This chapter is the first of two chapters devoted to the idea of incommensurability, which Kuhn devoted the lion’s share of his later philosophical career examining. The thesis of incommensurability originally posed in Structure posited a significant discontinuity between successive scientific theories: the content of earlier theories could not straightforwardly be assimilated to later ones, which had implications for notions like progress and rationality. This chapter examines the methodological and semantic aspects laid out in Structure, as well as his post-Structure reformulation of the concept, focusing as it did on Quine’s indeterminacy of translation thesis.