Topoi are closely related to intuitionistic type theories. Such a theory is equipped with certain types, terms, and theorems.

Among the types there should be a type Ω for truth-values, a type N for natural numbers, and, for each type A, a type ℘(A) for all sets of entities of type A.

Among the terms there should be in particular:

1. The formulas a = a′ and a ∊ α of type Ω, if a and a′ are of type A and α is of type ℘(A)
2. The numerals 0 and Sn of type N, if the numeral n is of type N
3. The comprehension term {x ∊ A|ϕ(x)} of type ℘(A), if ϕ(x) is a formula of type Ω containing a free variable x of type A

The set of theorems should contain certain obvious axioms and be closed under certain obvious rules of inference, neither of which will be spelled out here.

At this point the reader may wonder what happened to the usual logical symbols. These can all be defined—for example, universal quantification ∀_{x ∊ A}ϕ(x) as {x ∊ A|ϕ(x)} = {x ∊ A|x = x} and disjunction p ∨ q as ∀_{t ∊ Ω}((p ⊃ t) ⊃ ((q ⊃ t) ⊃ t)). For a formal definition of implication, see formal logic.

In general, the set of theorems will not be recursively enumerable. However, this will be the case for pure intuitionistic type theory ℒ₀, in which types, terms, and theorems are all defined inductively. In ℒ₀ there are no types, terms, or theorems other than those that follow from the definition of type theory. ℒ₀ is adequate for the constructive part of the usual elementary mathematics—arithmetic and analysis—but not for metamathematics, if this is to include a proof of Gödel’s completeness theorem, and not for category theory, if this is to include the Yoneda embedding of a small category into a set-valued functor category.

Internal language

It turns out that each topos 𝒯 has an internal language L(𝒯), an intuitionistic type theory whose types are objects and whose terms are arrows of 𝒯. Conversely, every type theory ℒ generates a topos T(ℒ), by the device of turning (equivalence classes of) terms into objects, which may be thought of as denoting sets.

Nominalists may be pleased to note that every topos 𝒯 is equivalent (in the sense of category theory) to the topos generated by a language—namely, the internal language of 𝒯. On the other hand, Platonists may observe that every type theory ℒ has a conservative extension to the internal language of a topos—namely, the topos generated by ℒ, assuming that this topos exists in the real (ideal) world. Here, the phrase “conservative extension” means that ℒ can be extended to LT(ℒ) without creating new theorems. The types of LT(ℒ) are names of sets in ℒ and the terms of LT(ℒ) may be identified with names of sets in ℒ for which it can be proved that they have exactly one element. This last observation provides a categorical version of Russell’s theory of descriptions: if one can prove the unique existence of an x of type A in ℒ such that ϕ(x), then this unique x has a name in LT(ℒ).

The interpretation of a type theory ℒ in a topos 𝒯 means an arrow ℒ → L(𝒯) in the category of type theories or, equivalently, an arrow T(ℒ) → 𝒯 in the category of topoi. Indeed, T and L constitute a pair of adjoint functors.

Gödel and category theory

It is now possible to reexamine Gödel’s theorems from a categorical point of view. In a sense, every interpretation of ℒ in a topos 𝒯 may be considered as a model of ℒ, but this notion of model is too general, for example, when compared with the models of classical type theories studied by Henkin. Therefore, it is preferable to restrict 𝒯 to being a special kind of topos called local. Given an arrow p into Ω in 𝒯, then, p is true in 𝒯 if p coincides with the arrow true in 𝒯, or, equivalently, if p is a theorem in the internal language of 𝒯. 𝒯 is called a local topos provided that (1) 0 = 1 is not true in 𝒯, (2) p ∨ q is true in 𝒯 only if p is true in 𝒯 or q is true in 𝒯, and (3) ∃_{x ∊ A}ϕ(x) is true in 𝒯 only if ϕ(a) is true in 𝒯 for some arrow a ∶ 1 → A in 𝒯. Here the statement 0 = 1 in provision 1 can be replaced by any other contradiction—e.g., by ∀_{t ∊ Ω}t, which says that every proposition is true.

A model of ℒ is an interpretation of ℒ in a local topos 𝒯. Gödel’s completeness theorem, generalized to intuitionistic type theory, may now be stated as follows: A closed formula of ℒ is a theorem if and only if it is true in every model of ℒ.

Gödel’s incompleteness theorem, generalized likewise, says that, in the usual language of arithmetic, it is not enough to look only at ω-complete models: Assuming that ℒ is consistent and that the theorems of ℒ are recursively enumerable, with the help of a decidable notion of proof, there is a closed formula g in ℒ, which is true in every ω-complete model, yet g is not a theorem in ℒ.

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). 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.