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see identity of indiscernibles Leader of the Warsaw school of logic and philosophy between the wars, where he taught many excellent logicians, most notably T arski . Leśniewski's publications and unorthodox logical systems are characterized by extreme care and rigor. His major efforts went into constructing and improving his three logical systems. These are: protothetic, a system of propositional logic with quantifiers and higher-order functors; ontology, a generalized term logic also constructible to any finite order; and mereology, a formal theory of PART/WHOLE and aggregates. Leiniewski created his system in response to R ussell's Paradox, as a foundation for mathematics without the P latonism and sloppiness of W hitehead and Russell's (1910–13) Principia Mathematica or the intuitive incomprehensibility of Zermelo's sets. ( See class, collection, set .) Like his hero F rege , Leiniewski decried formalism, insisting that his logical systems are interpreted and true. He believed logic should be ontologically neutral, and took care that no thesis of his system imply the existence of anything. This caution extended to his metalogical presentation: systems are taken not as tendentious abstract structures but as visible collections of marks growing in time by the addition of new theses (theorems and definitions) according to perfectly exact metalogical directives. Ontology ... log in or subscribe to read full text