  Mereology, assumes protothetic and ontology as logically prior theories, and has the term “part of” as its sole primitive concept. Being a part is a non-reflexive and transitive relation. There are no empty classes. Moreover, the class consisting of a single element is identical with it. In general, mereology is a theory of sets in the collective (mereological) sense, contrary to ordinary set theory, which describes sets in the distributive sense. The main difference between the two interpretations of the term “set” consists in the fact that the membership relation is transitive under the mereological reading, but non-transitive under the distributive one. Leśniewski believed that his theory classes would perform all the tasks of ordinary set theory without generating paradoxes. In fact, he invented mereology when he tried to solved the Russell paradox. Since there are no mereological classes which are not their own elements, the question which led to the Russell paradox simply makes no sense in Leśniewski’s systems. On the other hand, mereology is weaker than set theory.