 Two formulations of unification in the logical positivist tradition of the ideal logical structure of science placed the question of unity at the core of philosophy of science: Carl Hempel’s deductive-nomological model of explanation and Ernst Nagel’s model of reduction. Both are fundamentally epistemological models, and both are specifically explanatory, at least in the sense that explanation serves unification. The emphasis on language and logical structure makes explanatory reduction a form of unity of the synchronic kind. Still, Nagel’s model of reduction is a model of scientific structure and explanation as well as of scientific progress. It is based on the problem of relating different theories as different sets of theoretical predicates.