Inquiry in the sense of knowledge-seeking becomes inquiry in the sense of interrogation. Using this new logic, Hintikka establishes a result that will be considered among the fundamental theorems of all epistemology, of the virtual identity of optimal strategies of pure discovery with optimal deductive strategies. Hintikka shows, in fact, how the logic of experimental inquiry can be understood from the interrogative vantage point. Other topics examined include induction (in a forgotten sense that has nevertheless played a role in science), explanation, the incommensurability of theories, theory-ladenness of observations, and identifiability.