This argument might be challenged in two different ways.  First, one might deny that sentences containing numerical singular terms are (literally) true; one might, for example, endorse a fictionalist account of mathematics (Field 1980). For our purposes, we can sidestep this objection and focus on the other premise of the argument, that extensional sentences containing numerical singular terms, if true, refer to numbers—which is to say, in other words, that such sentences, whether true or not, are ontologically committed to numbers. If this is to serve as a criterion of ontological commitment, there must be some purely syntactic way of determining which expressions are genuine singular terms; if genuine singular terms are characterized semantically—for example, as expressions that refer to objects when they occur in true extensional sentences—then the account of ontological commitment goes in a circle. Thus, Fregean accounts are committed to a syntactic priority thesis: ontology depends on ontological commitment plus truth; and ontological commitment in turn depends on syntax (see Wright 1983 for extended discussion).