 Stanisław Kazimierz Leśniewski was born on 28 March 1886 at Serphukhov, near Moscow, to Izydor, an engineer working on the construction of the Trans-Siberian Railway, and Helena, née Palczewska. He was baptised at St. Stanislav’s church in St. Petersburg. His mother died when he was young and his father remarried. He attended classical Gimnazjum (grammar school studying classical languages) in Irkutsk in Siberia. Between 1904 and 1910 Leśniewski studied Philosophy and Mathematics in Germany, Switzerland and Russia, at Leipzig, Zurich, Heidelberg, St. Petersburg, and at Munich, where he heard lectures by Hans Cornelius, Moritz Geiger and Alexander Pfänder. In 1910 he went as a doctoral student to Lwów University, then in Austria-Hungary, where the foremost Polish philosopher of the day, Kazimierz Twardowski, a student of Franz Brentano, was professor and was building up a cadre of outstanding young philosophers. In 1912 he obtained his doctorate with a dissertation Przyczynek do analizy zdań egzystencjalnych [Contributions to the Analysis of Existential Propositions]. This was published the previous year in the leading Polish-language philosophy journal Przegląd Filozoficzny. Several further publications followed until the outbreak of war, which Leśniewski spent in Moscow teaching mathematics at Polish schools. During this period he developed what later became known as Mereology. After the Bolshevik Revolution in Russia, Leśniewski left Russia for Poland. During the 1919–21 Polish–Bolshevik War he worked under Lt. Col. Jan Kowalewski as a codebreaker for the Polish General Staff’s Cipher Section, in part because of his knowledge of Russian, helping Poland to survive Russia’s attempt to reconquer it. He tried unsuccessfully to obtain his habilitation in Lwów, where he was blocked, getting it instead in the University of Warsaw, where in 1919 he became (Extraordinary—Associate) Professor of the Foundations of Mathematics, a position especially created for him. From then until his final illness Leśniewski lectured regularly on logical and mathematical topics, and built up his logical systems. Though his logic never became widely accepted, even in Poland, with Jan Łukasiewicz he was respected as one of the co-founders of the Warsaw School of Logic. Leśniewski’s only doctoral student, Alfred Tarski (Leśniewski used to boast of having 100% geniuses as doctoral students) joined them to form the world’s pre-eminent centre of mathematical logic in the interwar years. Leśniewski was made a full professor in 1936. Always a smoker, he contracted thyroid cancer and died on 13 May 1939 at the age of 53. His papers, entrusted to his student Bolesław Sobociński, included unfinished works on logical antinomies and on many-valued logic. All these papers were destroyed during the Warsaw Rising of 1944.