ontological argument See PHILOSOPHY OF RELI -. GIO. ontological commitment, the object or objects common to the ontology fulfilling some (regimented) theory (a term fashioned by Quine). The ontology of a (regimented) theory consists in the objects the theory assumes there to be. In order to show that a theory assumes a given object, or objects of a given class, we must show that the theory would be true only if that object existed, or if that class is not empty. This can be shown in two different but equivalent ways: if the notation of the theory contains the existential quantifier ‘(Ex)’ of first-order predicate logic, then the theory is shown to assume a given object, or objects of a given class, provided that object is required among the values of the bound variables, or (additionally) is required among the values of the domain of a given predicate, in order for the theory to be true. Thus, if the theory entails the sentence ‘(Ex)(x is a dog)’, then the values over which the bound variable ‘x’ ranges must include at least one dog, in order for the theory to be true. Alternatively, if the notation of the theory contains for each predicate a complementary predicate, then the theory assumes a given object, or objects of a given class, provided some predicate is required to be true of that object, in order for the theory to be true. Thus, if the theory contains the predicate ‘is a dog’, then the extension of ‘is a dog’ cannot be empty, if the theory is to be true. However, it is possible for different, even mutually exclusive, ontologies to fulfill a theory equally well. Thus, an ontology containing collies to the exclusion of spaniels and one containing spaniels to the exclusion of collies might each fulfill a theory that entails ‘(Ex) (x is a dog)’. It follows that some of the objects a theory assumes (in its ontology) may not be among those to which the theory is ontologically committed. A theory is ontologically committed to a given object only if that object is common to all of the ontologies fulfilling the theory. And the theory is ontologically committed to objects of a given class provided that class is not empty according to each of the ontologies fulfilling the theory. See also QUANTIFICATION, THEORY OF DESCRIPTION. R.F.G.