 The term-introducing theory could be a scientific theory, such as the theory that introduces terms like ‘electron’, and the language of the theory could be observation language. Or, more interestingly, the term-introducing theory could be folk psychology, and the language of the theory could be the language of physics. If we have a tool for deriving the denotations of terms introduced by a theory, and we have a way of treating  folk psychology as a theory  (i.e., a conjunction of sentences to which folk wisdom is committed), we can derive the denotations of terms like ‘belief’, ‘pain’, and so on using this theory. Some of Lewis’s important early work on the metaphysics of mind was concerned with systematising the progress positivists, especially Ramsey and Carnap, had made on just this problem. The procedure is introduced in “An Argument for the Identity Theory”, “Psychophysical and Theoretical Identifications” (1972a) and “How to Define Theoretical Terms” (1970c). There are important later discussions of it in “Reduction of Mind” and “Naming the Colours” (1997c), among many others.