Stanisław Leśniewski in the context of University of Warsaw

Mereology (/mɪəriˈɒlədʒi/; from Greek μέρος 'part' (root: μερε-, mere-) and the suffix -logy, 'study, discussion, science') is the philosophical study of part-whole relationships, also called parthood relationships. As a branch of metaphysics, mereology examines the connections between parts and their wholes, exploring how components interact within a system. This theory has roots in ancient philosophy, with significant contributions from Plato, Aristotle, and later, medieval and Renaissance thinkers like Thomas Aquinas and John Duns Scotus. Mereology was formally axiomatized in the 20th century by Polish logician Stanisław Leśniewski, who introduced it as part of a comprehensive framework for logic and mathematics, and coined the word "mereology".

Mereological ideas were influential in early § Set theory, and formal mereology has continued to be used by a minority in works on the § Foundations of mathematics. Different axiomatizations of mereology have been applied in § Metaphysics, used in § Linguistic semantics to analyze "mass terms", used in the cognitive sciences, and developed in § General systems theory. Mereology has been combined with topology, for more on which see the article on mereotopology. Mereology is also used in the foundation of Whitehead's point-free geometry, on which see Tarski 1956 and Gerla 1995. Mereology is used in discussions of entities as varied as musical groups, geographical regions, and abstract concepts, demonstrating its applicability to a wide range of philosophical and scientific discourses.

The Lwów–Warsaw School (Polish: Szkoła Lwowsko-Warszawska) was an interdisciplinary school (mainly philosophy, logic and psychology) founded by Kazimierz Twardowski in 1895 in Lwów, Austro-Hungary (now Lviv, Ukraine).

Though its members represented a variety of disciplines, from mathematics through logic to psychology, the Lwów–Warsaw School is widely considered to have been a philosophical movement. It has produced some of the leading logicians of the twentieth century such as Jan Łukasiewicz, Stanisław Leśniewski, and Alfred Tarski, among others. Its members did not only contribute to the techniques of logic but also to various domains that belong to the philosophy of language.