The term "categorial grammar' was introduced by Bar-Hillel (1964, page 99) as a handy way of grouping together some of his own earlier work (1953) and the work of the Polish logicians and philosophers Leśniewski (1929) and Ajdukiewicz (1935), in contrast to approaches to linguistic analysis based on phrase structure grammars. The most accessible of these earlier works was the paper of Ajdukiewicz, who, under the influence of Husserl's Bedeutungskategorien and the type theory that Russell had introduced to fend off foundational problems in set theory, proposed a mode of grammatical analysis in which every element of the vocabulary of a language belongs to one or more categories, and each category is either basic or defined in terms of simpler categories in a way which fixes the combinatorial properties of complex categories.