foundations of mathematics The search for a distinguished model

A Platonist might still ask whether, among all the models of the language of mathematics, there is a distinguished model, which may be considered to be the world of mathematics. Take as the language ℒ₀ pure intuitionistic type theory (see above Intuitionistic type theories). It turns out, somewhat surprisingly, that the topos generated by ℒ₀ is a local topos; hence, the unique interpretation of ℒ₀ in the topos generated by it may serve as a distinguished model.

This so-called free topos has been constructed linguistically to satisfy any formalist, but it should also satisfy a moderate Platonist, one who is willing to abandon the principle of the excluded third, inasmuch as the free topos is the initial object in the category of all topoi. Hence, the free topos may be viewed, in the words of Leibniz, as the best of all possible worlds. More modestly speaking, the free topos is to an arbitrary topos like the ring of integers is to an arbitrary ring.

The language ℒ₀ should also satisfy any constructivist: if an existential statement ∃_{x ∊ A}ϕ(x) can be proved in ℒ₀, then ϕ(a) can be proved for some term a of type A; moreover, if p ∨ q can be proved, then either p can be proved or q can be proved.

The above argument would seem to make a strong case for the acceptance of pure intuitionistic type theory as the language of elementary mathematics—that is, of arithmetic and analysis—and hence for the acceptance of the free topos as the world of mathematics. Nonetheless, most practicing mathematicians prefer to stick to classical mathematics. In fact, classical arguments seem to be necessary for metamathematics—for example, in the usual proof of Gödel’s completeness theorem—even for intuitionistic type theory.

In this connection, one celebrated consequence of Gödel’s incompleteness theorem may be recalled, to wit: the consistency of ℒ cannot be proved (via arithmetization) within ℒ. This is not to say that it cannot be proved in a stronger metalanguage. Indeed, to exhibit a single model of ℒ would constitute such a proof.

It is more difficult to make a case for the classical world of mathematics, although this is what most mathematicians believe in. This ought to be a distinguished model of pure classical type theory ℒ₁. Unfortunately, Gödel’s argument shows that the interpretation of ℒ₁ in the topos generated by it is not a model in this sense.