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):
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:
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:
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:
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.
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]