  It was in Mereology, Leśniewski’s oldest system, that there were the most varied developments. The 1927–30 article series on Mereology as part of the foundations of mathematics rang the changes in possible primitives. After scathing attacks on Whitehead’s and Russell’s use/mention confusions in Principia, and on standard set theories, he retraces the formal development of his 1916 paper, noting in a long footnote the similarities with Whitehead’s theory of events, whose formal development he also criticises. He then summarizes the development up to 1920, continuing with additional results unpublished in 1916, taking the number of theorems up to 198. Further chapters tidy up the axiomatization in terms of ‘part’, and show that ‘ingredient’ can be taken as primitive. The theorems are taken up to 264, then ‘exterior’ is shown to be a possible primitive. There the development is arrested, and a final section discusses singular propositions of the form ‘\(A \isa b\)’, with a note on how to understand propositions about a thing which changes. Using the example ‘Warsaw of 1830 is smaller than Warsaw of 1930’, Leśniewski proposes to consider ‘Warsaw in 1830’ and ‘Warsaw in 1930’ as denoting time-slices of the temporally much longer object he calls ‘Warsaw from the beginning to the end of its existence’. In this way he claims to bring many uses of ‘is’ in ordinary language within the scope of an analysis using his Ontology. This four-dimensional understanding of ordinary objects is now commonplace, but at that time it was something of a rarity. The discussion is one of the few places in Leśniewski’s mature work where he indulges in anything similar to the philosophical logic of his early years. Otherwise, when not discussing formal systems and proving theorems, his prose discussions tend to be intemperate though frequently justified criticisms of the statements of others, most especially the proponents of standard set theory.