10,000 pages of correspondence, notes, and lectures and Carnap's library of 2,000 books and 1,600 issues of periodicals. Carnap worked in the areas of constructional theories, physicalism, the epistomological foundations of physics and mathematics, the syntactical structure of language, semantics, nodal logic, ontology and probability. Correspondents include Feigl, Hempel, Kaufmann, Neurath, and Schlick.