This argument becomes more problematic if it is applied to class models of ZFC, such as the von Neumann universe . The argument that applies to set models cannot be directly generalized to class models in ZFC because the property "the real number "x" is definable over the class model "N"" cannot be expressed as a formula of ZFC. Similarly, the question of whether the von Neumann universe contains real numbers that it cannot define cannot be expressed as a sentence in the language of ZFC. Moreover, there are countable models of ZFC in which all real numbers, all sets of real numbers, functions on the reals, etc. are definable .