 Carnap’s first major publication in English, soon after his arrival in the United States, was “Testability and Meaning” (1936–37), discussed in  sections 1.5–6  above, the first in a series of attempts to explicate “empirical content” or “empirical reducibility” more loosely than the strict reducibility envisaged in the Aufbau. By the late 1930s, Carnap was willing to countenance a theoretical language whose primitive terms are not constructed bottom-up from observation sentences at all but entirely top-down, “floating in the air, so to speak” so that these basic theoretical terms can have “only an indirect interpretation, which is incomplete in a certain sense” (Carnap 1939: 65). With these steps, Carnap began his return to a consideration of the theoretical language, which he had left behind during the Vienna years. But his main focus, after “Testability and Meaning”, was the semantic framework itself. In the slim volume on Foundations of Logic and Mathematics for the Encyclopedia of Unified Science (1939), the tripartite classification of linguistic theory into syntax, semantics, and pragmatics was first laid out. A five-volume series of Studies in Semantics was projected, of which the first two volumes, Introduction to Semantics and Formalization of Logic were published in 1942–3. Carnap then pursued a long-standing interest in modal logic and intensional languages, and published “Modalities and Quantification” in 1946, Meaning and Necessity in 1947. More details on Carnap’s work in semantics can be found in the supplement  Semantics.