Between the two world wars, Stanislaw Lesniewski (1886-1939), created the famous and important system of foundations of mathematics that comprises three deductive theories: Protothetic, Ontology, and Mereology.
His research started in 1914 with studies on the general theory of sets (later named Mereology'). Ontology followed between 1919 and 1921, and was the next step towards an integrated system. In order to combine these two systematically he constructed Protothetic - the system of first principles'. Together they amount to what Z. Jordan called ... most thorough, original, and...