Although he had presented his first ideas on a new theory of classes which would avoid the paradoxes while he was in Lwów, it was during his time in Moscow that Lesniewski published his formal theory called mereology.
Kahit na siya ay ipinapahayag sa kanyang unang mga ideya sa isang bagong teorya ng mga klase kung saan ay maiwasan ang paradoxes habang siya ay sa Lwów, ito ay sa panahon ng kanyang panahon sa Moscow na Lesniewski-publish na ang kanyang pormal na teorya na tinatawag na mereology.