The majority of the correspondence is divided into two categories, personal and professional correspondence. The category "personal correspondence" covers mostly private topics, such as finance, health, or issues relating to family and friends. The category "professional correspondence" comprises topics such as research, publications, employment, or grants. Among the correspondents are Ina and Rudolf Carnap, Herbert Feigl, Carl G. Hempel, Viktor Kraft, Herta Leng, Heinz Neider, Otto Neurath, Karl Popper, Moritz Schlick, Susan Stebbing, Gregory Vlastos, and Ludwig Wittgenstein. Exchanges with institutions include Fonds zur Hilfeleistung an politisch Verfolgte (Vienna), Institute for Advanced Study, and Princeton University.