 A related debate is the debate about free logic. Free logics are formal systems that drop the assumption made in standard first and higher order logic that every closed term denotes an object in the domain of the model. Free logic allows for terms that denote nothing, and in free logic certain rules about the inferential interaction between quantifiers and terms have to be modified. Whether free or un-free (standard) logic is the better formal model for natural language logical inference is a further question. For more discussion of logic with an empty domain see (Quine 1954) and (Williamson 1999). For a sound and complete proof system for logic with an empty domain, see (Tennant 1990). For a survey article on free logic, see (Lambert 2001).