  With Protothetic launched, Leśniewski could now look back on his system of foundations and see that it consisted of a hierarchy of three systems, developed in reverse order: Protothetic, introducing connectives, quantifiers and higher functions; Ontology, introducing the new category of names, with the new primitive ‘is’, and Mereology, based on a primitive mereological functor such as ‘part of’ or ‘ingredient of’, but introducing no new categories of expressions not already foreseen in Ontology.