  Stanisław Leśniewski (1886–1939) was one of the principal founders and movers of the school of logic that flourished in Warsaw between the two world wars. He was the originator of an unorthodox system of the foundations of mathematics, based on three formal systems: Protothetic, a logic of propositions and their functions; Ontology: a logic of names, and functors of arbitrary order; and Mereology, a general theory of part and whole. His concern for utmost rigor in the formalization and execution of logic, coupled with a nominalistic rejection of abstract entities, led to a precise but highly unusual metalogic. His strictures on correctly distinguishing use from mention of expressions, his canons of correct definition, and his mereology, have all informed the logical mainstream, but the majority of his logical views and innovations have not been widely adopted. Despite this, his influence as a teacher and as a motor for logical innovation are widely acknowledged. He remains one of logic’s most original figures.