics, only a physical state P could possibly fill causal role R.

3. Therefore, M = P. [1,2]


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Article Summary: “An Argument for the Identity Theory” by David Lewis


David Lewis presents an argument for the identity theory of mind – the view that mental states (Lewis here calls them ‘experiences,’ but I shall call them mental states since that is a more common term) are equivalent (in the strict sense of identity) to neural states. His argument deploys the functional understanding of mental states, which says that such states are characterized by the causal roles they fill. Lewis then advocates for the explanatory adequacy of physics – the view that physics can explain all of the causal relations of physical phenomena. Since mental states cause physical phenomena, and only physical things are needed to explain such a causal relation, mental states just are physical phenomena causing other physical phenomena (e.g., behavior). In simpler terms, take a mental state M. M fills causal role R. Due to the explanatory adequacy of physics, only a physical state P can fill R. Therefore, M = P.

Lewis begins by sketching his theory in relation to an example of a lock. The state of being unlocked is at first characterized in functional terms. That is, ‘being unlocked’ is a functional state. Then, with regard to a specific lock, we can see just what it is which fulfills that functional state. And in the case of, say, a cylindrical combination lock for bicycle chains, the functional state of ‘being unlocked’ will be found to be a proper alignment of slotted discs (a purely physical state). Lewis wants to say that the same is true of the mental states which fill functional states in theories of mind.

Lewis then clarifies the nature of the identity theory of mind in general, defending it against objection. The first point Lewis makes is that the identity theory intends for certain physical states to be mental states, but not that the physical state is the object of the experience. So the physical state of seeing red is not itself red. Lewis then diffuses the objection that since mental states are by analytic necessity unlocated, and physical states meanwhile are located, that mental states cannot be physical states. Lewis thinks this is an unwarranted objection because he sees no basis for claiming any form of necessity for the claim that that mental states are unlocated.

Lewis then proceeds to objections relating to the difference between neural-state and mental state-ascriptions. Identity theory claims that the two sorts of ascriptions refer to the same underlying phenomena, not that they do so in the same sense. So just because in some instances the truth value of the two sorts of ascription are different, the identity theory is not necessarily false. The two sorts of ascription can refer to the same phenomena, albeit in difference sense.

Additionally, for Lewis it is not the case that the identity theory is false just because mental-state and neural-state ascriptions are not synonymous, while the identity of attributes predicated by such ascriptions is established via such synonymy. For ‘having‘ an experience is being in a definitive state which fulfills a specific causal role, whereas having the attribute predicated of someone when it is said they are having the experience is the attribute of being in that state.

Lewis then presents the first premise of his argument: mental states’ defining characteristics are their causal roles. Lewis says that a mental state is in fact the causal role that it fills. He says that this notion is an expansion of the ‘topic-neutral’ analyses of mental states presented by JJC Smart; the only difference is that Lewis makes explicit the causal connections of the state. Such a theory is in opposition to both epiphenomenalism and behaviorism because it explicitly holds that mental states are causally efficacious, unlike these theories.

Lewis characterizes this view of mental states as a successor theory to behaviorism: like behaviorism, Lewis’s theory recognizes that the causal roles of a mental state are analytic; unlike behaviorism, his theory allows for mental states to be causes and effects, allows for the interdefinition of mental states in terms of each other, and can handle exceptions. Behaviorism could not handle an exception because it insisted that mental states were mere behavioral dispositions. Yet the complete paralytic could still be in pain despite not being able to exhibit any behavior. Lewis’s theory skirts the issue of exceptions by characterizing mental states as the typical (rather than exceptionless) occupants of specific causal roles.

Lewis concludes his defense of his first premise by saying that he is relying upon the analytic statements of mental states inherited from behaviorism when he says that such states are characterized primarily by causal role occupation. It is these causal-centric statements that he is utilizing, all the while avoiding the pitfalls of behaviorism.

Lewis’s second premise is that physics can explain all physical phenomena. That is to say, when a phenomena occurs in, say, a special science such as cognitive science, that phenomena can be explained in terms of a more fundamental science, and in turn that explanation can be explained in terms of a still more fundamental science, until the phenomena is explainable purely in terms of fundamental physics. As such, all causal occurrences can be explained in physical terms. Lewis does not mean to say that necessarily no non-physical phenomena exist. Rather, he thinks that physical states can explain all physical phenomena. If non-physical phenomena exist, they are explanatorily (causally) superfluous. Lewis’s justification for this assertion is that it is a working hypothesis of natural scientists.

It follows from the two premises of Lewis’s argument that mental states just are physical states. For mental states occupy causal roles, and according to the explanatory adequacy of physics, only physical states fill causal roles. As such mental states just are physical states. Further, it is very likely on this view that the specific sort of physical states that mental states are are neural states. The argument for identity theory is thus complete.

Lewis concludes by tackling a potential epiphenomenalist alternative which is consistent with his premises. This view would hold that mental states are non-physical correlates of physical states which are causally efficacious just like the underlying physical states since they are perfectly correlated with them. Lewis denies that such states would be causally efficacious because again, due to the adequacy of physics, there is just no need to posit such a non-physical causal force. Further, even if the theory were correct, it would actually implicate such mental states as duplicates of the physical mental states, rather than as mere correlates of them. And this is a very different position than what the epiphenomenalist wants to argue for.

Logical Outline

Argument Outline – Mental States Equal Physical States

  1. For any given mental state M, M fills causal role R.

  2. Because of the explanatory adequacy of physics, only a physical state P could possibly fill causal role R.

  3. Therefore, M = P.

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