I will give precise versions of the foregoing verbal definitions, by formalizing them within a formal metalanguage.
In order to avoid the necessity of formulating a meta-language, I will make use of VI 112, (i.e., Gödel's The consistency of the continuum hypothesis). Actually, the system there developed is unnecessarily strong for the limited purpose of this illustration. It will suffice to choose for the system of VI 112 without axioms C4 and E (axioms of replacement and choice); this system will be the logical basis of , and to this I will add some set-constants and some meaning postulates. The logical basis is roughly as strong as Zermelo's system with an axiom of infinity.
Within I will set up analogues of the previous definitions, applicable to a wide variety of object-languages. For the sake of concreteness it will be convenient to think of as having Zermelo set theory, without the axiom of infinity, as its logical basis – but it must be kept in mind that this is not an essential feature of the forthcoming definitions.
Our is certainly strong enough to have all primitive recursive functions calculable within it. This supplies us with the needed tool for talking about , within , by introducing a number of primitive recursive functions, properties, and relations describing the structure of . The exact definitions of these will depend on the exact form of , but for the forthcoming definitions it will suffice to know that these functions, properties, and relations are representable in for the given . This makes the resulting definitions most useful, since they can be applied to any of the type described.