 The TEs are a means to an end: that of formulating a system’s directives. A directive sounds like an imperative, but its illocutionary force is subtler. Assume a logical system has been developed up to a certain point, that is, up to a certain last written thesis (Leśniewski’s word covering axioms, theorems and definitions). At the very least, the axiom or axioms will have been written down. Assume for the sake of argument that the development has been fine up to now. There is no categorical imperative to extend the system by adding another thesis, but let us suppose the system’s author (or indeed any assistant) wishes to do so. He or she writes down a new collection of marks. Not just anything goes however. The marks have to be legible (obviously), unequivocally dissectible into elementary symbols (that Leśniewski calls words), grammatically well-formed as a sentence (no unbound variables are allowed), and finally admissible according to the directives. The directives lay down what may be admitted next after a given sequence of theses. For example, the next thesis may be an instantiation from a previous universally quantified thesis, or a modus ponens from two previous theses, or a quantifier distribution from a previous thesis, or a definition acceptable according to the canons, or a thesis of extensionality. Admissibility is always relativized to the prior sequence: the order of introduction matters. If the new candidate thesis is admissible according to one of the directives, it passes the test and becomes part of the system, which can then be extended still further. Otherwise it is rejected and the system is not extended.