In this letter, Lewis provides comments on a draft of Colin McGinn’s 1980 Functionalism and Phenomenalism: A Critical Note. Australasian Journal of Philosophy 58 (March):35-46. This letter is important for understanding Lewis’s theory of mind but also for understanding his doctrine of theoretical terms, which informs his philosophical method in general.

The letter begins with some remarks specific to Lewis’s theory of mind. He states the sense in which he is a functionalist and the sense in which he is not. He then explains what it means to say that mental terms are theoretical or newly introduced. Mental terms are not theoretical (newly introduced) simpliciter.

From these specific comments he moves to ‘bigger’ comments that pertain to his doctrine of theoretical terms. He corrects McGinn about the upshot of defining theoretical terms. In defining some term we may well eliminate it from our primitive vocabulary but that does not mean we have eliminated the term altogether such that we have eliminated talk of entities referred to by that term. For Lewis, defining a theoretical term T is, as he says in 1970, a ‘vindication’ of that term (How to Define Theoretical Terms, p. 427). This remark especially highlights the reductionist character of his philosophy and provides a backdrop for understanding how to situate his stance against eliminativist theories such as strands of anti-realism, eliminative materialism, and nihilism.

Two other big comments stand out in this letter. First, Lewis points out in what contexts circularity in philosophical analyses is and is not problematic. Second, he rejects any notion of conceptual priority in the service of conceptual analysis. We might define one concept in terms of another but this does not entail that the defined concept is more fundamental than or more basic than the defining concept. A system of concepts does not exhibit the sort of objective hierarchical structure that is becoming more prevalent in contemporary philosophy.