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We formulate a Hilbert-style axiomatic system and a tableau calculus for the STIT-based logic of imagination recently proposed in Wansing (2015). Completeness of the axiom system is shown by the method of canonical models; completeness of the tableau system is also shown by using standard methods.

In our everyday life we are holding each other responsible for what we believe and for how we form beliefs. Since there are many similarities between holding an agent responsible for her actions and holding an agent responsible for her beliefs, some epistemologists think that we can model epistemic responsibility, i.e., responsibility for a belief, along the lines ofmoral responsibility, i.e., responsibility for performing an action. Research onmoral responsibility suggests that a person can be held responsible for an action she has performed only if she had control over performing that action. Even though our practice of holding each other responsible for our beliefs is similar to our practice of holding each other responsible for our actions, our formation of a belief appears to be different to our performance of an action when it comes to our capacity to exercise control over it. Beliefs are mostly taken to be more or less passive responses to how the world appears to us, while in performing an action we shape the world, often as we intend it to be. This difference between actions and beliefs raises a problem when it comes to establishing a viable notion of epistemic responsibility along the lines of the notion of moral responsibility. Thus, we need to rethink our notion of epistemic responsibility, the kind of control necessary for responsibility, as well as the connection between epistemic responsibility and doxastic control. This special issue partially documents results of the two-day workshop “Doxastic Agency and Epistemic Responsibility” which took place at the Ruhr University Bochum in June 2014. It addresses questions such as: do we have the same kind of control over our doxastic attitudes as we have over our actions, what kind of control do we have over our doxastic attitudes and how is epistemic responsibility related to notions such as epistemic justification and practical responsibility?Most of the authors

Graham Priest takes the relation of identity to be non-transitive. In this paper, we are going to discuss several consequences of identity as a non-transitive relation. We will consider the Henkin-style completeness proof for classical first-order logic with a non-transitive “identity” predicate, Leibniz-identity in Priest’s second-order minimal logic of paradox, and the question whether or not identity of individuals should be defined as Leibniz-identity.

In this introductory note, we place the new essays on Belnap-Dunn logic, FDE, of the present volume against the background of the development of FDE. This note is an invitation to study the volume. It presents a chronological perspective on Belnap-Dunn logic and a slightly idiosyncratic list of further research topics.

Imagination has recently attracted considerable attention from epistemologists and is recognized as a source of belief and even knowledge. One remarkable feature of imagination is that it is often and typically agentive: agents decide to imagine. In cases in which imagination results in a belief, the agentiveness of imagination may be taken to give rise to indirect doxastic control and epistemic responsibility. This observation calls for a proper understanding of agentive imagination. In particular, it calls for the development of a semantics of imagination ascriptions. In the present paper an earlier suggestion by Ilkka Niiniluoto for a logic of imagination is considered. This proposal does not capture the agentive nature of imagination, and an alternative semantics is suggested. The new semantics combines the modal logic of agency with the neighbourhood semantics from alethic modal logic.

Curry’s paradox is well known. The original version employed a conditional connective, and is not forthcoming if the conditional does not satisfy contraction. A newer version uses a validity predicate, instead of a conditional, and is not forthcoming if validity does not satisfy structural contraction. But there is a variation of the paradox which uses “external validity” (that is, essentially, preservation of theoremhood). And since external validity contracts, one might expect the appropriate version of the Curry paradox to be inescapable. In this paper we show that this is not the case. We consider two ways of formalising the notion of external validity, and show that in both of these the paradox is not forthcoming without the appropriate forms of contraction.

This paper presents an overview of the methods of hypersequents and display sequents in the proof theory of non-classical logics. In contrast with existing surveys dedicated to hypersequent calculi or to display calculi, our aim is to provide a unified perspective on these two formalisms highlighting their differences and similarities and discussing applications and recent results connecting and comparing them.

This paper sheds light on the relationship between the logic of generalized truth values and the logic of bilattices. It suggests a definite solution to the problem of axiomatizing the truth and falsity consequence relations, $${\models_t}$$ ? t and $${\models_f}$$ ? f , considered in a language without implication and determined via the truth and falsity orderings on the trilattice SIXTEEN 3 (Shramko and Wansing, J Philos Logic, 34:121---153, 2005). The solution is based on the fact that a certain algebra isomorphic to SIXTEEN 3 generates the variety of commutative and distributive bilattices with conflation (Rivieccio, 2010).

In this introduction, we offer an overview of main systems developed in the growing literature on connexive logic, and also point to a few topics that seem to be collecting attention of many of those interested in connexive logic. We will also make clear the context to which the papers in this special issue belong and contribute.

The present paper studies two approaches to the expressiveness of propositional modal logics based on first-degree entailment logic, FDE. We first consider the basic FDE-based modal logic BK and certain systems in its vicinity, and then turn to some FDE-based modal logics in a richer vocabulary, including modal bilattice logic, MBL. On the one hand, model-theoretic proofs of the definability of connectives along the lines of [McCullough, “Logical connectives for intuitionistic propositional logic”, Journal of Symbolic Logic 36, 1 (1971): 15–20. DOI: 10.2307/2271511] and [[17] Wansing, “Logical connectives for constructive modal logic”, Synthese 150, 3 (2006): 459–482. DOI: 10.1007/s11229-005-5518-5] are given for various FDE-based modal logics. On the other hand, building on [Odintsov and Wansing, “Disentangling FDE-based paraconsistent modal logics, Studia Logica 105, 6 (2017): 1221–1254. DOI: 10.1007/s11225-017-9753-9], expressibility is considered in terms of mutual faithful embeddability of one logic into another logic. A distinction is drawn between definitional equivalence, which is defined with respect to a pair of structural translations between two languages, and weak definitional equivalence, which is defined with respect to a weaker notion of translations. Moreover, the definitional equivalence of some FDE-based modal logics is proven, especially the definitional equivalence of MBL and a conservative extension of the logic BK □ ×BK □ , which underlines the central role played by BK among FDE-based modal logics.

Starting from the notions of q-entailment and p-entailment, a two-dimensional notion of entailment is developed with respect to certain generalized q-matrices referred to as B-matrices. After showing that every purely monotonic singleconclusion consequence relation is characterized by a class of B-matrices with respect to q-entailment as well as with respect to p-entailment, it is observed that, as a result, every such consequence relation has an inferentially four-valued characterization. Next, the canonical form of B-entailment, a two-dimensional multiple-conclusion notion of entailment based on B-matrices, is introduced, providing a uniform framework for studying several different notions of entailment based on designation, antidesignation, and their complements. Moreover, the two-dimensional concept of a B-consequence relation is defined, and an abstract characterization of such relations by classes of B-matrices is obtained. Finally, a contribution to the study of inferential many-valuedness is made by generalizing Suszko’s Thesis and the corresponding reduction to show that any B-consequence relation is, in general, inferentially four-valued.

Anti-realistic conceptions of truth and falsity are usually epistemic or inferentialist. Truth is regarded as knowability, or provability, or warranted assertability, and the falsity of a statement or formula is identified with the truth of its negation. In this paper, a non-inferentialist but nevertheless anti-realistic conception of logical truth and falsity is developed. According to this conception, a formula (or a declarative sentence) A is logically true if and only if no matter what is told about what is told about the truth or falsity of atomic sentences, A always receives the top-element of a certain partial order on non-ontic semantic values as its value. The ordering in question is a told-true order. Analogously, a formula A is logically false just in case no matter what is told about what is told about the truth or falsity of atomic sentences, A always receives the top-element of a certain told-false order as its value. Here, truth and falsity are pari passu, and it is the treatment of truth and falsity as independent of each other that leads to an informational interpretation of these notions in terms of a certain kind of higher-level information.

The aim of this paper is to obtain a theoretical foundation of inconsistency-tolerant (or paraconsistent) reasoning by presenting a comprehensive study of the structural proof-theory of David Nelson’s paraconsistent logic. Inconsistency handling has a growing importance in Computer Science since inconsistencies may frequently occur in knowledge-based and intelligent information systems. Paraconsistent, inconsistency-tolerant logics have been studied to cope with such inconsistencies. In this paper, proof systems for Nelson’s paraconsistent logic N4 are comprehensively studied. The logic N4 is a fundamental system and known to be a common basis for various extended and useful paraconsistent logics. Some basic theorems including cut-elimination, normalization and completeness are uniformly proved using various embedding theorems. A variety of sequent calculi and natural deduction systems for N4 and some closely related systems are presented and compared.

In this paper, first some propositional conditional logics based on Belnap and Dunn’s useful four-valued logic of first-degree entailment are introduced semantically, which are then turned into systems of weakly and unrestrictedly connexive conditional logic. The general frame semantics for these logics makes use of a set of allowable (or admissible) extension/antiextension pairs. Next, sound and complete tableau calculi for these logics are presented. Moreover, an expansion of the basic conditional connexive logics by a constructive implication is considered, which gives an opportunity to discuss recent related work, motivated by the combination of indicative and counterfactual conditionals. Tableau calculi for the basic constructive connexive conditional logics are defined and shown to be sound and complete with respect to their semantics. This semantics has to ensure a persistence property with respect to the preorder that is used to interpret the constructive implication.

We take up a suggestion by Odintsov (2009, Studia Logica, 91, 407–428) and define intuitionistic variants of certain logics arising from the trilattice SIXTEEN3 introduced in Shramko and Wansing (2005, Journal of Philosophical Logic, 34, 121–153 and 2006, Journal of Logic, Language and Information, 15, 403–424). In a first step, a logic I16 is presented as a Gentzen-type sequent calculus for an intuitionistic version of Odintsov’s Hilbert-style axiom system LT (Kamide and Wansing, 2009, Review of Symbolic Logic, 2, 374–395; Odintsov, 2009, Studia Logica, 91, 407–428). The cut-elimination theorem for I16 is proved using an embedding of I16 into Gentzen’s LJ. The completeness theorem with respect to a Kripke-style semantics is also proved for I16. The framework of I16 is regarded as plausible and natural for the following reasons: (i) the properties of constructible falsity and paraconsistency with respect to some negation connectives hold for I16, and (ii) sequent calculi for Belnap and Dunn’s four-valued logic (Anderson et al., 1992, Entailment: The Logic of Relevance and Necessity; Belnap, 1977, A useful four-valued logic, In Modern uses of Multiple Valued Logic, pp. 5–37; Dunn, 1976, Philosophical Studies, 29, 149–168) and for Nelson’s constructive four-valued logic (Almukdad and Nelson, 1984, Journal of Symbolic Logic, 49, 231–233) are included as natural subsystems of I16. In a second step, a logic IT16 is introduced as a tableau calculus. The tableau system IT16 is an intuitionistic counterpart of Odintsov’s axiom system for truth entailment ⊨t in SIXTEEN3 and of the sequent calculus for ⊨t presented in Wansing (2010, Journal of Philosophical Logic, to appear). The tableau calculus is also shown to be sound and complete with respect to a Kripke-style semantics. A tableau calculus for falsity entailment can be obtained by suitably modifying the notion of provability.

The famous “slingshot argument” developed by Church, Godel, Quine and Davidson is often considered to be a formally strict proof of the Fregean conception that all true sentences, as well as all false ones, have one and the same denotation, namely their corresponding truth value: the true or the false. In this paper we examine the analysis of the slingshot argument by means of a non-Fregean logic undertaken recently by A.Woitowicz and put to the test her claim that the slingshot argument is in fact circular and presupposes what it intends to prove. We show that this claim is untenable. Nevertheless, the language of non-Fregean logic can serve as a useful tool for representing the slingshot argument, and several versions of the slingshot argument in non-Fregean logics are presented. In particular, a new version of the slingshot argument is presented, which can be circumvented neither by an appeal to a Russellian theory of definite descriptions nor by resorting to an analogous “Russellian” theory of λ–terms.

The present note contains a critical discussion of the methodology of paraconsistent logic in general and “the central optimisation problem of paraconsistent logics” in particular. It is argued that there exist several reasons not to consider classical logic as the reference logic for developing systems of paraconsistent logic, and it is suggested to weaken a certain maximality condition that may be seen as essential for “optimisation”, which is a methodology in the tradition of Newton da Costa. It is argued that the guiding motivation for the development of paraconsistent logics should be neither epistemological nor ontological, but informational. Moreover, it is pointed out that there are other notions of maximality and other methodologies. A methodology due to Graham Priest and Richard Routley and another methodology that focuses on a minimal shrinkage of expressiveness relative to a given reference logic are considered in some detail.

This paper deals with some criticism that has been put forward against strong, constructive negation in comparison to a certain example of Galois connected negations. The general background to this discussion is the informational interpretation of substructural logics, and the key issue is whether there exists an asymmetry or not between positive and negative information and between verification and falsification. The present paper confirms the view that a symmetrical conception is adequate for both direct and indirect variants of verification and falsification.

A prominent issue in mainstream epistemology is the controversy about doxastic obligations and doxastic voluntarism. In the present paper it is argued that this discussion can benefit from forging links with formal epistemology, namely the combined modal logic of belief, agency, and obligation. A stit-theory-based semantics for deontic doxastic logic is suggested, and it is claimed that this is helpful and illuminating in dealing with the mentioned intricate and important problems from mainstream epistemology. Moreover, it is argued that this linking is of mutual benefit. The discussion of doxastic voluntarism directs the attention of doxastic logicians to the notion of belief formation and thus to dynamic aspects of beliefs that have hitherto been neglected. The development of a formal language and semantics for ascriptions of belief formation may contribute to clarifying the contents and the implications of voluntaristic claims. A simple observation concerning other-agent nestings of stit-operators, for instance, may help illuminating the notions of making belief and responsibility for beliefs of others. In this way, stit-theory may serve as a bridge between mainstream and formal epistemology.

In Belnap’s useful 4-valued logic, the set 2={T,F} of classical truth values is generalized to the set 4=℘(2)={∅,{T},{F},{T,F}}. In the present paper, we argue in favor of extending this process to the set 16=℘(4) (and beyond). It turns out that this generalization is well-motivated and leads from the bilattice FOUR2 with an information and a truth-and-falsity ordering to another algebraic structure, namely the trilattice SIXTEEN3 with an information ordering together with a truth ordering and a (distinct) falsity ordering. Interestingly, the logics generated separately by the algebraic operations under the truth order and under the falsity order in SIXTEEN3 coincide with the logic of FOUR2, namely first degree entailment. This observation may be taken as a further indication of the significance of first degree entailment. In the present setting, however, it becomes rather natural to consider also logical systems in the language obtained by combining the vocabulary of the logic of the truth order and the falsity order. We semantically define the logics of the two orderings in the extended language and in both cases axiomatize a certain fragment comprising three unary operations: a negation, an involution, and their combination. We also suggest two other definitions of logics in the full language, including a bi-consequence system. In other words, in addition to presenting first degree entailment as a useful 16-valued logic, we define further useful 16-valued logics for reasoning about truth and (non-)falsity. We expect these logics to be an interesting and useful instrument in information processing, especially when we deal with a net of hierarchically interconnected computers. We also briefly discuss Arieli’s and Avron’s notion of a logical bilattice and state a number of open problems for future research.

An extension L+ of the non-associative Lambek calculus Lis defined. In L+ the restriction to formula-conclusion sequents is given up, and additional left introduction rules for the directional implications are introduced. The system L+ is sound and complete with respect to a modification of the ternary frame semantics for L.