Tarski, Alfred (tärˈskē) [key], 1902–, Polish-American mathematician and philosopher, Ph.D. Univ. of Warsaw, 1924. He lectured at Warsaw until 1939, emigrated to the United States, and then taught at the Univ. of California, Berkeley (1942–68). Tarski made extensive, basic contributions to the field of metamathematics, a branch of mathematical logic. His most important contribution to logic is the semantic method, a method that allows a more exacting study of formal scientific languages. His work is characterized by a basic acceptance and free use of the assumptions of set theory. For this reason he is regarded by some as a nominalist. His publications include A Decision Method for Elementary Algebra and Geometry (1948, rev. ed. 1957) and Undecidable Theories (with others, 1953; repr. 1968).