Kazimierz Ajdukiewicz was an eminent representative of the Lvov-Warsaw School. His main interest was the cognitive role of language. His radical conventionalism intended to explain rapid and fundamental changes in science. He used the method of philosophical paraphrase to make traditional metaphysical questions decidable. Then he drew metaphysical conclusions from the so called "semantic epistemology" based—according to his programme—on semantics (which played an important role in his research) and formal logic. His categorial grammar aiming to formulate the general criteria of syntactic coherence was the first grammar based exclusively on the structural properties of expressions. He also undertook a number of methodological issues, both general and detailed. He was interested in the theory of definition, the theory of questions, the problem of rationality of fallible inferential methods, the foundation of sentences, classification of reasonings, of sciences and of axiomatic systems, and in the reconstruction and evaluation of scientific procedures.