Stanisław Leśniewski (30 March 1886 – 13 May 1939) was a Polish mathematician, philosopher and logician.

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Stanisław Leśniewski
Born	(1886-03-30)30 March 1886 Serpukhov, Russian Empire
Died	13 May 1939(1939-05-13) (aged 53) Warsaw, Poland
Nationality	Polish
Alma mater	Lviv University
Known for	Calculus of names Mereology
Scientific career
Fields	Mathematics
Institutions	University of Warsaw
Doctoral advisor	Kazimierz Twardowski
Doctoral students	Alfred Tarski

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He was born on 28 March 1886 at Serpukhov, near Moscow, to father Izydor, an engineer working on the construction of the Trans-Siberian Railway, and mother Helena (née Palczewska).^[1] Leśniewski went to a high school in Irkutsk. Later he attended lectures by Hans Cornelius at the Ludwig Maximilian University of Munich^[2] and lectures by Wacław Sierpiński at Lviv University.^[3]

Leśniewski belonged to the first generation of the Lwów–Warsaw School of logic founded by Kazimierz Twardowski. Together with Alfred Tarski and Jan Łukasiewicz, he formed a trio which made the University of Warsaw, during the interbellum, perhaps the most important research center in the world for formal logic.^[4] Despite that, Leśniewski's growing anti-semitism later caused the deterioration of the relationship with Tarski.^[5]

His main contribution was the construction of three nested formal systems, to which he gave the Greek-derived names of protothetic, ontology, and mereology.^[6] ("Calculus of names" is sometimes used instead of ontology, a term widely employed in metaphysics in a very different sense.) A good textbook presentation of these systems is that by Simons (1987), who compares and contrasts them with the variants of mereology, more popular nowadays, descending from the calculus of individuals of Leonard and Goodman. Simons clarifies something that is very difficult to determine by reading Leśniewski and his students, namely that Polish mereology is a first-order theory equivalent to what is now called classical extensional mereology (modulo choice of language).

While he did publish a fair body of work (Leśniewski, 1992, is his collected works in English translation), some of it in German, the leading language for mathematics of his day, his writings had limited impact because of their enigmatic style and highly idiosyncratic notation. Leśniewski was also a radical nominalist: he rejected axiomatic set theory at a time when that theory was in full flower. He pointed to Russell's paradox and the like in support of his rejection, and devised his three formal systems as a concrete alternative to set theory. Even though Alfred Tarski was his sole doctoral pupil, Leśniewski nevertheless strongly influenced an entire generation of Polish logicians and mathematicians via his teaching at the University of Warsaw. It is mainly thanks to the writings of his students (e.g., Srzednicki and Rickey 1984) that Leśniewski's thought is known.^[7]

During the Polish–Soviet War of 1919-21, Leśniewski served the cause of Poland's independence by breaking Soviet Russian ciphers for the Polish General Staff's Cipher Bureau.^[8]

Leśniewski died suddenly of cancer, shortly before the German invasion of Poland, which resulted in the destruction of his Nachlass. He was buried at Warsaw's Powązki Cemetery.^[9]

• 1988. Lecture Notes in Logic. Kluwer. Table of Contents.
• 1992. Collected Works. 2 vols. Kluwer. Table of Contents.
• 1929, "Über Funktionen, deren Felder Gruppen mit Rücksicht auf diese Funktionen sind", Fundamenta Mathematicae 13: 319-32.
• 1929, "Grundzüge eines neuen Systems der Grundlagen der Mathematik", Fundamenta Mathematicae 14: 1-81.
• 1929, "Über Funktionen, deren Felder Abelsche Gruppen in bezug auf diese Funktionen sind", Fundamenta Mathematicae 14: 242-51.

• History of philosophy in Poland
• List of Poles