  Leśniewski’s supreme achievement as a logician, in his own eyes, was to formulate the metalogical requirements for extending a concrete system by directives, relative to the previous state of the system. Since the system is not planned or fixed in advance, the TEs and directives have to be schematically flexible enough to accommodate any future additions, while not being so liberal as to allow contradictions or nonsense to arise. Finding that balance, most especially in the rules for admissible definitions, was a considerable feat. The definition of ‘definition’ for Protothetic run to 18 separate complex clauses, and in Ontology, which adds a second style of definition, another 18 clauses.