 One of the main challenges set by the model-theoretic definition of logical consequence is to distinguish between the logical and the nonlogical vocabulary. The logical vocabulary is defined in all models by the recursive clauses (such as those mentioned above for conjunction and the universal quantifier), and in that sense its meaning is fixed. The choice of the logical vocabulary determines the class of models considered when evaluating validity, and thus it determines the class of the logically valid arguments. Now, while each formal language is typically defined with a choice of a logical vocabulary, one can ask for a more principled characterization of logical vocabulary. Tarski left the question of a principled distinction open in his 1936, and only gave the lines of a relativistic stance, by which different choices of the logical vocabulary may be admissible. Others have proposed criteria for logicality, demanding that logical constants be appropriately formal, general or topic neutral (for references and details, see the entry on  logical constants).  Note that a choice of the logical vocabulary is a special case of setting constraints on the class of models to be used. It has been suggested that the focus on criteria for the logical vocabulary misses this point, and that more generally the question is which semantic constraints should be adopted, limiting the admissible models for a language (Sagi 2014a, Zinke 2017). 