Although explicit discussion of ontological commitment postdates Frege's writings, an account of ontological commitment can be garnered from his defense of arithmetical platonism in The Foundations of Arithmetic (Frege 1884). That account is embedded in the following argument for the claim that numbers exist as “self-subsistent objects” (see Wright (1990), who calls the argument “deceptively simple”). The semantic role of singular terms (Frege's proper names) is to refer to objects. When singular terms occur in true sentences of an appropriate kind­—extensional sentences generally, and identity sentences in particular—they cannot help but to refer; for that is their contribution to the truth of the sentence. Appropriate sentences involving numerical singular terms are sometimes true; witness ‘the number of planets is nine’ and ‘1+1=2’. Therefore, these singular terms refer to objects. Moreover, the argument may be continued, since some of these singular terms—those of the form ‘the number of Fs’—figure in the identity condition definitive of the concept of number (Hume's Principle, see below), the objects referred to are properly called “numbers”. Therefore, numbers exist.