  During World War I, when Leśniewski lived in Moscow, he completed “Foundations of a General Theory of Sets I” (1916). Despite the use of the term ‘mnogość’ (set), this was a first rigorous deductive presentation of the theory of parts, wholes and concrete collections. Leśniewski later dropped the term ‘mnogość’ and instead invented the term ‘Mereology’, meaning ‘theory of parts’, an irregular coinage from the Greek μερος, part, in order to differentiate his view from what he ironically called “official” set theory. In this paper the language is a highly regimented Polish supplemented with variables, because at the time Leśniewski did not trust symbolic methods, having found the use/mention confusions in Whitehead and Russell’s Principia Mathematica a barrier to understanding, and assuming any symbolic logic had to be like that. Only later did Leśniewski discover the much tidier work of Frege, which he thereafter held up as the foremost example of symbolic logic to that date. Leśniewski was one of the earliest logicians to appreciate the virtues of Frege’s logical work as largely independent of its inconsistency. In later years, Leśniewski proved that Frege’s “way out” of the paradox, which involved a restriction of the fatal abstraction principle Basic Law V, had the unacceptable consequence that there could not be more than one object. This work was not published, but was reconstructed after World War II by Sobociński (1949).