From intersubjective correspondence to the counterpart relation—motifs from Carnap’s Aufbau in Lewis’s counterpart theory and his philosophical methodology

In 'Counterpart Theory and Quantified Modal Logic', David Lewis remarks that his counterpart relation 'is very like' the relation of intersubjective correspondence in Rudolf Carnap's 'Der logische Aufbau der Welt'. Given Lewis's status as the most eminent contemporary metaphysician, and Carnap's as the most eminent twentieth century critic of metaphysics, this reference may not only be surprising, but it is also suggestively opaque. In which respect could the two relations from two wildly distinct philosophical frameworks resemble each other? The main aim of this paper is to argue that Lewis's claim indeed points to a significant similarity: both relations act as replacements for identity in their respective frameworks. This main argument is supplemented by a discussion of three further similarities between aspects of Lewis's methodology and Carnap's methodology of the Aufbau. The paper hence supports recent efforts to show that Carnap's book was not the philosophical misstep it was taken to be by Quine and Goodman, but that it rather should be seen as an important step in the development of analytic philosophy.