Advances in Proof-Theoretic Semantics pp 211-229 | Cite as

# Self-contradictory Reasoning

## Abstract

This paper concerns the characterization of paradoxical reasoning in terms of structures of proofs. The starting point is the observation that many paradoxes use self-reference to give a statement a double meaning and that this double meaning results in a contradiction. Continuing by constraining the concept of meaning by the inferences of a derivation “self-contradictory reasoning” is formalized as reasoning with statements that have a double meaning, or equivalently, cannot be given any meaning. The “meanings” derived this way are global for the argument as a whole. That is, they are not only constraints for each separate inference step of the argument. It is shown that the basic examples of paradoxes, the liar paradox and Russell’s paradox, are self-contradictory. Self-contradiction is not only a structure of paradoxes but is found also in proofs using self-reference. Self-contradiction is formalized in natural deduction systems for naïve set theory, and it is shown that self-contradiction is related to normalization. Non-normalizable deductions are self-contradictory.

## Keywords

Paradox Proof structure Self-contradiction Proof theory Russell’s paradox## 1 Introduction

Let us consider Russell’s paradox:

Let

tbe the set of all sets not containing themselves. Assume thattcontains itself. Hence, by the definition oft,tdoes not contain itself. This contradicts the assumption thattcontains itself and hencetdoes not contain itself. Sincetdoes not contain itself, it follows from the definition oftthattcontains itself. This is a contradiction.

Let us take a closer look at the part of Russell’s paradox that proves that *t**does not contain itself*. Let \(\mathscr {E_R}\) be this part of Russell’s paradox. We observe that the assumption that *t**contains itself* is used twice in \(\mathscr {E_R}\). We shall now distinguish the use of an assumption from how it is used. Let us therefore, to express that an assumption is used in an argument, say that the assumption *occurs* in an argument. Thus there are two occurrences of the assumption that *t**contains itself* in \(\mathscr {E_R}\). One of these two occurrences of the assumption that *t**contains itself* is used together with the definition of *t* to derive that *t**does not contain itself*. To contradict this last proposition the other occurrence of the assumption that *t**contains itself* is used. Hence, there are two occurrences of the assumption that *t**contains itself* in \(\mathscr {E_R}\), and they are used in such a way that they contradict each other. In the last step of \(\mathscr {E_R}\), the conclusion that *t**does not contain itself* is drawn from the contradiction that the assumption that *t**contains itself* leads to. In a sense the two occurrences of the assumption that *t**contains itself* are identified in this step. Considering the two occurrences of the assumption that *t**contains itself* as one and the same proposition, we have that there in \(\mathscr {E_R}\) is a proposition which is used in two ways and that the two ways of using the proposition are incompatible.

A *self-contradictory argument* is, informally, an argument, as \(\mathscr {E_R}\) above, in which there is a proposition which is used in two or more ways such that not all of the ways of using the proposition are compatible. In this article we aim to make those ideas more precise and formally express the notion of self-contradictory reasoning in some formal systems.

## 2 Meaning Conditions

The notion of a self-contradictory argument as introduced in the previous section is based on “the way in which a proposition is used in an argument.” In this section we aim at making it more precise what we mean by this, and we will outline how the notion of a self-contradictory argument will be formally expressed in the succeeding sections. Given an argument and a proposition of this argument we shall in the following consider *the meaning forced on the proposition, by the steps of the argument*. The meaning forced on a proposition, by the steps of the argument, expresses precisely the way in which the proposition is used in the argument.

Let us consider an example. Let \(\mathscr {D}\) be the following argument: *The wind is blowing* because *it’s snowing and the wind is blowing*. Let *A* be the proposition *it’s snowing and the wind is blowing* and let *B* be the proposition *the wind is blowing*. Thus \(\mathscr {D}\) consists of one step and *A* and *B* are the premise and the conclusion, respectively, of this step. If we forget about which propositions *A* and *B* represent we still know something about them by remembering what kind of step the inference of \(\mathscr {D}\) is. That is, knowing only that the inference of \(\mathscr {D}\) is of the kind that informally corresponds to one of the \( \mathbin { \& }\mathrm {E}\) inference schemata in natural deduction for naïve set theory \(\mathbf {N}\) (see Appendix below), we know that since *A* is the premise of the step, *A* is \(A_1\)*and*\(A_2\) for some propositions \(A_1\) and \(A_2\). Moreover, if *A* is \(A_1\)*and*\(A_2\) then *B* is \(A_2\). The meaning forced on the propositions *A* and *B* by the inference of \(\mathscr {D}\) is this knowledge about *A* and *B* given by the knowledge about what kind of step the inference of \(\mathscr {D}\) is. Hence the meaning of *the meaning forced on the proposition, by the steps of the argument* depends on what is considered to be known, when knowing only what kind of steps the steps of the argument are.

In the previous section, “a self-contradictory argument” was explained to be an argument in which there is a proposition which is used in two or more ways such that not all of the ways of using the proposition are compatible. In this section “the meaning forced on a proposition, by the steps of the argument” expresses precisely the way in which the proposition is used in the argument. Hence, we can explain what “a self-contradictory argument” is by saying that it is an argument such that the steps of the argument force several meanings on one of the propositions of the argument and that not all of these meanings are compatible. Yet another way to put this is to say that an argument is self-contradictory if and only if the steps of the argument force an *ambiguous meaning* on one of the propositions of the argument. Note that, as is clear from the example above, the meaning forced on a proposition by an argument is not an interpretation of the proposition but a constraint on how it may be interpreted.

*the meaning of a proposition*mean an interpretation of the proposition. For instance,

*the wind is blowing*is the meaning of the proposition

*B*in the example above. Let

*A*be a formula occurrence in a deduction in some formal system. To denote that

*A*has a certain meaning,

*m*say, we decorate

*A*with

*m*. More precisely, we shall write

*m*:

*A*to denote that

*A*has the meaning

*m*. We use these decorations to define

*meaning conditions*. Meaning conditions are formal representations of the constraints given by the meaning forced on a proposition by an argument. For every formal system considered in this article we shall do the following. We shall define what the set of formal meanings is for decorating the formulas in deductions in the formal system and we shall give the meaning conditions associated with the formal system. Thus, through the meaning conditions we formally define what is informally described by “the way in which a proposition is used in an argument.” By an

*assignment*of meanings to the formulas in a deduction we mean a decoration of all of the formulas in the deduction. That a meaning is

*assigned*to a formula means that the formula has been decorated with the meaning. The meaning conditions are given as constraints on the decorations, by formal meanings, of the formulas in the deductions. As an example let us consider, in the formal system \(\mathbf {N}\), a deduction consisting of an \(\mathbin {\supset }\mathrm {E}\) inference, \(\alpha \) say. Let

*X*,

*Y*and

*Z*be the major premise, the minor premise and the conclusion, respectively, of \(\alpha \). Let \(m_x\), \(m_y\) and \(m_z\) denote some meanings assigned to

*X*,

*Y*and

*Z*, respectively. We decorate the formulas in the deduction as follows.Reasoning in the same way as in the previous example, we know that since

*X*is the major premise of an \(\mathbin {\supset }\mathrm {E}\) inference,

*X*must be \(X_1 \mathbin {\supset }X_2\) for some propositions \(X_1\) and \(X_2\). We express this constraint by requiring the meaning \(m_x\) to be \(m \Rightarrow n\) for some meanings

*m*and

*n*, where thus \(\Rightarrow \) means “implies that.” Moreover we require \(m_y\) to be

*m*and \(m_z\) to be

*n*. Thus, \(m_x\) may not be

*it’s snowing and the wind is blowing*. However \(m_y\) may be

*it’s snowing and the wind is blowing*and \(m_z\) may be

*the wind is blowing*. In this case \(m_x\) must be

*it’s snowing and the wind is blowing implies that the wind is blowing*. We express meaning conditions given for any \(\mathbin {\supset }\mathrm {E}\) inference in any deduction in the formal system \(\mathbf {N}\) by the schemaHence the meaning condition for the major premise

*A*of an \(\mathbin {\supset }\mathrm {E}\) inference is that

*A*must have the meaning \(m \Rightarrow n\) for some meanings

*m*and

*n*. Moreover, the meanings of the major premise, the minor premise and the conclusion respectively must have the relation to each other expressed by the schema. The notion of a self-contradictory deduction in a formal system is defined as follows.

### **Definition 1**

Assume that \(\mathbf {F}\) is a formal system. Assume that the set of formal meanings for decorating the formulas in the deductions in \(\mathbf {F}\) are defined, and assume that the meaning conditions associated with the formal system are given in some way. Then a deduction \(\mathscr {D}\) in \(\mathbf {F}\) is *self-contradictory* if there is no assignment of formal meanings to the formulas in \(\mathscr {D}\) such that this assignment satisfies the meaning conditions.

The meaning conditions, as we shall give them, are related to the *inversion principle* of Prawitz. In Prawitz (1965) [6] we can read the following.

Observe that an elimination rule is, in a sense, the inverse of the corresponding introduction rule: by an application of an elimination rule one essentially only restores what had already been established if the major premise of the application was inferred by an application of an introduction rule.

We may say that, for a given deduction, the constraint expressed by the meaning conditions is an attempt to make the inversion principle global, in the deduction. But this attempt is successful if and only if the deduction is not self-contradictory, since otherwise there is no assignment of formal meanings to the formulas in the deduction such that this assignment satisfies the meaning conditions.

The Curry-Howard interpretation may resemble what designates meanings in the meaning conditions. However, the similarity is only superficial. In general, it is not the case that the assignment of Curry-Howard interpretations to the formula occurrences in a deduction satisfies the meaning conditions. Since the Curry-Howard interpretation is just a representation of an argument, there are always Curry-Howard interpretations of the formula occurrences in a deduction, but there need not be an assignment of formal meanings to the formulas in the deduction such that this assignment satisfies the meaning conditions.

## 3 The Liar Paradox

In this section we shall study the liar paradox as an example of a self-contradictory argument. The liar paradox is the following.

Let

Pbe the sentence “This sentence is false.” That is,Pis the sentence “Pis false.” AssumeP. Hence, by the definition ofP,Pis false. This contradicts the assumptionP, and hencePis false. SincePis false,Pfollows from the definition ofP. This is a contradiction.

*P*are formulas, and if

*A*and

*B*are formulas, then \(A \mathbin {\supset }B\) is a formula; \(\lnot A\) is defined to be \(A \mathbin {\supset }\mathord {\perp }\). The inference schemata of \(\mathbf {FP}\) are the following.

*x*is a meaning, and if

*m*and

*n*are meanings, then

*pm*and \(m \Rightarrow n\) are meanings. We may interpret the meanings as follows: \(m \Rightarrow n\) means “

*m*implies that

*n*,” and

*pm*means “This sentence is false,” where “This” refers to the sentence expressed by

*m*. The meaning conditions associated with the formal system \(\mathbf {FP}\) are the following.

Now assume that there is an assignment of formal meanings to the formulas in the deduction \(\mathscr {F}\) above such that this assignment satisfies the meaning conditions.

*m*is the meaning of the minor premise

*P*of the \(\mathbin {\supset }\mathrm {E}\) inference and that

*n*is the meaning of the conclusion \(\mathord {\perp }\) of the \(\mathbin {\supset }\mathrm {E}\) inference. Then, by the conditions above we conclude that the meaning of the premise

*P*of the \(\mathrm {PE}\) inference must be \(p(m \Rightarrow n)\).The condition given for the \(\mathbin {\supset }\mathrm {I}\) inference schema requires both of the formulas cancelled at the \(\mathbin {\supset }\mathrm {I}\) inference in \(\mathscr {F}\) to have the same meaning. However, no matter how we choose

*m*and

*n*the meanings

*m*and \(p(m \Rightarrow n)\) are not the same. Hence, there is no assignment of formal meanings to the formulas in \(\mathscr {F}\) such that this assignment satisfies the meaning conditions. Hence, \(\mathscr {F}\) is self-contradictory.

## 4 Self-contradictory Reasoning in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\)

Let \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) be the fragment of \(\mathbf {N}\) obtained by removing the symbols \(\mathord {\forall }\), \(\mathord {\exists }\) and \(=\) and the inference schemata corresponding to these symbols from \(\mathbf {N}\). In this section we shall study the notion of self-contradictory deductions in the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\). We shall also prove the following theorem.

### **Theorem 1**

Every non-self-contradictory deduction in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) is normalizable.

In this section and the two succeeding ones we shall use the terminology of Ekman (1994) [2, Sect. 3.1], see Appendix below. Hence, by “normalizable” in Theorem 1 we mean normalizable as defined in Ekman (1994) [2, Sect. 3.1], see Appendix below. As in the formal system \(\mathbf {FP}\), *m* and *n* denote meanings.

*A*is a formula such that there is no normal proof of

*A*in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\). Then, by Proposition 3.1.4 in Ekman (1994) [2] there is no normalizable proof of

*A*in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\). Hence by Theorem 1 every proof of

*A*is self-contradictory. Since there is no normal proof of \(\mathord {\perp }\) in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) it follows that every paradox in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) is self-contradictory, if by paradox we mean a proof of \(\mathord {\perp }\). In Ekman (1994) [2, Sect. 2.1] it is shown that there is no normal proof of the formula \(t \notin u\), where

*t*is the term defined byHence, every proof of \(t \notin u\) in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) is self-contradictory. In Ekman (1994) [2, Sect. 2.1] also a proof, named Crabbe’s counterexample (see Crabbé (1974) [1]), of the formula \(t \notin u\) is presented. This proof is a proof in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) and hence Crabbe’s counterexample is a self-contradictory proof. It is also argued in Ekman (1994) [2, Sect. 2.1] that Crabbe’s counterexample expresses a correct argument in \(\mathbf {ZF}\). Hence the formula \(t \notin u\), or the proposition that informally corresponds to \(t \notin u\), serves as an example of a proposition provable in \(\mathbf {ZF}\), but only by self-contradictory proofs, unless we use proof principles not expressible in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\).

*meaning variables*. The set of formal meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) is inductively defined as follows. The meaning variable

*x*and \( false \) are meanings, and if

*m*and

*n*are meanings, then \(\epsilon m\), \(m \Rightarrow n\), \(m \mathbin {\wedge }n\) and \(m + n\) are meanings. The meaning conditions associated with the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) are the following.

### **Lemma 1**

If a deduction \(\mathscr {D}\) is non-self-contradictory and \(\mathscr {D}\) reduces to \(\mathscr {E}\), then also the deduction \(\mathscr {E}\) is non-self-contradictory.

### *Proof*

Let \(\theta \) be an assignment of formal meanings to the formula occurrences in \(\mathscr {D}\) such that this assignment satisfies the meaning conditions. Then \(a(\theta ,\mathscr {E},\mathscr {D})\) is an assignment of formal meanings to the formula occurrences in \(\mathscr {E}\) such that this assignment satisfies the meaning conditions.\(\square \)

*P*in the language of \(\mathbf {P}\). Let \(^*\) be the function from the set of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) onto the set of formulas of \(\mathbf {P}\), defined as follows.We extend \(^*\) to a function from the set of sets of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) onto the set of sets of formulas of \(\mathbf {P}\) by letting \(\varGamma ^*\) denote the set of formulas \(A^*\) such that

*A*belongs to \(\varGamma \), for all sets of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\).

*m*:

*A*, then \(\mathscr {D}^*\) is the open assumption \(m^*\):Observe that there is no open assumption of the form \(\mathord {\perp }\mathbin {\supset }\mathord {\perp }\) in \(\mathscr {D}^*\), cancelled at the \(\mathbin {\supset }\mathrm {I}\) inference, in the deduction to the right above.For all other cases of the end inference of a deduction \(\mathscr {D}\), the definition of \(\mathscr {D}^*\) commutes with the definition of deduction. For instance, for the case that an \(\mathbin {\supset }\mathrm {I}\) is the last inference of a deduction, we have the following clause defining the image under \(^*\) of this deduction:

### **Proposition 1**

Assume that \(\mathscr {D}\) is a non-self-contradictory deduction, \(\theta \) is an assignment of formal meanings to the formula occurrences in \(\mathscr {D}\) such that this assignment satisfies the meaning conditions and \(\mathscr {D} \Longrightarrow \mathscr {E}\). Let \(\mathscr {D}\) also denote the deduction obtained from \(\mathscr {D}\) by decorating the formula occurrences in \(\mathscr {D}\) with \(\theta \). Let \(\mathscr {E}\) also denote the deduction obtained from \(\mathscr {E}\) by decorating the formula occurrences in \(\mathscr {E}\) with \(a(\theta ,\mathscr {E},\mathscr {D})\). Then \(\mathscr {D}^* \Longrightarrow \mathscr {E}^*\).

Since \(\mathbf {P}\) is strongly normalizable (see Prawitz (1965) [6]), we have Theorem 1 as a consequence of Proposition 1.

## 5 Self-contradictory Reasoning in \(\mathbf {N}_{-\mathord {\exists }=}\)

Under the assumption that meaning conditions formally express the way in which a proposition is used, as outlined in Sect. 2, it is a bit more complicated to define the meaning conditions associated with a formal system with quantifiers than it is to define the meaning conditions associated with a quantifier-free formal system. In this section we shall study the notion of self-contradictory deductions in the formal system \(\mathbf {N}_{-\mathord {\exists }=}\), which is the fragment of \(\mathbf {N}\) obtained by removing the symbols \(\mathord {\exists }\) and \(=\) and the inference schemata corresponding to these symbols from \(\mathbf {N}\). We shall also prove the following theorem.

### **Theorem 2**

Every non-self-contradictory deduction in \(\mathbf {N}_{-\mathord {\exists }=}\) is normalizable.

Let *A* be any formula. To define the meaning conditions associated with the formal system \(\mathbf {N}_{-\mathord {\exists }=}\) we shall informally consider \(\mathord {\forall }x A\) to represent the informally given infinitely long formula \( A[t_1/x] \mathbin { \& }(A[t_2/x] \mathbin { \& }(A[t_3/x] \mathbin { \& }\ldots ))\), where \(t_1,t_2,t_3,\ldots \) are all terms of the formal system \(\mathbf {N}_{-\mathord {\exists }=}\).

*m*.With meaning conditions given this way, we require that there is a one to one correspondence between the meaning of the premise and the conclusion both for \(\mathord {\forall }\mathrm {I}\) inferences and for \(\mathord {\forall }\mathrm {E}\) inferences. This condition is however too strong, if we consider \(\mathord {\forall }x A\) to represent the informally given infinitely long formula above, since the meaning conditions given for \( \mathbin { \& }\mathrm {E}\) inferences does not require that there is a one to one correspondence between the meaning of the premise and the conclusion of an \( \mathbin { \& }\mathrm {E}\) inference. As an example, consider the following deduction.This deduction is non-self-contradictory independently of which formulas

*A*and

*C*are. It is straightforward to assign meanings to the formula occurrences of the deduction above such that this assignment satisfies the meaning conditions. Assume that

*C*is \(\mathord {\forall }x (r \in x)\) and let us consider

*C*to represent the informally given formula \( (r \in t_1) \mathbin { \& }((r \in t_2) \mathbin { \& }((r \in t_3) \mathbin { \& }\ldots ))\), where \(t_1,t_2,t_3,\ldots \) are all terms of the formal system \(\mathbf {N}_{-\mathord {\exists }=}\). Then \( r \in \{y \mid A\} \mathbin { \& }(r \in \{y \mid \lnot A\} \mathbin { \& }C)\) and

*C*informally represent the same formula. We have the following proof of \(\lnot C\), which from an informal point of view is another presentation of the deduction above.This deduction is self-contradictory if the meaning conditions are given as above.

*x*and \( false \) are meanings, and if

*m*and

*n*are meanings, then \(\epsilon m\), \(m \Rightarrow n\), \(m \mathbin {\wedge }n\), \(m + n\) and \(\lambda x.m\) are meanings. The meaning conditions are the following and in addition the meaning conditions associated with the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\).We have the restriction on the meaning variable, designated

*x*, in the \(\mathord {\forall }\mathrm {I}\) meaning condition schema that it may not occur free in any meaning assigned to an open assumption in \(\mathscr {D}\). This restriction excludes, for instance, the following decoration of a deduction.Remember that the aim is to define the meaning conditions so that the meaning conditions express a constraint given by the meaning forced on a proposition given by an argument, in the sense of Sect. 2. Remember also that the meaning forced on a proposition given by an argument is arbitrary so far as what is considered to be known is arbitrary, when knowing only what kind of steps the steps of the argument are. We do not claim that the meaning conditions given are the only possible. The given meaning conditions express constraints which we judge as accurate.

*x*does not occur free in

*A*. Then the constraint defined by the meaning conditions can be strengthened so that \(m_2\) and \(m_1\) are required to be syntactically equal. More generally, if

*x*occurs free in

*A*we can strengthen the constraint defined by the meaning conditions so that, in an informal sense, if one “submeaning” of \(m_2\) and one “submeaning” of \(m_1\) “correspond” to the same subformula of

*A*, and

*x*does not occur free in this subformula, then these “submeanings” of \(m_1\) and \(m_2\), respectively, are required to be syntactically equal.

In the following we shall not assume this last restriction to be added. Of course, if Theorem 2 holds without this restriction added to the restrictions of the meaning conditions, then this theorem also is true with this restriction added.

*r*is an arbitrary term. We then have the \(\mathord {\perp }\mathrm {E}\) inference schema as a derived schema, derived as follows, where

*x*is supposed to be chosen so that

*x*does not occur free in

*A*.Then if we also take \( false \) to be defined by \(\lambda x.\epsilon x\) we have the \(\mathord {\perp }\mathrm {E}\) meaning condition schema as a derived meaning condition schema, derived from the meaning condition schemata \(\mathord {\forall }\mathrm {E}\) and \(\mathord {\in } \mathrm {E}\).

### **Lemma 2**

If a deduction \(\mathscr {D}\) is non-self-contradictory and \(\mathscr {D}\) reduces to \(\mathscr {E}\) then also the deduction \(\mathscr {E}\) is non-self-contradictory.

*A*and

*B*are formulas, then \(A \mathbin {\supset }B\), \( A \mathbin { \& }B\), \(A \mathbin {\vee }B\) and \(\mathord {\forall }X A\) are formulas. The \( \mathord {\perp }, \mathbin {\supset }, \mathbin { \& }\) and \(\mathbin {\vee }\) inference schemata are the same for \(\mathbf {P^2}\) as for the formal system \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\). The \(\mathord {\forall }\) inference schemata for \(\mathbf {P^2}\) are the following.We have the restriction on deductions in \(\mathbf {P^2}\) that the variable designated

*X*in the \(\mathord {\forall }\mathrm {I}\) schema may not occur free in any open assumption in the deduction designated \(\mathscr {D}\). The reduction rules for deductions in \(\mathbf {P^2}\) are the same as the reduction rules for deductions in \(\mathbf {N}_{-\mathord {\exists }=}\) except that the substitution of a term for a variable in the \(\mathord {\forall }\)-reduction in \(\mathbf {N}_{-\mathord {\exists }=}\) corresponds, in \(\mathbf {P^2}\), to a substitution of a proposition for a propositional variable. We presuppose that the set of variables of \(\mathbf {N}_{-\mathord {\exists }=}\) and the set of propositional variables of \(\mathbf {P^2}\) have the same cardinality. Hence there is a one to one correspondence, \(^{*}\) say, between the set of variables of \(\mathbf {N}_{-\mathord {\exists }=}\) and the set of propositional variables of \(\mathbf {P^2}\). For any variable

*x*of \(\mathbf {N}_{-\mathord {\exists }=}\) we let the propositional variable

*X*of \(\mathbf {P^2}\) denote \(x^{*}\). The function \(^*\) from the set of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\exists }=}\) onto the set of formulas of \(\mathbf {P^2}\) is defined as follows.The function \(^*\) is extended to a function from the set of sets of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\exists }=}\) onto the set of sets of formulas of \(\mathbf {P^2}\) by letting \(\varGamma ^*\) denote the set of formulas \(A^*\) such that

*A*belongs to \(\varGamma \), for all sets \(\varGamma \) of meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-\mathord {\exists }=}\). In a similar way as in Sect. 4 we extend \(^*\) once more, to a function from the set of non-self-contradictory deductions in \(\mathbf {N}_{-\mathord {\exists }=}\) to the set of deductions in \(\mathbf {P^2}\). To define this function we add the following clauses to the definition of the function \(^*\) in Sect. 4.The definition of \(a(\theta ,\mathscr {E},\mathscr {D})\), given in Sect. 4, extends from deductions in \(\mathbf {N}_{-\mathord {\forall }\mathord {\exists }=}\) to deductions in \(\mathbf {N}_{-\mathord {\exists }=}\) by defining \(a(\theta ,\mathscr {E},\mathscr {D})\) also in the case \(\mathscr {D}\) reduces to \(\mathscr {E}\) via an \(\mathord {\forall }\) reduction. This is done in a similar way as for the other cases of the kind of reduction that takes \(\mathscr {D}\) to \(\mathscr {E}\).

### **Proposition 2**

Assume that \(\mathscr {D}\) is a non-self-contradictory deduction, \(\theta \) is an assignment of formal meanings to the formula occurrences in \(\mathscr {D}\) such that this assignment satisfies the meaning conditions and \(\mathscr {D} \Longrightarrow \mathscr {E}\). Let \(\mathscr {D}\) also denote the deduction obtained from \(\mathscr {D}\) by decorating the formula occurrences in \(\mathscr {D}\) with \(\theta \). Let \(\mathscr {E}\) also denote the deduction obtained from \(\mathscr {E}\) by decorating the formula occurrences in \(\mathscr {E}\) with \(a(\theta ,\mathscr {E},\mathscr {D})\). Then \(\mathscr {D}^* \Longrightarrow \mathscr {E}^*\).

From Girard (1971) [4] it is known that deductions in \(\mathbf {P^2}\) are strongly normalizable; see also Martin-Löf (1971) [5]. From this together with Proposition 2, Theorem 2 follows.

## 6 Self-contradictory Reasoning in \(\mathbf {N}_{-=}\)

*x*and \( false \) are meanings, and if

*m*and

*n*are meanings, then \(\epsilon m\), \(m \Rightarrow n\), \(m \mathbin {\wedge }n\), \(\lambda x.m\) and \(\mu x.m\) are meanings. The meaning conditions associated with the formal system \(\mathbf {N}_{-=}\) are the following, and in addition the meaning conditions associated with the formal system \(\mathbf {N}_{-\mathord {\exists }=}\).We have the restriction on the meaning variable designated

*x*in the \(\mathord {\exists }E\) meaning condition schema that neither may it occur free in the meaning designated

*n*assigned to the subsequent premise of the \(\mathord {\exists }E\) nor may it occur free in any meaning assigned to an open assumption of the deduction of the subsequent premise \(\mathscr {E}\) other than the open assumption designated

*A*.

### **Theorem 3**

Every non-self-contradictory deduction in \(\mathbf {N}_{-=}\) is normalizable.

Let \(\mathbf {PR}\) be the formal system with the same language as \(\mathbf {N}_{-=}\), obtained by removing the \(\in \)-inferences from \(\mathbf {N}\). We have the following result concerning \(\mathbf {PR}\).

### **Proposition 3**

Every deduction in \(\mathbf {PR}\) is non-self-contradictory.

### *Proof*

Let \(\mathscr {D}\) be any given deduction in \(\mathbf {PR}\). We shall define an assignment of formal meanings to the formulas in \(\mathscr {D}\) such that this assignment satisfies the meaning conditions. This assignment is defined by decorating every formula occurrence *A* in \(\mathscr {D}\) with the formal meaning \(A^\circ \), where \(^\circ \) is a function from the set of formulas of \(\mathbf {PR}\) to the set of formal meanings to be assigned to formulas in deductions in the formal system \(\mathbf {N}_{-=}\). The bijection \(^\circ \) is defined as follows.

## References

- 1.Crabbé, M.: Non-normalisation de la théorie de Zermelo. http://www.logic-center.be/Publications/Bibliotheque/contreexemple.pdf (sketch of the result presented at the Proof Theory Symposium held in Kiel in 1974)
- 2.Ekman, J.: Normal proofs in set theory. Ph.D. thesis, Department of Computing Science, University of Göteborg (1994)Google Scholar
- 3.Gentzen, G.: Untersuchungen über das logische Schließen. Mathematische Zeitschrift, 39, 176-210, 405-431 (1934/35) (English translation in: The Collected Papers of Gerhard Gentzen (Szabo, M.E. (ed.). Amsterdam, North Holland (1969), pp. 68-131)Google Scholar
- 4.Girard, J.-Y.: Une extension de l’interprétation de Gödel à l’analyse, et son application à l’élimination des coupures dans l’analyse et la théorie des types. In: Fenstad, J.E. (ed.) Proceedings of the Second Scandinavian Logic Symposium, pp. 63-92. North-Holland, Amsterdam (1971)Google Scholar
- 5.Martin-Löf, P.: Hauptsatz for the theory of species. In: Fenstad, J.E. (ed.) Proceedings of the Second Scandinavian Logic Symposium, pp. 217-233. North-Holland, Amsterdam (1971)Google Scholar
- 6.Prawitz, D.: Natural Deduction: A Proof-Theoretical Study. Almqvist & Wiksell, Stockholm (1965). Reprinted Dover Publ., Mineola 2006Google Scholar
- 7.Prawitz, D.: Ideas and results in proof theory. In: Fenstad, J.E. (ed.) Proceedings of the Second Scandinavian Logic Symposium (Oslo 1970), pp. 235–307. North-Holland, Amsterdam (1971)CrossRefGoogle Scholar

## Copyright information

**Open Access** This chapter is distributed under the terms of the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.