Proof Theory

This paper surveys the common approach to quantification and generalised quantification in formal linguistics and philosophy of language. We point out how this general setting departs from empirical linguistic data, and give some hints for a different view based on proof theory, which on many aspects gets closer to the language itself. We stress the importance of Hilbert's oper- ator epsilon and tau for, respectively, existential and universal quantifications. Indeed, these operators help a lot to construct semantic representation close to (...) natural language, in particular with quantified noun phrases as individual terms. We also define guidelines for the design of the proof rules corresponding to generalised quantifiers. (shrink)

Reverse mathematics studies which subsystems of second order arithmetic are equivalent to key theorems of ordinary, non-set-theoretic mathematics. The main philosophical application of reverse mathematics proposed thus far is foundational analysis, which explores the limits of different foundations for mathematics in a formally precise manner. This paper gives a detailed account of the motivations and methodology of foundational analysis, which have heretofore been largely left implicit in the practice. It then shows how this account can be fruitfully applied in the (...) evaluation of major foundational approaches by a careful examination of two case studies: a partial realization of Hilbert’s program due to Simpson [1988], and predicativism in the extended form due to Feferman and Schütte. -/- Shore [2010, 2013] proposes that equivalences in reverse mathematics be proved in the same way as inequivalences, namely by considering only omega-models of the systems in question. Shore refers to this approach as computational reverse mathematics. This paper shows that despite some attractive features, computational reverse mathematics is inappropriate for foundational analysis, for two major reasons. Firstly, the computable entailment relation employed in computational reverse mathematics does not preserve justification for the foundational programs above. Secondly, computable entailment is a Pi-1-1 complete relation, and hence employing it commits one to theoretical resources which outstrip those available within any foundational approach that is proof-theoretically weaker than Pi-1-1-CA0. (shrink)

While non-classical theories of truth that take truth to be transparent have some obvious advantages over any classical theory that evidently must take it as non-transparent, several authors have recently argued that there's also a big disadvantage of non-classical theories as compared to their “external” classical counterparts: proof-theoretic strength. While conceding the relevance of this, the paper argues that there is a natural way to beef up extant internal theories so as to remove their proof-theoretic disadvantage. It is suggested that (...) the resulting internal theories should seem preferable to their external counterparts. (shrink)

I give an account of proof terms for derivations in a sequent calculus for classical propositional logic. The term for a derivation δ of a sequent Σ≻Δ encodes how the premises Σ and conclusions Δ are related in δ. This encoding is many–to–one in the sense that different derivations can have the same proof term, since different derivations may be different ways of representing the same underlying connection between premises and conclusions. However, not all proof terms for a sequent Σ≻Δ (...) are the same. There may be different ways to connect those premises and conclusions. -/- Proof terms can be simplified in a process corresponding to the elimination of cut inferences in sequent derivations. However, unlike cut elimination in the sequent calculus, each proof term has a unique normal form (from which all cuts have been eliminated) and it is straightforward to show that term reduction is strongly normalising—every reduction process terminates in that unique normal form. Further- more, proof terms are invariants for sequent derivations in a strong sense—two derivations δ1 and δ2 have the same proof term if and only if some permutation of derivation steps sends δ1 to δ2 (given a relatively natural class of permutations of derivations in the sequent calculus). Since not every derivation of a sequent can be permuted into every other derivation of that sequent, proof terms provide a non-trivial account of the identity of proofs, independent of the syntactic representation of those proofs. (shrink)

The paper begins with a conceptual discussion of Michael Dummett's proof-theoretic justification of deduction or proof-theoretic semantics, which is based on what we might call Gentzen's thesis: 'the introductions constitute, so to speak, the "definitions" of the symbols concerned, and the eliminations are in the end only consequences thereof, which could be expressed thus: In the elimination of a symbol, the formula in question, whose outer symbol it concerns, may only "be used as that which it means on the basis (...) of the introduction of this symbol".' The intuitive philosophical content of Dummett's notions of harmony and stability is that harmony obtains if the grounds for asserting a proposition match the consequences of accepting it, and stability obtains if the converse also holds. Rules of inference define the meanings of a logical constant they govern if and only if they are stable. Gentzen observed that 'it should be possible to establish on the basis of certain requirements that the elimination rules are functions of the corresponding introduction rules.' One of the objectives of this paper is to specify such a function: I will specify a process by which it is possible to determine the elimination rules of logical constants from their introduction rules, and conversely, to determine the introduction rules from the elimination rules. I'll give the general forms of rules of inference and generalised reduction procedures for the normalisation of deduction. I'll give a formally precise characterisations of harmony and stability and show that deductions in logics that contain only constants governed by stable rules always normalise. (shrink)

This paper describes a decision procedure for disjunctions of conjunctions of anti-prenex normal forms of pure first-order logic (FOLDNFs) that do not contain V within the scope of quantifiers. The disjuncts of these FOLDNFs are equivalent to prenex normal forms whose quantifier-free parts are conjunctions of atomic and negated atomic formulae (= Herbrand formulae). In contrast to the usual algorithms for Herbrand formulae, neither skolemization nor unification algorithms with function symbols are applied. Instead, a procedure is described that rests on (...) nothing but equivalence transformations within pure first-order logic (FOL). This procedure involves the application of a calculus for negative normal forms (the NNF-calculus) with A -||- A & A (= &I) as the sole rule that increases the complexity of given FOLDNFs. The described algorithm illustrates how, in the case of Herbrand formulae, decision problems can be solved through a systematic search for proofs that reduce the number of applications of the rule &I to a minimum in the NNF-calculus. In the case of Herbrand formulae, it is even possible to entirely abstain from applying &I. Finally, it is shown how the described procedure can be used within an optimized general search for proofs of contradiction and what kind of questions arise for a &I-minimal proof strategy in the case of a general search for proofs of contradiction. (shrink)

The paper studies a cluster of systems for fully disquotational truth based on the restriction of initial sequents. Unlike well-known alternative approaches, such systems display both a simple and intuitive model theory and remarkable proof-theoretic properties. We start by showing that, due to a strong form of invertibility of the truth rules, cut is eliminable in the systems via a standard strategy supplemented by a suitable measure of the number of applications of truth rules to formulas in derivations. Next, we (...) notice that cut remains eliminable when suitable arithmetical axioms are added to the system. Finally, we establish a direct link between cut-free derivability in infinitary formulations of the systems considered and fixed-point semantics. Noticeably, unlike what happens with other background logics, such links are established without imposing any restriction to the premisses of the truth rules. (shrink)

This paper considers Rumfitt’s bilateral classical logic (BCL), which is proposed to counter Dummett’s challenge to classical logic. First, agreeing with several authors, we argue that Rumfitt’s notion of harmony, used to justify logical rules by a purely proof theoretical manner, is not sufficient to justify coordination rules in BCL purely proof-theoretically. For the central part of this paper, we propose a notion of proof-theoretical validity similar to Prawitz for BCL and proves that BCL is sound and complete respect to (...) this notion of validity. The major difficulty in defining validity for BCL is that validity of positive +A appears to depend on negative −A, and vice versa. Thus, the straightforward inductive definition does not work because of this circular dependance. However, Knaster-Tarski’s fixed point theorem can resolve this circularity. Finally, we discuss the philosophical relevance of our work, in particular, the impact of the use of fixed point theorem and the issue of decidability. (shrink)

When modeling informal proofs like that of Euclid’s Elements using a sound logical system, we go from proofs seen as somewhat unrigorous – even having gaps to be filled – to rigorous proofs. However, metalogic grounds the soundness of our logical system, and proofs in metalogic are not like formal proofs and look suspiciously like the informal proofs. This brings about what I am calling here the groundedness problem: how can we decide with certainty that our metalogical proofs are rigorous (...) and sustain our logical system? In this paper, I will expose this problem. I will not try to solve it here. (shrink)

We investigate involutive commutative residuated lattices without unit, which are commutative residuated lattice-ordered semigroups enriched with a unary involutive negation operator. The logic of this structure is discussed and the Genzten-style sequent calculus of it is presented. Moreover, we prove the decidability of this logic.

To appear in the Proceedings of Logic Colloquium 2006. (32 pages).

We present sound and complete sequent calculi for the modal mu-calculus with converse modalities, aka two-way modal mu-calculus. Notably, we introduce a cyclic proof system wherein proofs can be represented as finite trees with back-edges, i.e., finite graphs. The sequent calculi incorporate ordinal annotations and structural rules for managing them. Soundness is proved with relative ease as is the case for the modal mu-calculus with explicit ordinals. The main ingredients in the proof of completeness are isolating a class of non-wellfounded (...) proofs with sequents of bounded size, called slim proofs, and a counter-model construction that shows slimness suffices to capture all validities. Slim proofs are further transformed into cyclic proofs by means of re-assigning ordinal annotations. (shrink)

This article provides a language-theoretic rendering of Herbrand’s theorem. To each first-order proof is associated a higher-order recursion scheme that abstracts the computation of Herbrand sets obtained through Gentzen-style multicut elimination. The representation extends previous results in this area by lifting the prenex restriction on cut formulas and relaxing the cut-elimination strategies. Features of the new approach are the interpretation of cut as simple composition and contraction as ‘call with current continuation’.

The purpose of this paper is to present a formalization of the language, semantics and axiomatization of justification logic in Coq. We present proofs in a natural deduction style derived from the axiomatic approach of justification logic. Additionally, we present possible world semantics in Coq based on Fitting models to formalize the semantic satisfaction of formulas. As an important result, with this implementation, it is possible to give a proof of soundness for $\mathsf{L}\mathsf{P}$ with respect to Fitting models.

In this paper we provide a proof theoretical investigation of logical argumentation, where arguments are represented by sequents, conflicts between arguments are represented by sequent elimination rules, and deductions are made by dynamic proof systems extending standard sequent calculi. The idea is to imitate argumentative movements in which certain claims are introduced or withdrawn in the presence of counter-claims. This is done by a dynamic evaluation of sequences of sequents, in which the latter are considered ‘derived’ or ‘not derived’ according (...) to the content of the sequence. We show that decisive conclusions of such a process correspond to well-accepted consequences of the underlying argumentation framework. The outcome is therefore a general and modular proof-theoretical approach for paraconsistent and non-monotonic reasoning with argumentation systems. (shrink)

In this paper I will develop a lambda-term calculus, lambda-2Int, for a bi-intuitionistic logic and discuss its implications for the notions of sense and denotation of derivations in a bilateralist setting. Thus, I will use the Curry-Howard correspondence, which has been well-established between the simply typed lambda-calculus and natural deduction systems for intuitionistic logic, and apply it to a bilateralist proof system displaying two derivability relations, one for proving and one for refuting. The basis will be the natural deduction system (...) of Wansing's bi-intuitionistic logic 2Int, which I will turn into a term-annotated form. Therefore, we need a type theory that extends to a two-sorted typed lambda-calculus. I will present such a term-annotated proof system for 2Int and prove a Dualization Theorem relating proofs and refutations in this system. On the basis of these formal results I will argue that this gives us interesting insights into questions about sense and denotation as well as synonymy and identity of proofs from a bilateralist point of view. (shrink)

In this paper a framework to distinguish in a Fregean manner between sense and denotation of \(\lambda\)-term-annotated derivations will be applied to a bilateralist sequent calculus displaying two derivability relations, one for proving and one for refuting. Therefore, a two-sorted typed \(\lambda\)-calculus will be used to annotate this calculus and a Dualization Theorem will be given, stating that for any derivable sequent expressing a proof, there is also a derivable sequent expressing a refutation and vice versa. By having joint \(\lambda\)-term (...) annotations for proof systems in natural deduction and sequent calculus style, a comparison with respect to sense and denotation between derivations in those systems will be feasible, since the annotations elucidate the structural correspondences of the respective derivations. Thus, we will have a basis for determining in which cases, firstly, derivations expressing a proof vs. derivations expressing a refutation and, secondly, derivations in natural deduction vs. in sequent calculus can be identified and on which level. (shrink)

In derivations of a sequent system, $\mathcal{L}\mathcal{J}$, and a natural deduction system, $\mathcal{N}\mathcal{J}$, the trails of formulae and the subformula property based on these trails will be defined. The derivations of $\mathcal{N}\mathcal{J}$ and $\mathcal{L}\mathcal{J}$ will be connected by the map $g$, and it will be proved the following: an $\mathcal{N}\mathcal{J}$-derivation is normal $\Longleftrightarrow $ it has the subformula property based on trails $\Longleftrightarrow $ its $g$-image in $\mathcal{L}\mathcal{J}$ is without maximum cuts $\Longrightarrow $ that $g$-image has the subformula property based (...) on trails. In $\mathcal{L}\mathcal{J}$-derivations, another type of cuts, sub-cuts, will be introduced, and it will be proved the following: all cuts of an $\mathcal{L}\mathcal{J}$-derivation are sub-cuts $\Longleftrightarrow $ it has the subformula property based on trails. (shrink)

This paper clarifies, revises, and extends the account of the transmission of truthmakers by core proofs that was set out in chap. 9 of Tennant. Brauer provided two kinds of example making clear the need for this. Unlike Brouwer’s counterexamples to excluded middle, the examples of Brauer that we are dealing with here establish the need for appeals to excluded middle when applying, to the problem of truthmaker-transmission, the already classical metalinguistic theory of model-relative evaluations.

An original family of labelled sequent calculi $\mathsf {G3IL}^{\star }$ for classical interpretability logics is presented, modularly designed on the basis of Verbrugge semantics (a.k.a. generalised Veltman semantics) for those logics. We prove that each of our calculi enjoys excellent structural properties, namely, admissibility of weakening, contraction and, more relevantly, cut. A complexity measure of the cut is defined by extending the notion of range previously introduced by Negri w.r.t. a labelled sequent calculus for Gödel–Löb provability logic, and a cut-elimination (...) algorithm is discussed in detail. To our knowledge, this is the most extensive and structurally well-behaving class of analytic proof systems for modal logics of interpretability currently available in the literature. (shrink)

Lorenzen's article has immediately been recognised as a landmark in the history of infinitary proof theory. We propose a translation and this introduction in order to publicise its approach and method of proof, without any ordinal assignment. It is best known for providing a constructive proof of consistency for ramified type theory without axiom of reducibility by showing that it is a part of a trivially consistent ‘inductive calculus’ that describes our knowledge of arithmetic without detour; the proof resorts only (...) to the inductive definition of formulas and theorems. It proposes a definition of semilattices, of distributive lattices, of pseudocomplemented semilattices, and of countably complete boolean algebras as deductive calculi, and constructs conservatively the respective free objects over a given preordered set. It illustrates that lattice theory is a bridge between algebra and logic: the construction of an element corresponds to a step in a proof. The fruitfulness of its use of the ω-rule is immediately recognised by Schütte. It triggers the analysis by Ackermann (1951) of the infinitary inductive definition of the accessibility predicate with the goal of proving transfinite induction up to ordinal terms beyond ϵ0. (shrink)

This paper presents a comprehensive proof-theoretic analysis of Jaśkowski’s discussive (or discursive) logic, working with a set of connectives including classical negation and disjunction, as well as so-called (right-)discussive conjunction and discussive implication. By employing established techniques two labelled frameworks are introduced: sequent and natural deduction systems. The paper explores the ability of the proposed calculi to accurately represent Jaśkowski’s discussive logic, particularly in light of its paraconsistent nature, and establishes cut- admissibility and normalization theorems. Additionally, the introduced sequent calculus (...) – shown to allow terminating proof search – is employed to prove the embedding of discussive logic within modal logic S5. Finally, it is proved that the natural deduction calculus translates into the corresponding sequent system, with soundness and completeness established for both calculi. Concluding remarks highlight the potential for expanding this study and suggest directions for future research. (shrink)

In this paper, we investigate the proof theory of a modal expansion of intuitionistic propositional logic obtained by adding an 'actuality' operator among the connectives. This logic was initially considered by L. Humberstone, and, more recently, also by S. Niki and H. Omori to present a possible application of intuitionism to empirical discourse. Niki and Omori's idea to consider the notion of actuality based on intuitionistic logic was presented, among other things, using Gentzen sequents. Unfortunately, their proof system is not (...) cut-free. In this paper, we define another proof system for Niki and Omori’s logic, namely a hypersequent calculus, and present a proof of cut-elimination for it. (shrink)

On a bilateralist reading, sequents are interpreted as statements to the effect that, given the assertion of the antecedent it is incoherent to deny the succedent. This interpretation goes against its own ecumenical ambitions, endowing Cut with a meaning very close to that of tertium non datur and thus rendering it intuitionistically unpalatable. This paper explores a top-down route for arguing that, even intuitionistically, a prohibition to deny is as strong as a licence to assert.

The past decades have seen remarkable progress in the study of substructural logics, be it mathematically or philosophically oriented. This progress has a somewhat perplexing effect: the more subst...

In proof-theoretic semantics, meaning is based on inference. It may seen as the mathematical expression of the inferentialist interpretation of logic. Much recent work has focused on base-extension semantics, in which the validity of formulas is given by an inductive definition generated by provability in a ‘base’ of atomic rules. Base-extension semantics for classical and intuitionistic propositional logic have been explored by several authors. In this paper, we develop base-extension semantics for the classical propositional modal systems |$K$|⁠, |$KT$|⁠, |$K4$| and (...) |$S4$|⁠, with |$\square $| as the primary modal operator. We establish appropriate soundness and completeness theorems and establish the duality between |$\square $| and a natural presentation of |$\lozenge $|⁠. We also show that our semantics is in its current form not complete with respect to euclidean modal logics. Our formulation makes essential use of relational structures on bases. (shrink)

Según la sabiduría popular, la conectiva tonk definida por Prior trivializa cualquier teoría que la incluya. Sin embargo, no hay que olvidar que, si un argumento es lógicamente válido o no, depende en buena medida de la noción de consecuencia lógica subyacente. Casi siempre se asume que la consecuencia lógica es tarskiana, esto es, que es reflexiva, transitiva y monotónica. Sin embargo, Belnap había conjeturado que tonk podría no ser tan problemática en una lógica no transitiva, cosa que finalmente probó (...) Cook en 2005. En este artículo mejoramos el resultado de Cook en dos aspectos: nuestra hipótesis es más simple (a saber, usamos menos interpretaciones que él y no usamos una relación de consecuencia lógica disyuntiva en el definiendum) y no es ad hoc (a saber, nuestra relación de consecuencia lógica no está definida exclusivamente para evitar la trivialidad). (shrink)

Natural deduction inference rules seem to reflect the way we actually reason. Hence, many if not most inferentialist theories maintain that meaning is conferred on linguistic expressions by natural deduction rules, rather than the more abstract alternative of sequent rules. In the present paper, I argue, to the contrary, that an inferentialist theory of meaning must take a somewhat metainferential form, whereby the meanings of linguistic expressions—in particular, the logical constants—are conferred by sequent rules, conceived of as licensing inferences between (...) inferences. To that end, I propose that sequent rules are to be understood as inferential semantic clauses, i.e., as playing a role analogous to that of the semantic clauses in truth conditional theories of meaning. I establish this proposal on the basis of an argument to the effect that inferentialists must adopt an extended version of Frege's context principle, according to which only within a complete sentence potentially serving either as a premise or as a conclusion of an inference do words have meaning. The resultant inferentialist theory is not disconnected from the way we actually reason. Moreover, on its basis one can make a meaning-theoretic case for rather unintuitive features of many sequent calculi, such as multiple conclusions and various ways of going substructural. (shrink)

The roots of Kurt Gödel's functional interpretation in the Dialectica paper of 1958 go back to a lecture course from 1941 and a single lecture given at Yale in April 1941. Although the functional interpretation is first mentioned in 1938 in the context of the extended Hilbert Programme, the 1941 lectures mostly discuss the functional interpretation in the context of intuitionistic logic. In this article, I will examine the early development of the functional interpretation and its relationship to the 1938 (...) ‘Zilsel’ lecture, especially the critical remarks on Gentzen's consistency proof. It is shown that Gödel's functional interpretation was decidedly influenced by Gentzen, putting into question his earlier criticism of Gentzen's proof. (shrink)

Gentzen-style sequent calculi and Gentzen-style natural deduction systems are introduced for a family (C-family) of connexive logics over Wansing’s basic constructive connexive logic C. The C-family is derived from C by incorporating Peirce’s law, the law of excluded middle, and the generalized law of excluded middle. Natural deduction systems with general elimination rules are also introduced for the C-family. Theorems establishing the equivalence between the proposed sequent calculi and natural deduction systems are demonstrated. Cutelimination and normalization theorems are established for (...) the proposed sequent calculi and natural deduction systems, respectively. Additionally, similar results are obtained for a family (N-family) of paraconsistent logics over Nelson’s constructive four-valued logic N4. (shrink)

First, G0-style sequent calculi for extended Belnap–Dunn and intuitionistic logics, including Nelson and Gurevich logics, are introduced. A theorem establishing the equivalence between G0- and G3-style sequent calculi for these logics is then presented, and the cut-elimination theorem for these G0-style calculi is obtained as a result. Next, natural deduction systems with general elimination rules are introduced for these logics, and a full normalization theorem for these natural deduction systems is proved. This proof is achieved using bi-directional translations between the (...) proposed G0-style sequent calculi and the natural deduction systems with general elimination rules. (shrink)

G3-style sequent calculi are introduced for a family of logics with strong negation: Gurevich logic, Nelson logic, intuitionistic propositional logic, Avron logic, De-Omori logic, and classical propositional logic. Structural properties including cut elimination are established for these calculi. In addition, a Glivenko theorem for embedding classical propositional logic into Gurevich logic is shown.

This paper gives a broad account of the various sequent-based proof formalisms in the proof-theoretic literature. We consider formalisms for various modal and tense logics, intuitionistic logic, conditional logics, and bunched logics. After providing an overview of the logics and proof formalisms under consideration, we show how these sequent-based formalisms can be placed in a hierarchy in terms of the underlying data structure of the sequents. We then discuss how this hierarchy can be traversed using translations. Translating proofs up this (...) hierarchy is found to be relatively straightforward while translating proofs down the hierarchy is substantially more difficult. Finally, we inspect the prevalent distinction in structural proof theory between ‘internal calculi’ and ‘external calculi.’ We discuss the ambiguities involved in the informal definitions of these categories, and we critically assess the properties that (calculi from) these classes are purported to possess. (shrink)

Hilbert’s Program in the 1920s aimed to give finitary consistency proofs for infinitary mathematics, thus putting infinitary mathematics on a more secure footing. There is a popular narrative that Hilbert’s Program was decisively refuted by Gödel’s incompleteness theorems in 1931. However, Gödel himself, in a remarkable paper of 1958, pursues a modified version of Hilbert’s Program: he presents his Dialectica interpretation as a new, Hilbert-style consistency proof for arithmetic based on “an extension of the finitary standpoint,” and he clearly regards (...) this proof as epistemologically significant. So, is there any truth to the claim that Hilbert’s Program was refuted by the incompleteness theorems? In this essay, I explain and assess the epistemological project that Gödel sets out in his Dialectica paper. Ultimately, I argue that the Dialectica interpretation is best understood, not as a consistency proof, but as a way of assigning a constructive meaning to arithmetic. (shrink)

The paper provides a cut-free sequent calculus for the theory of tight apartness in a language where both apartness and equality are primitive notions. The result is obtained by aptly modifying the underlying logical calculus for intuitionistic logic and adding rules of inference corresponding to the axioms of apartness and the principles governing the mutual deductive relationships between apartness and equality. While the rules for apartness are found directly from the axioms by applying standard proof-theoretic methods, the others, especially the (...) rule corresponding to the principle of tight apartness, are found by exploiting the logical law of _consequentia mirabilis_. Along the way, we also provide a cut-free sequent calculus for the theory of weak tight apartness, also known as the theory of negated equality, thus answering in the positive to an open problem in the existing literature on the subject. (shrink)

In this paper we show how to extend the standard cut-elimination procedure from first-order intuitionistic stable logic to a class of intuitionistic stable theories. Building on previous works by Negri and von Plato, we aptly modify the underlying calculus for first-order intuitionistic logic so as to preserve the admissibility of all the structural rules, including cut, in the presence of a restricted version of the rule of classical reductio ad absurdum and of a special case of universal rules.

I show that the logic $\textsf {TJK}^{d+}$, one of the strongest logics currently known to support the naive theory of truth, is obtained from the Kripke semantics for constant domain intuitionistic logic by dropping the requirement that the accessibility relation is reflexive and only allowing reflexive worlds to serve as counterexamples to logical consequence. In addition, I provide a simplified natural deduction system for $\textsf {TJK}^{d+}$, in which a restricted form of conditional proof is used to establish conditionals.

It is an unusual property for a logic to prove a formula and its negation without ending up in triviality. Some systems have nonetheless been observed to satisfy this property: one group of such non-trivial negation inconsistent logics has its archetype in H. Wansing’s constructive connexive logic, whose negation-implication fragment already proves contradictions. N. Francez and Y. Weiss subsequently investigated relevant subsystems of this fragment, and Weiss in particular showed that they remain negation inconsistent. In this note, we take a (...) closer look at this phenomenon in the systems of Francez and Weiss, and point out two types of necessary conditions, one proof-theoretic and one relevant, which any contradictory formula must satisfy. As a consequence, we propose a nine-fold classification of provable contradictions for the logics. (shrink)

We show that the replacement rule of the sequent calculi ${\bf G3[mic]}^= $ in [8] can be replaced by the simpler rule in which one of the principal formulae is not repeated in the premiss.

To give a good account of deductive inference, we need to recognise two new relations, one in the realm of contents, the other in the psychological realm of mental action. When these new relations are properly coordinated, they can supply an account of what it is for a thinker to be making a deductive inference. The account endorses the condition that in deductive reasoning, a thinker must take the premises to support the conclusion. The account is distinguished from the positions (...) of Broome, Ryle, and Wright. (shrink)

This paper introduces nested sequent calculi for modal logics that include non-standard modalities as primitive operators in their languages. By non-standard modalities, we mean non-contingency, contingency, essence, accident, impossibility, and unnecessity. We consider basic normal modal logic K and its serial, reflexive, transitive, and symmetric extensions. Our research begins by using Poggiolesi’s nested sequent calculi as a foundation. These calculi are specifically designed for logics that are formulated in a language that includes the necessity operator. Next, we proceed to modify (...) their rules to accommodate non-standard modalities. We then establish the soundness and completeness of the resulting calculi. As a consequence, we get that the nested sequent calculus for K is cut-free. Subsequently, we provide a constructive cut admissibility proof for K. Finally, we discuss the issues pertaining to the cut admissibility for the extensions of K and their relationships with the so-called special structural rules as well as the potential for considering other forms of non-standard modalities. (shrink)