Intuitionistic Logicbuilds.openlogicproject.org/content/intuitionistic-logic/intuitionistic-logic.pdfnumber that is equal to 2 if the Riemann hypothesis is true, and 3 otherwise. Angrily,

Part I

Intuitionistic Logic

1

This is a brief introduction to intuitionistic logic produced by ZesenQian and revised by RZ. It is not yet well integrated with the rest of thetext and needs examples and motivations.

2 intuitionistic-logic rev: 534a139 (2020-06-25) by OLP / CC–BY

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Chapter 1

Introduction

1.1 Constructive Reasoning

In constrast to extensions of classical logic by modal operators or second-orderquantifiers, intuitionistic logic is “non-classical” in that it restricts classicallogic. Classical logic is non-constructive in various ways. Intuitionistic logicis intended to capture a more “constructive” kind of reasoning characteristicof a kind of constructive mathematics. The following examples may serve toillustrate some of the underlying motivations.

Suppose someone claimed that they had determined a natural number nwith the property that if n is even, the Riemann hypothesis is true, and if nis odd, the Riemann hypothesis is false. Great news! Whether the Riemannhypothesis is true or not is one of the big open questions of mathematics, andthey seem to have reduced the problem to one of calculation, that is, to thedetermination of whether a specific number is even or not.

What is the magic value of n? They describe it as follows: n is the naturalnumber that is equal to 2 if the Riemann hypothesis is true, and 3 otherwise.

Angrily, you demand your money back. From a classical point of view, thedescription above does in fact determine a unique value of n; but what youreally want is a value of n that is given explicitly.

To take another, perhaps less contrived example, consider the followingquestion. We know that it is possible to raise an irrational number to a rational

power, and get a rational result. For example,√

22

= 2. What is less clearis whether or not it is possible to raise an irrational number to an irrationalpower, and get a rational result. The following theorem answers this in theaffirmative:

Theorem 1.1. There are irrational numbers a and b such that ab is rational.

Proof. Consider√

2√2. If this is rational, we are done: we can let a = b =

√2.

Otherwise, it is irrational. Then we have

(√

2

√2)√2 =√

2

√2·√2

=√

22

= 2,

3

which is rational. So, in this case, let a be√

2√2, and let b be

√2.

Does this constitute a valid proof? Most mathematicians feel that it does.But again, there is something a little bit unsatisfying here: we have provedthe existence of a pair of real numbers with a certain property, without beingable to say which pair of numbers it is. It is possible to prove the same result,but in such a way that the pair a, b is given in the proof: take a =

√3 and

b = log3 4. Then

ab =√

3log3 4

= 31/2·log3 4 = (3log3 4)1/2 = 41/2 = 2,

since 3log3 x = x.Intuitionistic logic is designed to capture a kind of reasoning where moves

like the one in the first proof are disallowed. Proving the existence of an xsatisfying ϕ(x) means that you have to give a specific x, and a proof that itsatisfies ϕ, like in the second proof. Proving that ϕ or ψ holds requires thatyou can prove one or the other.

Formally speaking, intuitionistic logic is what you get if you restrict a proofsystem for classical logic in a certain way. From the mathematical point ofview, these are just formal deductive systems, but, as already noted, they areintended to capture a kind of mathematical reasoning. One can take this tobe the kind of reasoning that is justified on a certain philosophical view ofmathematics (such as Brouwer’s intuitionism); one can take it to be a kindof mathematical reasoning which is more “concrete” and satisfying (along thelines of Bishop’s constructivism); and one can argue about whether or notthe formal description captures the informal motivation. But whatever philo-sophical positions we may hold, we can study intuitionistic logic as a formallypresented logic; and for whatever reasons, many mathematical logicians find itinteresting to do so.

1.2 Syntax of Intuitionistic Logic

int:int:syn:sec

The syntax of intuitionistic logic is the same as that for propositional logic. Inclassical propositional logic it is possible to define connectives by others, e.g.,one can define ϕ→ψ by ¬ϕ∨ψ, or ϕ∨ψ by ¬(¬ϕ∧¬ψ). Thus, presentationsof classical logic often introduce some connectives as abbreviations for thesedefinitions. This is not so in intuitionistic logic, with two exceptions: ¬ϕ canbe—and often is—defined as an abbreviation for ϕ→⊥. Then, of course, ⊥must not itself be defined! Also, ϕ↔ ψ can be defined, as in classical logic, as(ϕ→ ψ) ∧ (ψ→ ϕ).

Formulas of propositional intuitionistic logic are built up from propositionalvariables and the propositional constant ⊥ using logical connectives. We have:

1. A denumerable set At0 of propositional variables p0, p1, . . .

2. The propositional constant for falsity ⊥.






3. The logical connectives: ∧ (conjunction), ∨ (disjunction), → (condi-tional)

4. Punctuation marks: (, ), and the comma.

Definition 1.2 (Formula).int:int:syn:

defn:formulas

The set Frm(L0) of formulas of propositionalintuitionistic logic is defined inductively as follows:

1. ⊥ is an atomic formula.

2. Every propositional variable pi is an atomic formula.

3. If ϕ and ψ are formulas, then (ϕ ∧ ψ) is a formula.

4. If ϕ and ψ are formulas, then (ϕ ∨ ψ) is a formula.

5. If ϕ and ψ are formulas, then (ϕ→ ψ) is a formula.

6. Nothing else is a formula.

In addition to the primitive connectives introduced above, we also use thefollowing defined symbols: ¬ (negation) and ↔ (biconditional). Formulas con-structed using the defined operators are to be understood as follows:

1. ¬ϕ abbreviates ϕ→⊥.

2. ϕ↔ ψ abbreviates (ϕ→ ψ) ∧ (ψ→ ϕ).

Although ¬ is officially treated as an abbreviation, we will sometimes giveexplicit rules and clauses in definitions for ¬ as if it were primitive. This ismostly so we can state practice problems.

1.3 The Brouwer-Heyting-Kolmogorov Interpretation

int:int:bhk:sec

Proofs of validity of intuitionistic propositions using the BHK interpre-tation are confusing; they have to be explained better.

There is an informal constructive interpretation of the intuitionist connectives,usually known as the Brouwer-Heyting-Kolmogorov interpretation. It uses thenotion of a “construction,” which you may think of as a constructive proof. (Wedon’t use “proof” in the BHK interpretation so as not to get confused with thenotion of a derivation in a formal proof system.) Based on this intuitive notion,the BHK interpretation explains the meanings of the intuitionistic connectives.

1. We assume that we know what constitutes a construction of an atomicstatement.

intuitionistic-logic rev: 534a139 (2020-06-25) by OLP / CC–BY 5





2. A construction of ϕ1 ∧ ϕ2 is a pair 〈M1,M2〉 where M1 is a constructionof ϕ1 and M2 is a construction of A2.

3. A construction of ϕ1 ∨ ϕ2 is a pair 〈s,M〉 where s is 1 and M is aconstruction of ϕ1, or s is 2 and M is a construction of ϕ2.

4. A construction of ϕ→ ψ is a function that converts a construction of ϕinto a construction of ψ.

5. There is no construction for ⊥ (absurdity).

6. ¬ϕ is defined as synonym for ϕ→⊥. That is, a construction of ¬ϕ is afunction converting a construction of ϕ into a construction of ⊥.

Example 1.3. Take ¬⊥ for example. A construction of it is a function which,given any construction of ⊥ as input, provides a construction of ⊥ as output.Obviously, the identity function Id is such a construction: given a construc-tion M of ⊥, Id(M) = M yields a construction of ⊥.

Generally speaking, ¬ϕ means “A construction of ϕ is impossible”.

Example 1.4. Let us prove ϕ→¬¬ϕ for any proposition ϕ, which is ϕ→((ϕ→⊥)→⊥). The construction should be a function f that, given a construction Mof ϕ, returns a construction f(M) of (ϕ→⊥)→⊥. Here is how f constructs theconstruction of (ϕ→⊥)→⊥: We have to define a function g which, when givena construction h of ϕ→⊥ as input, outputs a construction of ⊥. We can defineg as follows: apply the input h to the construction M of ϕ (that we receivedearlier). Since the output h(M) of h is a construction of ⊥, f(M)(h) = h(M)is a construction of ⊥ if M is a construction of ϕ.

Example 1.5. Let us give a construction for ¬(ϕ∧¬ϕ), i.e., (ϕ∧(ϕ→⊥))→⊥.This is a function f which, given as input a construction M of ϕ ∧ (ϕ→⊥),yields a construction of ⊥. A construction of a conjunction ψ1 ∧ ψ2 is a pair〈N1, N2〉 where N1 is a construction of ψ1 and N2 is a construction of ψ2. Wecan define functions p1 and p2 which recover from a construction of ψ1 ∧ ψ2

the constructions of ψ1 and ψ2, respectively:

p1(〈N1, N2〉) = N1

p2(〈N1, N2〉) = N2

Here is what f does: First it applies p1 to its input M . That yields a construc-tion of ϕ. Then it applies p2 to M , yielding a construction of ϕ→⊥. Such aconstruction, in turn, is a function p2(M) which, if given as input a constructionof ϕ, yields a construction of ⊥. In other words, if we apply p2(M) to p1(M),we get a construction of ⊥. Thus, we can define f(M) = p2(M)(p1(M)).

Example 1.6. Let us give a construction of ((ϕ ∧ ψ)→ χ)→ (ϕ→ (ψ→ χ)),i.e., a function f which turns a construction g of (ϕ∧ψ)→χ into a construction






of (ϕ→ (ψ→ χ)). The construction g is itself a function (from constructionsof ϕ ∧ ψ to constructions of C). And the output f(g) is a function hg fromconstructions of ϕ to functions from constructions of ψ to constructions of χ.

Ok, this is confusing. We have to construct a certain function hg, whichwill be the output of f for input g. The input of hg is a construction Mof ϕ. The output of hg(M) should be a function kM from constructions Nof ψ to constructions of χ. Let kg,M (N) = g(〈M,N〉). Remember that 〈M,N〉is a construction of ϕ ∧ ψ. So kg,M is a construction of ψ → χ: it mapsconstructions N of ψ to constructions of χ. Now let hg(M) = kg,M . That’s afunction that maps constructions M of ϕ to constructions kg,M of ψ→χ. Nowlet f(g) = hg. That’s a function that maps constructions g of (ϕ ∧ ψ)→ χ toconstructions of ϕ→ (ψ→ χ). Whew!

The statement ϕ∨¬ϕ is called the Law of Excluded Middle. We can proveit for some specific ϕ (e.g., ⊥ ∨ ¬⊥), but not in general. This is because theintuitionistic disjunction requires a construction of one of the disjuncts, butthere are statements which currently can neither be proved nor refuted (say,Goldbach’s conjecture). However, you can’t refute the law of excluded middleeither: that is, ¬¬(ϕ ∨ ¬ϕ) holds.

Example 1.7. To prove ¬¬(ϕ ∨ ¬ϕ), we need a function f that transforms aconstruction of ¬(ϕ∨¬ϕ), i.e., of (ϕ∨ (ϕ→⊥))→⊥, into a construction of ⊥.In other words, we need a function f such that f(g) is a construction of ⊥ if gis a construction of ¬(ϕ ∨ ¬ϕ).

Suppose g is a construction of ¬(ϕ∨¬ϕ), i.e., a function that transforms aconstruction of ϕ∨¬ϕ into a construction of ⊥. A construction of ϕ∨¬ϕ is apair 〈s,M〉 where either s = 1 and M is a construction of ϕ, or s = 2 and M isa construction of ¬ϕ. Let h1 be the function mapping a construction M1 of ϕto a construction of ϕ∨¬ϕ: it maps M1 to 〈1,M2〉. And let h2 be the functionmapping a construction M2 of ¬ϕ to a construction of ϕ ∨ ¬ϕ: it maps M2 to〈2,M2〉.

Let k be g ◦h1: it is a function which, if given a construction of ϕ, returns aconstruction of ⊥, i.e., it is a construction of ϕ→⊥ or ¬ϕ. Now let l be g ◦h2.It is a function which, given a construction of ¬ϕ, provides a construction of ⊥.Since k is a construction of ¬ϕ, l(k) is a construction of ⊥.

Together, what we’ve done is describe how we can turn a construction g of¬(ϕ∨¬ϕ) into a construction of ⊥, i.e., the function f mapping a construction gof ¬(ϕ ∨ ¬ϕ) to the construction l(k) of ⊥ is a construction of ¬¬(ϕ ∨ ¬ϕ).

As you can see, using the BHK interpretation to show the intuitionisticvalidity of formulas quickly becomes cumbersome and confusing. Luckily, thereare better derivation systems for intuitionistic logic, and more precise semanticinterpretations.

1.4 Natural Deduction

int:int:ntd:sec






Natural deduction without the ⊥C rules is a standard derivation system forintuitionistic logic. We repeat the rules here and indicate the motivation usingthe BHK interpretation. In each case, we can think of a rule which allows usto conclude that if the premises have constructions, so does the conclusion.

Since natural deduction derivations have undischarged assumptions, weshould consider such a derivation, say, of ϕ from undischarged assumptions Γ ,as a function that turns constructions of all ψ ∈ Γ into a construction of ϕ. Ifthere is a derivation of ϕ from no undischarged assumptions, then there is aconstruction of ϕ in the sense of the BHK interpretation. For the purpose ofthe discussion, however, we’ll suppress the Γ when not needed.

An assumption ϕ by itself is a derivation of ϕ from the undischarged as-sumption ϕ. This agrees with the BHK-interpretation: the identity functionon constructions turns any construction of ϕ into a construction of ϕ.

Conjunction

ϕ ψ∧Intro

ϕ ∧ ψ

ϕ ∧ ψ∧Elimϕ

ϕ ∧ ψ∧Elim

ψ

Suppose we have constructions N1, N2 of ϕ1 and ϕ2, respectively. Then wealso have a construction ϕ1 ∧ ϕ2, namely the pair 〈N1, N2〉.

A construction of ϕ1 ∧ϕ1 on the BHK interpretation is a pair 〈N1, N2〉. Soassume we have such a pair. Then we also have a construction of each conjunct:N1 is a construction of ϕ1 and N2 is a construction of ϕ2.

Conditional

[ϕ]u

ψu →Introϕ→ ψ

ϕ→ ψ ϕ→Elim

ψ

If we have a derivation of ψ from undischarged assumption ϕ, then there is afunction f that turns constructions of ϕ into constructions of ψ. That samefunction is a construction of ϕ→ψ. So, if the premise of→Intro has a construc-tion conditional on a construction of ϕ, the conclusion ϕ→ψ has a construction.

On the other hand, suppose there are constructions N of ϕ and f of ϕ→ψ. A construction of ϕ→ ψ is a function that turns constructions of ϕ into






constructions of ψ. So, f(N) is a construction of ψ, i.e., the conclusion of→Elim has a construction.

Disjunction

ϕ∨Intro

ϕ ∨ ψ

ψ∨Intro

ϕ ∨ ψ ϕ ∨ ψ

[ϕ]n

χ

[ψ]n

χn ∨Elimχ

If we have a construction Ni of ϕi we can turn it into a construction 〈i,Ni〉of ϕ1 ∨ϕ2. On the other hand, suppose we have a construction of ϕ1 ∨ϕ2, i.e.,a pair 〈i,Ni〉 where Ni is a construction of ϕi, and also functions f1, f2, whichturn constructions of ϕ1, ϕ2, respectively, into constructions of χ. Then fi(Ni)is a construction of χ, the conclusion of ∨Elim.

Absurdity

⊥ ⊥Iϕ

If we have a derivation of ⊥ from undischarged assumptions ψ1, . . . , ψn, thenthere is a function f(M1, . . . ,Mn) that turns constructions of ψ1, . . . , ψn intoa construction of ⊥. Since ⊥ has no construction, there cannot be any con-structions of all of ψ1, . . . , ψn either. Hence, f also has the property that ifM1, . . . , Mn are constructions of ψ1, . . . , ψn, respectively, then f(M1, . . . ,Mn)is a construction of ϕ.

Rules for ¬

Since ¬ϕ is defined as ϕ→⊥, we strictly speaking do not need rules for ¬. Butif we did, this is what they’d look like:

[ϕ]n

⊥n ¬Intro¬ϕ

¬ϕ ϕ¬Elim⊥






Examples of Derivations

1. ` ϕ→ (¬ϕ→⊥), i.e., ` ϕ→ ((ϕ→⊥)→⊥)

[ϕ]2 [ϕ→⊥]1→Elim⊥

1 →Intro(ϕ→⊥)→⊥

2 →Introϕ→ (ϕ→⊥)→⊥

2. ` ((ϕ ∧ ψ)→ χ)→ (ϕ→ (ψ→ χ))

[(ϕ ∧ ψ)→ χ]3[ϕ]2 [ψ]1

∧Introϕ ∧ ψ

→Elimχ1 →Introψ→ χ

2 →Introϕ→ (ψ→ χ)

3 →Intro((ϕ ∧ ψ)→ χ)→ (ϕ→ (ψ→ χ))

3. ` ¬(ϕ ∧ ¬ϕ), i.e., ` (ϕ ∧ (ϕ→⊥))→⊥

[ϕ ∧ (ϕ→⊥)]1∧Elim

ϕ→⊥[ϕ ∧ (ϕ→⊥)]1

∧Elimϕ→Elim⊥

1 →Intro(ϕ ∧ (ϕ→⊥))→⊥

4. ` ¬¬(ϕ ∨ ¬ϕ), i.e., ` ((ϕ ∨ (ϕ→⊥))→⊥)→⊥

[(ϕ ∨ (ϕ→⊥))→⊥]2

[(ϕ ∨ (ϕ→⊥))→⊥]2[ϕ]1

∨Introϕ ∨ (ϕ→⊥)

→Elim⊥1 →Introϕ→⊥

∨Introϕ ∨ (ϕ→⊥)

→Elim⊥2 →Intro

((ϕ ∨ (ϕ→⊥))→⊥)→⊥

Proposition 1.8. If Γ ` ϕ in intuitionistic logic, Γ ` ϕ in classical logic. Inparticular, if ϕ is an intuitionistic theorem, it is also a classical theorem.

Proof. Every natural deduction rule is also a rule in classical natural deduction,so every derivation in intuitionistic logic is also a derivation in classical logic.

Problem 1.1. Give derivations in intutionistic logic of the following.

1. (¬ϕ ∨ ψ)→ (ϕ→ ψ)

2. ¬¬¬ϕ→¬ϕ

3. ¬¬(ϕ ∧ ψ)↔ (¬¬ϕ ∧ ¬¬ψ)






1.5 Axiomatic Derivations

int:int:axd:sec

Axiomatic derivations for intuitionistic propositional logic are the conceptu-ally simplest, and historically first, derivation systems. They work just as inclassical propositional logic.

Definition 1.9 (Derivability). If Γ is a set of formulas of L then a derivationfrom Γ is a finite sequence ϕ1, . . . , ϕn of formulas where for each i ≤ n one ofthe following holds:

1. ϕi ∈ Γ ; or

2. ϕi is an axiom; or

3. ϕi follows from some ϕj and ϕk with j < i and k < i by modus ponens,i.e., ϕk ≡ ϕj → ϕi.

Definition 1.10 (Axioms). The set of Ax0 of axioms for the intuitionisticpropositional logic are all formulas of the following forms:

(ϕ ∧ ψ)→ ϕint:int:axd:

ax:land1

(1.1)

(ϕ ∧ ψ)→ ψint:int:axd:

ax:land2

(1.2)

ϕ→ (ψ→ (ϕ ∧ ψ))int:int:axd:

ax:land3

(1.3)

ϕ→ (ϕ ∨ ψ)int:int:axd:

ax:lor1

(1.4)

ϕ→ (ψ ∨ ϕ)int:int:axd:

ax:lor2

(1.5)

(ϕ→ χ)→ ((ψ→ χ)→ ((ϕ ∨ ψ)→ χ))int:int:axd:

ax:lor3

(1.6)

ϕ→ (ψ→ ϕ)int:int:axd:

ax:lif1

(1.7)

(ϕ→ (ψ→ χ))→ ((ϕ→ ψ)→ (ϕ→ χ))int:int:axd:

ax:lif2

(1.8)

⊥→ ϕint:int:axd:

ax:lfalse1

(1.9)

Definition 1.11 (Derivability). A formula ϕ is derivable from Γ , writtenΓ ` ϕ, if there is a derivation from Γ ending in ϕ.

Definition 1.12 (Theorems). A formula ϕ is a theorem if there is a deriva-tion of ϕ from the empty set. We write ` ϕ if ϕ is a theorem and 0 ϕ if it isnot.

Proposition 1.13. If Γ ` ϕ in intuitionistic logic, Γ ` ϕ in classical logic.In particular, if ϕ is an intuitionistic theorem, it is also a classical theorem.

Proof. Every intuitionistic axiom is also a classical axiom, so every derivationin intuitionistic logic is also a derivation in classical logic.






Chapter 2

Semantics

This chapter collects definitions for semantics for intuitionistic logic.So far only Kripke and topological semantics are covered. There are noexamples yet, either of how models make formulas true or of proofs thatformulas are valid.

2.1 Introduction

int:sem:int:sec

No logic is satisfactorily described without a semantics, and intuitionistic logicis no exception. Whereas for classical logic, the semantics based on valuations iscanonical, there are several competing semantics for intuitionistic logic. None ofthem are completely satisfactory in the sense that they give an intuitionisticallyacceptable account of the meanings of the connectives.

The semantics based on relational models, similar to the semantics formodal logics, is perhaps the most popular one. In this semantics, proposi-tional variables are assigned to worlds, and these worlds are related by anaccessibility relation. That relation is always a partial order, i.e., it is reflexive,antisymmetric, and transitive.

Intuitively, you might think of these worlds as states of knowledge or “evi-dentiary situations.” A state w′ is accessible from w iff, for all we know, w′ isa possible (future) state of knowledge, i.e., one that is compatible with what’sknown at w. Once a proposition is known, it can’t become un-known, i.e.,whenever ϕ is known at w and Rww′, ϕ is known at w′ as well. So “knowl-edge” is monotonic with respect to the accessibility relation.

If we define “ϕ is known” as in epistemic logic as “true in all epistemicalternatives,” then ϕ ∧ ψ is known at w if in all epistemic alternatives, both ϕand ψ are known. But since knowledge is monotonic and R is reflexive, thatmeans that ϕ ∧ ψ is known at w iff ϕ and ψ are known at w. For the samereason, ϕ ∨ ψ is known at w iff at least one of them is known. So for ∧ and ∨,the truth conditions of the connectives coincide with those in classical logic.

12

The truth conditions for the conditional, however, differ from classical logic.ϕ→ψ is known at w iff at no w′ with Rww′, ϕ is known without ψ also beingknown. This is not the same as the condition that ϕ is unknown or ψ is knownat w. For if we know neither ϕ nor ψ at w, there might be a future epistemicstate w′ with Rww′ such that at w′, ϕ is known without also coming to know ψ.

We know ¬ϕ only if there is no possible future epistemic state in whichwe know ϕ. Here the idea is that if ϕ were knowable, then in some possiblefuture epistemic state ϕ becomes known. Since we can’t know ⊥, in that futureepistemic state, we would know ϕ but not know ⊥.

On this interpretation the principle of excluded middle fails. For there aresome ϕ which we don’t yet know, but which we might come to know. For suchan ϕ, both ϕ and ¬ϕ are unknown, so ϕ ∨ ¬ϕ is not known. But we do know,e.g., that ¬(ϕ ∧ ¬ϕ). For no future state in which we know both ϕ and ¬ϕ ispossible, and we know this independently of whether or not we know ϕ or ¬ϕ.

Relational models are not the only available semantics for intuitionisticlogic. The topological semantics is another: here propositions are interpretedas open sets in a topological space, and the connectives are interpreted asoperations on these sets (e.g., ∧ corresponds to intersection).

2.2 Relational models

int:sem:rel:sec

In order to give a precise semantics for intuitionistic propositional logic, wehave to give a definition of what counts as a model relative to which we canevaluate formulas. On the basis of such a definition it is then also possible todefine semantics notions such as validity and entailment. One such semanticsis given by relational models.

Definition 2.1. A relational model for intuitionistic propositional logic is atriple M = 〈W,R, V 〉, where

1. W is a non-empty set,

2. R is a partial order (i.e., a reflexive, antisymmetric, and transitive binaryrelation) on W , and

3. V is a function assigning to each propositional variable p a subset of W ,such that

4. V is monotone with respect to R, i.e., if w ∈ V (p) and Rww′, thenw′ ∈ V (p).

Definition 2.2.int:sem:rel:

defn:true-at-w

We define the notion of ϕ being true at w in M, M, w ϕ,inductively as follows:

1. ϕ ≡ p: M, w ϕ iff w ∈ V (p).

2. ϕ ≡ ⊥: not M, w ϕ.

3. ϕ ≡ ¬ψ: M, w ϕ iff for no w′ such that Rww′, M, w′ ψ.






4. ϕ ≡ ψ ∧ χ: M, w ϕ iff M, w ψ and M, w χ.

5. ϕ ≡ ψ ∨ χ: M, w ϕ iff M, w ψ or M, w χ (or both).

6. ϕ ≡ ψ→ χ: M, w ϕ iff for every w′ such that Rww′, not M, w′ ψor M, w′ χ (or both).

We write M, w 1 ϕ if not M, w ϕ. If Γ is a set of formulas, M, w Γ meansM, w ψ for all ψ ∈ Γ .

Problem 2.1. Show that according to Definition 2.2, M, w ¬ϕ iff M, w ϕ→⊥.

Proposition 2.3. int:sem:rel:

prop:true-monotonic

Truth at worlds is monotonic with respect to R, i.e., ifM, w ϕ and Rww′, then M, w′ ϕ.

Proof. Exercise.

Problem 2.2. Prove Proposition 2.3.

2.3 Semantic Notions

int:sem:sem:secDefinition 2.4. We say ϕ is true in the model M = 〈W,R, V 〉, M ϕ, iff

M, w ϕ for all w ∈ W . ϕ is valid, � ϕ, iff it is true in all models. We saya set of formulas Γ entails ϕ, Γ � ϕ, iff for every model M and every w suchthat M, w Γ , M, w ϕ.

Proposition 2.5. int:sem:sem:

prop:sat-entails

1. int:sem:sem:

prop:sat-entails1

If M, w Γ and Γ � ϕ, then M, w ϕ.

2. int:sem:sem:

prop:sat-entails2

If M Γ and Γ � ϕ, then M ϕ.

Proof. 1. Suppose M Γ . Since Γ � ϕ, we know that if M, w Γ , thenM, w ϕ. Since M, u Γ for all every u ∈ W , M, w Γ . HenceM, w ϕ.

2. Follows immediately from (1).

2.4 Topological Semantics

int:sem:top:sec

Another way to provide a semantics for intuitionistic logic is using the mathe-matical concept of a topology.

Definition 2.6. Let X be a set. A topology on X is a set O ⊆ ℘(X) thatsatisfies the properties below. The elements of O are called the open sets ofthe topology. The set X together with O is called a topological space.






1. The empty set and the entire space open: ∅, X ∈ O.

2. Open sets are closed under finite intersections: if U , V ∈ O then U ∩V ∈O

3. Open sets are closed under arbitrary unions: if Ui ∈ O for all i ∈ I, then⋃{Ui : i ∈ I} ∈ O.

We may write X for a topology if the collection of open sets can be inferredfrom the context; note that, still, only after X is endowed with open sets canit be called a topology.

Definition 2.7. A topological model of intuitionistic propositional logic is atriple X = 〈X,O, V 〉 where O is a topology on X and V is a function assigningan open set in O to each propositional variable.

Given a topological model X, we can define [[ϕ]]X inductively as follows:

1. V (⊥) = ∅

2. [[p]]X = V (p)

3. [[ϕ ∧ ψ]]X = [[ϕ]]X ∩ [[ψ]]X

4. [[ϕ ∨ ψ]]X = [[ϕ]]X ∪ [[ψ]]X

5. [[ϕ→ ψ]]X = Int((X \ [[ϕ]]X) ∪ [[ψ]]X)

Here, Int(V ) is the function that maps a set V ⊆ X to its interior, that is, theunion of all open sets it contains. In other words,

Int(V ) =⋃{U : U ⊆ V and U ∈ O}.

Note that the interior of any set is always open, since it is a union of opensets. Thus, [[ϕ]]X is always an open set.

Although topological semantics is highly abstract, there are ways to thinkabout it that might motivate it. Suppose that the elements, or “points,” of Xare points at which statements can be evaluated. The set of all points where ϕis true is the proposition expressed by ϕ. Not every set of points is a potentialproposition; only the elements of O are. ϕ � ψ iff ψ is true at every point atwhich ϕ is true, i.e., [[ϕ]]X ⊆ [[ψ]]X, for all X. The absurd statement ⊥ is nevertrue, so [[⊥]]X = ∅. How must the propositions expressed by ψ ∧ χ, ψ ∨ χ, andψ→ χ be related to those expressed by ψ and χ for the intuitionistically validlaws to hold, i.e., so that ϕ ` ψ iff [[ϕ]]X ⊂ [[ψ]]X. ⊥ ` ϕ for any ϕ, and only∅ ⊆ U for all U . Since ψ∧χ ` ψ, [[ψ∧χ]]X ⊆ [[ψ]]X, and similarly [[ψ∧χ]]X ⊆ [[χ]]X.The largest set satisfying W ⊆ U and W ⊆ V is U ∩ V . Conversely, ψ ` ψ ∨ χand χ ` ψ ∨ χ, and so [[ψ]]X ⊆ [[ψ ∨ χ]]X and [[χ]]X ⊆ [[ψ ∨ χ]]X. The smallestset W such that U ⊆W and V ⊆W is U ∪ V . The definition for → is tricky:ϕ → ψ expresses the weakest proposition that, combined with ϕ, entails ψ.That ϕ→ ψ combined with ϕ entails ψ is clear from (ϕ→ ψ) ∧ ϕ ` ψ. So[[ϕ→ ψ]]X should be the greatest open set such that [[ϕ→ ψ]]X ∩ [[ϕ]]X ⊂ [[ψ]]X,leading to our definition.






Chapter 3

Soundness and Completeness

This chapter collects soundness and completeness results for propo-sitional intuitionistic logic. It needs an introduction. The completenessproof makes use of facts about provability that should be stated and provedexplicitly somehwere.

3.1 Soundness of Axiomatic Derivations

int:sc:sax:sec

The soundness proof relies on the fact that all axioms are intuitionisti-cally valid; this still needs to be proved, e.g., in the Semantics chapter.

Theorem 3.1 (Soundness). int:sc:sax:

thm:soundness

If Γ ` ϕ, then Γ � ϕ.

Proof. We prove that if Γ ` ϕ, then Γ � ϕ. The proof is by induction onthe number n of formulas in the derivation of ϕ from Γ . We show that if ϕ1,. . . , ϕn = ϕ is a derivation from Γ , then Γ � ϕn. Note that if ϕ1, . . . , ϕn isa derivation, so is ϕ1, . . . , ϕk for any k < n.

There are no derivations of length 0, so for n = 0 the claim holds vacuously.So the claim holds for all derivations of length < n. We distinguish casesaccording to the justification of ϕn.

1. ϕn is an axiom. All axioms are valid, so Γ � ϕn for any Γ .

2. ϕn ∈ Γ . Then for any M and w, if M, w Γ , obviously M Γϕn[w],i.e., Γ � ϕ.

3. ϕn follows by mp from ϕi and ϕj ≡ ϕi → ϕn. ϕ1, . . . , ϕi and ϕ1,. . . , ϕj are derivations from Γ , so by inductive hypothesis, Γ � ϕi andΓ � ϕi→ ϕn.

16

Suppose M, w Γ . Since M, w Γ and Γ � ϕi→ ϕn, M, w ϕi→ ϕn.By definition, this means that for all w′ such that Rww′, if M, w′ ϕithen M, w′ ϕn. Since R is reflexive, w is among the w′ such that Rww′,i.e., we have that if M, w ϕi then M, w ϕn. Since Γ � ϕi, M, w ϕi.So, M, w ϕn, as we wanted to show.

3.2 Soundness of Natural Deduction

int:sc:snd:sec

Theorem 3.2 (Soundness).int:sc:snd:

thm:soundness

If Γ ` ϕ, then Γ � ϕ.

Proof. We prove that if Γ ` ϕ, then Γ � ϕ. The proof is by induction on thederivation of ϕ from Γ .

1. If the derivation consists of just the assumption ϕ, we have ϕ ` ϕ, andwant to show that ϕ � ϕ. Consider any model M such that M ϕ.Then trivially M ϕ.

2. The derivation ends in ∧Intro: The derivations of the premises ψ fromundischarged assumptions Γ and of χ from undischarged assumptions ∆show that Γ ` ψ and ∆ ` χ. By induction hypothesis we have that Γ � ψand ∆ � χ. We have to show that Γ ∪∆ � ϕ∧ψ, since the undischargedassumptions of the entire derivation are Γ together with ∆. So supposeM Γ ∪∆. Then also M Γ . Since Γ � ψ, M ψ. Similarly, M χ.So M ψ ∧ χ.

3. The derivation ends in ∧Elim: The derivation of the premise ψ ∧ χ fromundischarged assumptions Γ shows that Γ ` ψ ∧ χ. By induction hy-pothesis, Γ � ψ ∧ χ. We have to show that Γ � ψ. So suppose M Γ .Since Γ � ψ ∧ χ, M ψ ∧ χ. Then also M ψ. Similarly if ∧Elim endsin χ, then Γ � χ.

4. The derivation ends in ∨Intro: Suppose the premise is ψ, and the undis-charged assumptions of the derivation ending in ψ are Γ . Then we haveΓ ` ψ and by inductive hypothesis, Γ � ψ. We have to show thatΓ � ψ ∨ χ. Suppose M Γ . Since Γ � ψ, M ψ. But then alsoM ψ ∨ χ. Similarly, if the premise is χ, we have that Γ � χ.

5. The derivation ends in ∨Elim: The derivations ending in the premisesare of ψ ∨ χ from undischarged assumptions Γ , of θ from undischargedassumptions ∆1∪{ψ}, and of θ from undischarged assumptions ∆2∪{χ}.So we have Γ ` ψ ∨ χ, ∆1 ∪ {ψ} ` θ, and ∆2 ∪ {χ} ` θ. By inductionhypothesis, Γ � ψ∨χ, ∆1∪{ψ} � θ, and ∆2∪{χ} � θ. We have to provethat Γ ∪∆1 ∪∆2 � θ.

Suppose M Γ ∪∆1∪∆2. Then M Γ and since Γ � ψ∨χ, M ψ∨χ.By definition of M , either M ψ or M χ. So we distinguish cases:






(a) M ψ. Then M ∆1 ∪ {ψ}. Since ∆1 ∪ ψ � θ, we have M θ. (b)M χ. Then M ∆2 ∪ {χ}. Since ∆2 ∪ χ � θ, we have M θ. So ineither case, M θ, as we wanted to show.

6. The derivation ends with →Intro concluding ψ→ χ. Then the premiseis χ, and the derivation ending in the premise has undischarged assump-tions Γ ∪{ψ}. So we have that Γ ∪{ψ} ` χ, and by induction hypothesisthat Γ ∪ {ψ} � χ. We have to show that Γ � ψ→ χ.

Suppose M, w Γ . We want to show that for all w′ such that Rww′, ifM, w′ ψ, then M, w′ χ. So assume that Rww′ and M, w′ ψ. ByProposition 2.3, M, w′ Γ . Since Γ ∪{ψ} � χ, M, w′ χ, which is whatwe wanted to show.

7. The derivation ends in→Elim and conclusion χ. The premises are ψ→χand ψ, with derivations from undischarged assumptions Γ , ∆. So wehave Γ ` ψ→ χ and ∆ ` ψ. By inductive hypothesis, Γ � ψ→ χ and∆ � ψ. We have to show that Γ ∪∆ � χ.

Suppose M, w Γ ∪∆. Since M, w Γ and Γ � ψ→ χ, M, w ψ→ χ.By definition, this means that for all w′ such that Rww′, if M, w′ ψthen M, w′ χ. Since R is reflexive, w is among the w′ such that Rww′,i.e., we have that if M, w ψ then M, w χ. Since M, w ∆ and∆ � ψ, M, w ψ. So, M, w χ, as we wanted to show.

8. The derivation ends in ⊥I , concluding ϕ. The premise is ⊥ and theundischarged assumptions of the derivation of the premise are Γ . ThenΓ ` ⊥. By inductive hypothesis, Γ � ⊥. We have to show Γ � ϕ.

We proceed indirectly. If Γ 2 ϕ there is a model M and world w suchthat M, w Γ and M, w 1 ϕ. Since Γ � ⊥, M, w ⊥. But that’simpossible, since by definition, M, w 1 ⊥. So Γ � ϕ.

9. The derivation ends in ¬Intro: Exercise.

10. The derivation ends in ¬Elim: Exercise.

Problem 3.1. Complete the proof of Theorem 3.2. For the cases for ¬Introand ¬Elim, use the definition of M, w ¬ϕ in Definition 2.2, i.e., don’t treat¬ϕ as defined by ϕ→⊥.

3.3 Lindenbaum’s Lemma

int:sc:lin:sec

Definition 3.3. int:sc:lin:

defn:prime

A set of formulas Γ is prime iff

1. int:sc:lin:

defn:prime1

Γ is consistent.

2. int:sc:lin:

defn:prime2

If Γ ` ϕ then ϕ ∈ Γ , and






3.int:sc:lin:

defn:prime3

If ϕ ∨ ψ ∈ Γ then ϕ ∈ Γ or ψ ∈ Γ .

Lemma 3.4 (Lindenbaum’s Lemma).int:sc:lin:

lem:lindenbaum

If Γ 0 ϕ, there is a Γ ∗ ⊇ Γ suchthat Γ ∗ is prime and Γ ∗ 0 ϕ.

Proof. Let ψ1 ∨ χ1, ψ2 ∨ χ2, . . . , be an enumeration of all formulas of theform ψ ∨ χ. We’ll define an increasing sequence of sets of formulas Γn, whereeach Γn+1 is defined as Γn together with one new formula. Γ ∗ will be theunion of all Γn. The new formulas are selected so as to ensure that Γ ∗ isprime and still Γ ∗ 0 ϕ. This means that at each step we should find the firstdisjunction ψi ∨ χi such that:

1. Γn ` ψi ∨ χi

2. ψi /∈ Γn and χi /∈ Γn

We add to Γn either ψi if Γn ∪ {ψi} 0 ϕ, or χi otherwise. We’ll have to showthat this works. For now, let’s define i(n) as the least i such that (1) and (2)hold.

Define Γ0 = Γ and

Γn+1 =

{Γn ∪ {ψi(n)} if Γn ∪ {ψi(n)} 0 ϕΓn ∪ {χi(n)} otherwise

If i(n) is undefined, i.e., whenever Γn ` ψ ∨ χ, either ψ ∈ Γn or χ ∈ Γn, we letΓn+1 = Γn. Now let Γ ∗ =

⋃∞n=0 Γn

First we show that for all n, Γn 0 ϕ. We proceed by induction on n. Forn = 0 the claim holds by the hypothesis of the theorem, i.e., Γ 0 ϕ. If n > 0,we have to show that if Γn 0 ϕ then Γn+1 0 ϕ. If i(n) is undefined, Γn+1 = Γnand there is nothing to prove. So suppose i(n) is defined. For simplicity, leti = i(n).

We’ll prove the contrapositive of the claim. Suppose Γn+1 ` ϕ. By con-struction, Γn+1 = Γn ∪ {ψi} if Γn ∪ {ψi} 0 ϕ, or else Γn+1 = Γn ∪ {χi}. Itclearly can’t be the first, since then Γn+1 0 ϕ. Hence, Γn ∪ {ψi} ` ϕ andΓn+1 = Γn ∪ {χi}. By definition of i(n), we have that Γn ` ψi ∨ χi. We haveΓn ∪ {ψi} ` ϕ. We also have Γn+1 = Γn ∪ {χi} ` ϕ. Hence, Γn ` ϕ, which iswhat we wanted to show.

If Γ ∗ ` ϕ, there would be some finite subset Γ ′ ⊆ Γ ∗ such that Γ ′ ` ϕ.Each θ ∈ Γ ′ must be in Γi for some i. Let n be the largest of these. SinceΓi ⊆ Γn if i ≤ n, Γ ′ ⊆ Γn. But then Γn ` ϕ, contrary to our proof above thatΓn 0 ϕ.

Lastly, we show that Γ ∗ is prime, i.e., satisfies conditions (1), (2), and (3)of Definition 3.3.

First, Γ ∗ 0 ϕ, so Γ ∗ is consistent, so (1) holds.We now show that if Γ ∗ ` ψ∨χ, then either ψ ∈ Γ ∗ or χ ∈ Γ ∗. This proves

(3), since if ψ ∈ Γ ∗ then also Γ ∗ ` ψ, and similarly for χ. So assume Γ ∗ ` ψ∨χbut ψ /∈ Γ ∗ and χ /∈ Γ ∗. Since Γ ∗ ` ψ ∨ χ, Γn ` ψ ∨ χ for some n. ψ ∨ χ






appears on the enumeration of all disjunctions, say as ψj ∨χj . ψj ∨χj satisfiesthe properties in the definition of i(n), namely we have Γn ` ψj ∨ χj , whileψj /∈ Γn and χj /∈ Γn. At each stage, at least one fewer disjunction ψi ∨ χisatisfies the conditions (since at each stage we add either ψi or χi), so at somestage m we will have j = i(Γm). But then either ψ ∈ Γm+1 or χ ∈ Γm+1,contrary to the assumption that ψ /∈ Γ ∗ and χ /∈ Γ ∗.

Now suppose Γ ∗ ` ϕ. Then Γ ∗ ` ϕ ∨ ϕ. But we’ve just proved that ifΓ ∗ ` ϕ ∨ ϕ then ϕ ∈ Γ ∗. Hence, Γ ∗ satisfies (2) of Definition 3.3.

3.4 The Canonical Model

int:sc:mod:sec

The worlds in our model will be finite sequences σ of natural numbers, i.e.,σ ∈ N∗. Note that N∗ is inductively defined by:

1. Λ ∈ N∗.

2. If σ ∈ N∗ and n ∈ N, then σ.n ∈ N∗ (where σ.n is σ _ 〈n〉 and σ _ σ′ isthe concatenation if σ and σ′).

3. Nothing else is in N∗.

So we can use N∗ to give inductive definitions.Let 〈ψ1, χ1〉, 〈ψ2, χs〉, . . . , be an enumeration of all pairs of formulas. Given

a set of formulas ∆, define ∆(σ) by induction as follows:

1. ∆(Λ) = ∆

2. ∆(σ.n) = {(∆(σ) ∪ {ψn})∗ if ∆(σ) ∪ {ψn} 0 χn∆(σ) otherwise

Here by (∆(σ) ∪ {ψn})∗ we mean the prime set of formulas which exists byLemma 3.4 applied to the set ∆(σ) ∪ {ψn} and the formula χn. Note that bythis definition, if ∆(σ) ∪ {ψn} 0 χn, then ∆(σ.n) ` ψn and ∆(σ.n) 0 χn. Notealso that ∆(σ) ⊆ ∆(σ.n) for any n. If ∆ is prime, then ∆(σ) is prime for all σ.

Definition 3.5. int:sc:mod:

defn:canonical-model

Suppose ∆ is prime. Then the canonical model M(∆) for ∆is defined by:

1. W = N∗, the set of finite sequences of natural numbers.

2. R is the partial order according to which Rσσ′ iff σ is an initial segmentof σ′ (i.e., σ′ = σ _ σ′′ for some sequence σ′′).

3. V (p) = {σ : p ∈ ∆(σ)}.

It is easy to verify that R is indeed a partial order. Also, the monotonic-ity condition on V is satisfied. Since ∆(σ) ⊆ ∆(σ.n) we get ∆(σ) ⊆ ∆(σ′)whenever Rσσ′ by induction on σ.






3.5 The Truth Lemma

int:sc:tru:sec

Lemma 3.6.int:sc:tru:

lem:truth

If ∆ is prime, then M(∆), σ ϕ iff ∆(σ) ` ϕ.

Proof. By induction on ϕ.

1. ϕ ≡ ⊥: Since ∆(σ) is prime, it is consistent, so ∆(σ) 0 ϕ. By definition,M(∆), σ 1 ϕ.

2. ϕ ≡ p: By definition of , M(∆), σ ϕ iff σ ∈ V (p), i.e., ∆(σ) ` ϕ.

3. ϕ ≡ ¬ψ: exercise.

4. ϕ ≡ ψ ∧ χ: M(∆), σ ϕ iff M(∆), σ ψ and M(∆), σ χ. Byinduction hypothesis, M(∆), σ ψ iff ∆(σ) ` ψ, and similarly for χ.But ∆(σ) ` ψ and ∆(σ) ` χ iff ∆(σ) ` ϕ.

5. ϕ ≡ ψ∨χ: M(∆), σ ϕ iff M(∆), σ ψ or M(∆), σ χ. By inductionhypothesis, this holds iff ∆(σ) ` ψ of ∆(σ) ` χ. We have to show thatthis in turn holds iff ∆(σ) ` ϕ. The left-to-right direction is clear. Theright-to-left direction follows since ∆(σ) is prime.

6. ϕ ≡ ψ→χ: First the contrapositive of the left-to-right direction: Assume∆(σ) 0 ψ→ χ. Then also ∆(σ) ∪ {ψ} 0 χ. Since 〈ψ, χ〉 is 〈ψn, χn〉 forsome n, we have∆(σ.n) = (∆(σ)∪{ψ})∗, and∆(σ.n) ` ψ but∆(σ.n) 0 χ.By inductive hypothesis, M(∆), σ.n ψ and M(∆), σ.n 1 χ. SinceRσ(σ.n), this means that M(∆), σ 1 ϕ.

Now assume ∆(σ) ` ψ → χ, and let Rσσ′. Since ∆(σ) ⊆ ∆(σ′), wehave: if ∆(σ′) ` ψ, then ∆(σ′) ` χ. In other words, for every σ′ suchthat Rσσ′, either ∆(σ′) 0 ψ or ∆(σ′) ` χ. By induction hypothesis, thismeans that whenever Rσσ′, either M(∆), σ′ 1 ψ or M(∆), σ′ χ, i.e.,M(∆), σ ϕ.

3.6 The Completeness Theorem

int:sc:cpl:sec Theorem 3.7.int:sc:cpl:

thm:completeness

If Γ � ϕ then Γ ` ϕ.

Proof. We prove the contrapositive: Suppose Γ 0 ϕ. Then by Lemma 3.4, thereis a prime set Γ ∗ ⊇ Γ such that Γ ∗ 0 ϕ. Consider the canonical model M(Γ ∗)for Γ ∗ as defined in Definition 3.5. For any ψ ∈ Γ , Γ ∗ ` ψ. Note thatΓ ∗(Λ) = Γ ∗. By the Truth Lemma (Lemma 3.6), we have M(Γ ∗), Λ ψ forall ψ ∈ Γ and M(Γ ∗), Λ 1 ϕ. This shows that Γ 2 ϕ.






Chapter 4

Propositions as Types

This is a very experimental draft of a chapter on the Curry-Howardcorrespondence. It needs more explanation and motivation, and thereare probably errors and omissions. The proof of normalization should bereviewed and expanded. There are no examples for the product type.Permuation and simplification conversions are not covered. It will make alot more sense once there is also material on the (typed) lambda calculuswhich is basically presupposed here. Use with extreme caution.

4.1 Introduction

int:pty:int:sec

Historically the lambda calculus and intuitionistic logic were developed sepa-rately. Haskell Curry and William Howard independently discovered a closesimilarity: types in a typed lambda calculus correspond to formulas in intu-itionistic logic in such a way that a derivation of a formula corresponds directlyto a typed lambda term with that formula as its type. Moreover, beta reduc-tion in the typed lambda calculus corresponds to certain transformations ofderivations.

For instance, a derivation of ϕ→ψ corresponds to a term λxϕ. Nψ, which hasthe function type ϕ→ ψ. The inference rules of natural deduction correspondto typing rules in the typed lambda calculus, e.g.,

[ϕ]x

ψx →Introϕ→ ψ corresponds to

x : ϕ ⇒ N : ψλ

⇒ λxϕ. Nψ : ϕ→ ψ

where the rule on the right means that if x is of type ϕ and N is of type ψ,then λxϕ. N is of type ϕ→ ψ.

The →Elim rule corresponds to the typing rule for composition terms, i.e.,

22

ϕ→ ψ ϕ→Elim

ψ corresponds to⇒ P : ϕ→ ψ ⇒ Q : ϕ

app⇒ Pϕ→ψQϕ : ψ

If a →Intro rule is followed immediately by a →Elim rule, the derivationcan be simplified:

[ϕ]x

ψx →Introϕ→ ψ ϕ

→Elimψ

−→ ϕ

ψ

which corresponds to the beta reduction of lambda terms

(λxϕ. Pψ)Q −→ P [Q/x].

Similar correspondences hold between the rules for ∧ and “product” types,and between the rules for ∨ and “sum” types.

This correspondence between terms in the simply typed lambda calculusand natural deduction derivations is called the “Curry-Howard”, or “proposi-tions as types” correspondence. In addition to formulas (propositions) corre-sponding to types, and proofs to terms, we can summarize the correspondencesas follows:

logic programproposition type

proof termassumption variable

discharged assumption bind variablenot discharged assumption free variable

implication function typeconjunction product typedisjunction sum typeabsurdity bottom type

The Curry-Howard correspondence is one of the cornerstones of automatedproof assistants and type checkers for programs, since checking a proof witness-ing a proposition (as we did above) amounts to checking if a program (term)has the declared type.

4.2 Sequent Natural Deduction

int:pty:snd:sec

Let us write Γ ⇒ ϕ if there is a natural deduction derivation with Γ as undis-charged assumptions and ϕ as conclusion; or ⇒ ϕ if Γ is empty.






We write Γ, ϕ1, . . . , ϕn for Γ ∪ {ϕ1, . . . , ϕn}, and Γ,∆ for Γ ∪∆.

Observe that when we have Γ ⇒ ϕ∧ϕ, meaning we have a derivation withΓ as undischarged assumptions and ϕ ∧ ϕ as end-formula, then by applying∧Elim at the bottom, we can get a derivation with the same undischargedassumptions and ϕ as conclusion. In other words, if Γ ⇒ ϕ ∧ ψ, then Γ ⇒ ϕ.

Γ ⇒ ϕ ∧ ψ∧Elim

Γ ⇒ ϕ

Γ ⇒ ϕ ∧ ψ∧Elim

Γ ⇒ ψ

The label ∧Elim hints at the relation with the rule of the same name in naturaldeduction.

Likewise, suppose we have Γ, ϕ ⇒ ψ, meaning we have a derivation withundischarged assumptions Γ, ϕ and end-formula ψ. If we apply the →Introrule, we have a derivation with Γ as undischarged assumptions and ϕ→ ψ asthe end-formula, i.e., Γ ⇒ ϕ→ ψ. Note how this has made the discharge ofassumptions more explicit.

Γ, ϕ ⇒ ψ→Intro

Γ ⇒ ϕ→ ψ

We can draw conclusions from other rules in the same fashion, which isspelled out as follows:

Γ ⇒ ϕ ∆ ⇒ ψ∧Intro

Γ,∆ ⇒ ϕ ∧ ψΓ ⇒ ϕ ∧ ψ

∧Elim1Γ ⇒ ϕ

Γ ⇒ ϕ ∧ ψ∧Elim2Γ ⇒ ψ

Γ ⇒ ϕ ∨Intro1Γ ⇒ ϕ ∨ ψΓ ⇒ ψ ∨Intro2Γ ⇒ ϕ ∨ ψ

Γ ⇒ ϕ ∨ ψ ∆,ϕ ⇒ χ ∆′, ψ ⇒ χ∨Elim

Γ,∆,∆′ ⇒ χ

Γ, ϕ ⇒ ψ→Intro

Γ ⇒ ϕ→ ψ

∆ ⇒ ϕ→ ψ Γ ⇒ ϕ→Elim

Γ,∆ ⇒ ψ

Γ ⇒ ⊥ ⊥IΓ ⇒ ϕ

Any assumption by itself is a derivation of ϕ from ϕ, i.e., we always haveϕ⇒ ϕ.

ϕ ⇒ ϕ

Together, these rules can be taken as a calculus about what natural de-duction derivations exist. They can also be taken as a notational variant ofnatural deduction, in which each step records not only the formula derived butalso the undischarged assumptions from which it was derived.






ϕ ⇒ ϕ

ϕ ⇒ ϕ ∨ (ϕ→⊥) ψ ⇒ ψ

ϕ,ψ→ ⇒ ⊥(ψ ⇒ ϕ→⊥(ψ ⇒ ϕ ∨ (ϕ→⊥) (ψ ⇒ ψ

(ψ ⇒ ⊥⇒ ψ→⊥

where ψ is short for (ϕ ∨ (ϕ→⊥))→⊥.

4.3 Proof Terms

int:pty:ter:sec

We give the definition of proof terms, and then establish its relation withnatural deduction derivations.

Definition 4.1 (Proof terms). Proof terms are inductively generated by thefollowing rules:

1. A single variable x is a proof term.

2. If P and Q are proof terms, then PQ is also a proof term.

3. If x is a variable, ϕ is a formula, and N is a proof term, then λxϕ. N isalso a proof term.

4. If P and Q are proof terms, then 〈P,Q〉 is a proof term.

5. If M is a proof term, then pi(M) is also a proof term, where i is 1 or 2.

6. If M is a proof term, and ϕ is a formula, then inϕi (M) is a proof term,where i is 1 or 2.

7. IfM,N1, N2 is proof terms, and x1, x2 are variables, then case(M,x1.N1, x2.N2)is a proof term.

8. If M is a proof term and ϕ is a formula, then contrϕ(M) is proof term.

Each of the above rules corresponds to an inference rule in natural deduc-tion. Thus we can inductively assign proof terms to the formulas in a deriva-tion. To make this assignment unique, we must distinguish between the twoversions of ∧Elim and of ∨Intro. For instance, the proof terms assigned to theconclusion of ∨Intro must carry the information whether ϕ∨ψ is inferred fromϕ or from ψ. Suppose M is the term assigned to ϕfrom which ϕ∨ψ is inferred.Then the proof term assigned to ϕ ∨ ψ is inϕ1 (M). If we instead infer ψ ∨ ϕthen the proof term assigned is inϕ2 (M).

The term λxϕ. N is assigned to the conclusion of→Intro. The ϕ representsthe assumption being discharged; only have we included it can we infer theformula of λxϕ. N based on the formula of N .






Definition 4.2 (Typing context). A typing context is a mapping from vari-ables to formulas. We will call it simply the “context” if there is no confusion.We write a context Γ as a set of pairs 〈x, ϕ〉.

A pair Γ ⇒M where M is a proof term represents a derivation of a formulawith context Γ .

Definition 4.3 (Typing pair). A typing pair is a pair 〈Γ,M〉, where Γ is atyping context and M is a proof term.

Since in general terms only make sense with specific contexts, we will speaksimply of “terms” from now on instead of “typing pair”; and it will be apparentwhen we are talking about the literal term M .

4.4 Converting Derivations to Proof Terms

int:pty:pt:sec

We will describe the process of converting natural deduction derivations topairs. We will write a proof term to the left of each formula in the derivation,resulting in expressions of the form M : ϕ. We’ll then say that, M witnesses ϕ.Let’s call such an expression a judgment.

First let us assign to each assumption a variable, with the following con-straints:

1. Assumptions discharged in the same step (that is, with the same numberon the square bracket) must be assigned the same variable.

2. For assumptions not discharged, assumptions of different formulas shouldbe assigned different variables.

Such an assignment translates all assumptions of the form

ϕ into x : ϕ.

With assumptions all associated with variables (which are terms), we can nowinductively translate the rest of the deduction tree. The modified naturaldeduction rules taking into account context and proof terms are given below.Given the proof terms for the premise(s), we obtain the corresponding proofterm for conclusion.

M1 : ϕ1 M2 : ϕ2 ∧Intro〈M1,M2〉 : ϕ1 ∧ ϕ2

M : ϕ1 ∧ ϕ2 ∧Elim1pi(M) : ϕ1

M : ϕ1 ∧ ϕ2 ∧Elim2pi(M) : ϕ2

In ∧Intro we assume we have ϕ1 witnessed by term M1 and ϕ2 witnessedby term M2. We pack up the two terms into a pair 〈M1,M2〉 which witnessesϕ1 ∧ ϕ2.






In ∧Elimi we assume that M witnesses ϕ1 ∧ ϕ2. The term witnessing ϕiis pi(M). Note that M is not necessary of the form 〈M1,M2〉, so we cannotsimply assign M1 to the conclusion ϕi.

Note how this coincides with the BHK interpretation. What the BHKinterpretation does not specify is how the function used as proof for ϕ→ ψ issupposed to be obtained. If we think of proof terms as proofs or functions ofproofs, we can be more explicit.

[x : ϕ]

N : ψ→Intro

λxϕ. N : ϕ→ ψ

P : ϕ→ ψ Q : ϕ→Elim

PQ : ψ

The λ notation should be understood as the same as in the lambda calculus,and PQ means applying P to Q.

M1 : ϕ1 ∨Intro1inϕ1

1 (M1) : ϕ1 ∨ ϕ2

M2 : ϕ2 ∨Intro2inϕ2

2 (M2) : ϕ1 ∨ ϕ2

M : A1 ∨ ϕ2

[x1 : ϕ1]

N1 : χ

[x2 : ϕ2]

N2 : χ∨Elim

case(M,x1.N1, x2.N2) : χ

The proof term inϕ1

1 (M1) is a term witnessing ϕ1∨ϕ2, where M1 witnesses ϕ1.The term case(M,x1.N1, x2.N2) mimics the case clause in programming

languages: we already have the derivation of ϕ∨ψ, a derivation of χ assuming ϕ,and a derivation of χ assuming ψ. The case operator thus select the appropriateproof depending on M ; either way it’s a proof of χ.

N : ⊥ ⊥Icontrϕ(N) : ϕ

contrϕ(N) is a term witnessing ϕ, whenever N is a term witnessing ⊥.Now we have a natural deduction derivation with all formulas associated

with a term. At each step, the relevant typing context Γ is given by the list ofassumptions remaining undischarged at that step. Note that Γ is well defined:since we have forbidden assumptions of different undischarged assumptions tobe assigned the same variable, there won’t be any disagreement about theformulas mapped to which a variable is mapped.

We now give some examples of such translations:Consider the derivation of ¬¬(ϕ ∨ ¬ϕ), i.e., ((ϕ ∨ (ϕ→⊥))→⊥)→⊥. Its

translation is:






[y : (ϕ ∨ (ϕ→⊥))→⊥]2

[y : (ϕ ∨ (ϕ→⊥))→⊥]2

[x : ϕ]1

inϕ→⊥1 (x) : ϕ ∨ (ϕ→⊥)

y(inϕ→⊥1 (x)) : ⊥1

λxϕ. y(inϕ→⊥1 (x)) : ϕ→⊥inϕ2 (λxϕ. y(inϕ→⊥1 (x))) : ϕ ∨ (ϕ→⊥)

y(inϕ2 (λxϕ. yinϕ→⊥1 (x))) : ⊥2

λy(ϕ∨(ϕ→⊥))→⊥. y(inϕ2 (λxϕ. yinϕ→⊥1 (x))) : ((ϕ ∨ (ϕ→⊥))→⊥)→⊥

The tree has no assumptions, so the context is empty; we get:

` λy(ϕ∨(ϕ→⊥))→⊥. y(inϕ2 (λxϕ. yinϕ→⊥1 (x))) : ((ϕ ∨ (ϕ→⊥))→⊥)→⊥

If we leave out the last →Intro, the assumptions denoted by y would be in thecontext and we would get:

y : ((ϕ ∨ (ϕ→⊥))→⊥) ` y(inϕ2 (λxϕ. yinϕ→⊥1 (x))) : ⊥

Another example: ` ϕ→ (ϕ→⊥)→⊥

[x : ϕ]2 [y : ϕ→⊥]1

yx : ⊥1λyϕ→⊥. yx : (ϕ→⊥)→⊥

2λxϕ. λyϕ→⊥. yx : ϕ→ (ϕ→⊥)→⊥

Again all assumptions are discharged and thus the context is empty, the re-sulting term is

` λxϕ. λyϕ→⊥. yx : ϕ→ (ϕ→⊥)→⊥

If we leave out the last two →Intro inferences, the assumptions denoted byboth x and y would be in context and we would get

x : ϕ, y : ϕ→⊥ ` yx : ⊥

4.5 Recovering Derivations from Proof Terms

int:pty:tp:sec

Now let us consider the other direction: translating terms back to naturaldeduction trees. We will use still use the double refutation of the excludedmiddle as example, and let S denote this term, i.e.,

λy(ϕ∨(ϕ→⊥))→⊥. y(inϕ2 (λxϕ. yinϕ→⊥1 (x))) : ((ϕ ∨ (ϕ→⊥))→⊥)→⊥

For each natural deduction rule, the term in the conclusion is always formedby wrapping some operator around the terms assigned to the premise(s). Rules






correspond uniquely to such operators. For example, from the structure of theS we infer that the last rule applied must be →Intro, since it is of the formλy.... . . ., and the λ operator corresponds to→Intro. In general we can recoverthe skeleton of the derivation solely by the structure of the term, e.g.,

[y :]2

[y :]2

[x]1∨Intro1

inϕ→⊥1 (x) :→Elim

y(inϕ→⊥1 (x)) :1 →Introλxϕ. y(inϕ→⊥1 (x)) :

∨Intro2inϕ2 (λxϕ. yinϕ→⊥1 (x)) :

→Elimy(inϕ2 (λxϕ. yinϕ→⊥1 (x))) :

2 →Introλy(ϕ∨(ϕ→⊥))→⊥. y(inϕ2 (λxϕ. y(inϕ→⊥1 (x)))) :

Our next step is to recover the formulas these terms witness. We define afunction F (Γ,M) which denotes the formula witnessed by M in context Γ , byinduction on M as follows:

F (Γ, x) = Γ (x)

F (Γ, 〈N1, N2〉 = F (Γ,N1) ∧ F (Γ,N2)

F (Γ,pi(N) = ϕi if F (Γ,N) = ϕ1 ∧ ϕ2

F (Γ, inϕi (N) =

{F (N) ∨ ϕ if i = 1

ϕ ∨ F (N) if i = 2

F (Γ, case(M,x1.N1, x2.N2)) = F (Γ ∪ {xi : F (Γ,M)}, Ni)F (Γ, λxϕ. N) = ϕ→ F (Γ ∪ {x : ϕ}, N)

F (Γ,NM) = ψ if F (Γ,N) = ϕ→ ψ

where Γ (x) means the formula mapped to by x in Γ and Γ ∪ {x : ϕ} is acontext exactly as Γ except mapping x to ϕ, whether or not x is already in Γ .

Note there are cases where F (Γ,M) is not defined, for example:

1. In the first line, it is possible that x is not in Γ .

2. In recursive cases, the inner invocation may be undefined, making theouter one undefined too.

3. In the third line, its only defined when F (Γ,M) is of the form ϕ1 ∨ ϕ2,and the right hand is independent on i.

As we recursively compute F (Γ,M), we work our way up the natural deduc-tion derivation. The every step in the computation of F (Γ,M) corresponds toa term in the derivation to which the derivation-to-term translation assigns M ,and the formula computed is the end-formula of the derivation. However, theresult may not be defined for some choices of Γ . We say that such pairs 〈Γ,M〉are ill-typed, and otherwise well-typed. However, if the term M results from






translating a derivation, and the formulas in Γ correspond to the undischargedassumptions of the derivation, the pair 〈Γ,M〉 will be well-typed.

Proposition 4.4. If D is a derivation with undischarged assumptions ϕ1, . . . ,ϕn, M is the proof term associated with D and Γ = {x1 : ϕ1, . . . , xn : ϕn},then the result of recovering derivation from M in context Γ is D.

In the other direction, if we first translate a typing pair to natural deductionand then translate it back, we won’t get the same pair back since the choice ofvariables for the undischarged assumptions is underdetermined. For example,consider the pair 〈{x : ϕ, y : ϕ→ ψ}, yx〉. The corresponding derivation is

ϕ→ ψ ϕ→Elim

ψ

By assigning different variables to the undischarged assumptions, say, u toϕ → ψ and v to ϕ, we would get the term uv rather than yx. There is aconnection, though: the terms will be the same up to renaming of variables.

Now we have established the correspondence between typing pairs and nat-ural deduction, we can prove theorems for typing pairs and transfer the resultto natural deduction derivations.

Similar to what we did in the natural deduction section, we can make someobservations here too. Let Γ ` M : ϕ denote that there is a pair (Γ,M)witnessing the formula ϕ. Then always Γ ` x : ϕ if x : ϕ ∈ Γ , and thefollowing rules are valid:

Γ `M1 : ϕ1 ∆ `M2 : ϕ2 ∧IntroΓ,∆ ` 〈M1,M2〉 : ϕ1 ∧ ϕ2

Γ `M : ϕ1 ∧ ϕ2 ∧ElimiΓ ` pi(M) : ϕi

Γ `M1 : ϕ1 ∨Intro1Γ ` inϕ2

1 (M) : ϕ1 ∨ ϕ2

Γ `M2 : ϕ2 ∨Intro2Γ ` inϕ1

2 (M) : ϕ1 ∨ ϕ2

Γ `M : ϕ ∨ ψ ∆1, x1 : ϕ1 ` N1 : χ ∆2, x2 : ϕ2 ` N2 : χ∨Elim

Γ,∆,∆′ ` case(M,x1.N1, x2.N2) : χ

Γ, x : ϕ ` N : ψ→Intro

Γ ` λxϕ. N : ϕ→ ψ

Γ ` Q : ϕ ∆ ` P : ϕ→ ψ→Elim

Γ,∆ ` PQ : ψ

Γ `M : ⊥ ⊥ElimΓ ` contrϕ(M) : ϕ

These are the typing rules of the simply typed lambda calculus extendedwith product, sum and bottom.

In addition, the F (Γ,M) is actually a type checking algorithm; it returnsthe type of the term with respect to the context, or is undefined if the term isill-typed with respect to the context.

4.6 Reduction

int:pty:red:sec






In natural deduction derivations, an introduction rule that is followed by anelimination rule is redundant. For instance, the derivation

ϕ ϕ→ ψ→Elim

ψ [χ]∧Intro

ψ ∧ χ∧Elim

ψ→Intro

χ→ ψ

can be replaced with the simpler derivation:

ϕ ϕ→ ψ→Elim

ψ→Intro

χ→ ψ

As we see, an ∧Intro followed by ∧Elim “cancel out.” In general, we see thatthe conclusion of ∧Elim is always the formula on one side of the conjunction,and the premises of ∧Intro requires both sides of the conjunction, thus if weneed a derivation of either side, we can simply use that derivation withoutintroducing the conjunction followed by eliminating it.

Thus in general we have

D1

ϕ1

D2

ϕ2 ∧Introϕ1 ∧ ϕ2 ∧Elimiϕi −→

Di

ϕi

The −→ symbol has a similar meaning as in the lambda calculus, i.e., asingle step of a reduction. In the proof term syntax for derivations, the abovereduction rule thus becomes:

(Γ, pi〈Mϕ1

1 ,Mϕ2

2 〉) −→ (Γ,Mi)

In the typed lambda calculus, this is the beta reduction rule for the producttype.

Note the type annotation on M1 and M2: while in the standard term syntaxonly λxϕ. N has such notion, we reuse the notation here to remind us of theformula the term is associated with in the corresponding natural deductionderivation, to reveal the correspondence between the two kinds of syntax.

In natural deduction, a pair of inferences such as those on the left, i.e., apair that is subject to cancelling is called a cut. In the typed lambda calculusthe term on the left of −→ is called a redex, and the term to the right is called thereductum. Unlike untyped lambda calculus, where only (λx.N)Q is consideredto be redex, in the typed lambda calculus the syntax is extended to termsinvolving 〈N,M〉, pi(N), inϕi (N), case(N, x1.M1, x2.M2), and contrN (), withcorresponding redexes.






Similarly we have reduction for disjunction:

D

ϕi ∨Introϕ1 ∨ ϕ2

[ϕ1]u

D1

χ

[ϕ2]u

D2

χu ∨Elimχ −→

D

ϕi

Di

χ

This corresponds to a reduction on proof terms:

(Γ, case(inϕi

i (Mϕi), xϕ1

1 .Nχ1 , x

ϕ2

2 .Nχ2 )) −→ (Γ,Nχ

i [Mϕi/xϕi

i ])

This is the beta reduction rule of for sum types. Here, M [N/x] means replacingall assumptions denoted by variable x in M with N ,

It would be nice if we pass the context Γ to the substitution function sothat it can check if the substitution makes sense. For example, xy[ab/y] doesnot make sense under the context {x : ϕ→ θ, y : ϕ, a : ψ→ χ, b : ψ} since thenwe would be substituting a derivation of χ where a derivation of ϕ is expected.However, as long as our usage of substitution is careful enough to avoid sucherrors, we won’t have to worry about such conflicts. Thus we can define itrecursively as we did for untyped lambda calculus as if we are dealing withuntyped terms.

Finally, the reduction of the function type corresponds to removal of adetour of a →Intro followd by a →Elim.

[ϕ]u

D

ψu →Introϕ→ ψ

D′

ϕ→Elim

ψ −→

D′

ϕ

D

ψ

For proof terms, this amounts to ordinary beta reduction:

(Γ, (λxϕ. Nψ)Qϕ) −→ (Γ,Nψ[Qϕ/xϕ])

Absurdity has only an elimination rule and no introduction rule, thus thereis no such reduction for it.

Note that the above notion of reduction concerns only deductions with a cutat the end of a derivation. We would of course like to extend it to reductionof cuts anywhere in a derivation, or reductions of subterms of proof termswhich constitute redexes. Note that, however, the conclusion of the reductiondoes not change after reduction, thus we are free to continue applying rules toboth sides of −→. The resulting pairs of trees constitutes an extended notion ofreduction; it is analogous to compatibility in the untyped lambda calculus.






It’s easy to see that the context Γ does not change during the reduction(both the original and the extended version), thus it’s unnecessary to mentionthe context when we are discussing reductions. In what follows we will assumethat every term is accompanied by a context which does no change duringreduction. We then say “proof term” when we mean a proof term accompaniedby a context which makes it well-typed.

As in lambda calculus, the notion of normal-form term and normal deduc-tion is given:

Definition 4.5. A proof term with no redex is said to be in normal form;likewise, a derivation without cuts is a normal derivation. A proof term is innormal form if and only if its counterpart derivation is normal.

4.7 Normalization

int:pty:nor:sec

In this section we prove that, via some reduction order, any deduction canbe reduced to a normal deduction, which is called the normalization property.We will make use of the propositions-as-types correspondence: we show thatevery proof term can be reduced to a normal form; normalization for naturaldeduction derivations then follows.

Firstly we define some functions that measure the complexity of terms. Thelength len(ϕ) of a formulas is defined by

len(p) = 0

len(ϕ ∧ ψ) = len(ϕ) + len(ψ) + 1

len(ϕ ∨ ψ) = len(ϕ) + len(ψ) + 1

len(ϕ→ ψ) = len(ϕ) + len(ψ) + 1.

The complexity of a redex M is measured by its cut rank cr(M):

cr((λxϕ. Nψ)Q) = len(ϕ) + len(ψ) + 1

cr(pi(〈Mϕ, Nψ〉)) = len(ϕ) + len(ψ) + 1

cr(case(inϕi

i (Mϕi), xϕ1

1 .Nχ1 , x

ϕ2

2 .Nχ2 )) = len(ϕ) + len(ψ) + 1

The complexity of a proof term is measured by the most complex redex in it,and 0 if it is normal:

mr(M) = max{cr(N)|N is a sub term of M and is redex}

Lemma 4.6.int:pty:nor:

lem:subst

If M [Nϕ/xϕ] is a redex and M 6≡ x, then one of the followingcases holds:

1. M is itself a redex, or

2. M is of the form pi(x), and N is of the form 〈P1, P2〉

3. M is of the form case(i, x1.P1, x2.P2), and N is of the form ini(Q)






4. M is of the form xQ, and N is of the form λx. P

In the first case, cr(M [N/x]) = cr(M); in the other cases, cr(M [N/x]) =len(ϕ)).

Proof. Proof by induction on M .

1. If M is a single variable y and y 6≡ x, then y[N/x] is y, hence not a redex.

2. If M is of the form 〈N1, N2〉, or λx.N , or inϕi (N), then M [Nϕ/xϕ] is alsoof that form, and so is not a redex.

3. If M is of the form pi(P ), we consider two cases.

a) If P is of the form 〈P1, P2〉, then M ≡ pi(〈P1, P2〉) is a redex, andclearly

M [N/x] ≡ pi(〈P1[N/x], P2[N/x]〉)

is also a redex. The cut ranks are equal.

b) If P is a single variable, it must be x to make the substitution aredex, and N must be of the form 〈P1, P2〉. Now consider

M [N/x] ≡ pi(x)[〈P1, P2〉/x],

which is pi(〈P1, P2〉). Its cut rank is equal to cr(x), which is len(ϕ).

The cases of case(N, x1.N1, x2.N2) and PQ are similar.

Lemma 4.7. If M contracts to M ′, and cr(M) > cr(N) for all proper redexsub-terms N of M , then cr(M) > mr(M ′).

Proof. Proof by cases.

1. If M is of the form pi(〈M1,M2〉), then M ′ is Mi; since any sub-term ofMi is also proper sub-term of M , the claim holds.

2. If M is of the form (λxϕ. N)Qϕ, then M ′ is N [Qϕ/xϕ]. Consider a redexin M ′. Either there is corresponding redex in N with equal cut rank,which is less than cr(M) by assumption, or the cut rank equals len(ϕ),which by definition is less than cr((λxϕ. N)Q).

3. If M is of the form

case(ini(Nϕi), xϕ1

1 .Nχ1 , x

ϕ2

2 .Nχ2 ),

then M ′ ≡ Ni[N/xϕi

i ]. Consider a redex in M ′. Either there is corre-sponding redex in Ni with equal cut rank, which is less than cr(M) byassumption; or the cut rank equals len(ϕi), which by definition is lessthan cr(case(ini(N

ϕi), xϕ1

1 .Nχ1 , x

ϕ2

2 .Nχ2 )).






Theorem 4.8. All proof terms reduce to normal form; all derivations reduceto normal derivations.

Proof. The second follows from the first. We prove the first by complete in-duction on m = mr(M), where M is a proof term.

1. If m = 0, M is already normal.

2. Otherwise, we proceed by induction on n, the number of redexes in Mwith cut rank equal to m.

a) If n = 1, select any redex N such that m = cr(N) > cr(P ) for anyproper sub-term P which is also a redex of course. Such a redexmust exist, since any term only has finitely many subterms.

Let N ′ denote the reductum of N . Now by the lemma mr(N ′) <mr(N), thus we can see that n, the number of redexes with cr(=)mis decreased. So m is decreased (by 1 or more), and we can applythe inductive hypothesis for m.

b) For the induction step, assume n > 1. the process is similar, exceptthat n is only decreased to a positive number and thus m does notchange. We simply apply the induction hypothesis for n.

The normalization of terms is actually not specific to the reduction orderwe chose. In fact, one can prove that regardless of the order in which redexesare reduced, the term always reduces to a normal form. This property is calledstrong normalization.

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