We study the problem of explaining observations about the probabilities of events such as ‘it rains 20% of the time’, ‘rain and snow are equally likely’, etc. We explain these statements with a probability distribution or a statement about probabilities of (other) events that are consistent with our knowledge and entail the observation. We formalise this problem in a fuzzy probabilistic logic FP. We define and motivate the notions of abduction problems and their solutions. We analyse the complexity of solution recognition and existence for a given abduction problem in FP for the case of full language and its disjunctive-clause fragments. We also obtain a translation of classical probabilistic abduction (finding the most likely explanation of a given event) to FP.

We explore a fuzzy modal logic that can formalise probabilistic reasoning about actions and knowledge. In particular, we deal with contexts involving statements about events expressed via modal formulas, e.g., ‘after doing a, the probability of A knowing that p holds increases / decreases / is equal to 0.25’, ‘according to A, p is equally likely to happen after doing a or b’, etc. We define the semantics of the logic on Kripke frames equipped with probability measures. We analyse the complexity of deciding the satisfiability of formulas of our logic over finitely branching models, for the full language and its fragments of varying expressivity. In particular, we identify several fragments of our logic where satisfiability is decidable in polynomial time.

We present a novel approach to querying classically inconsistent description logic (DL) knowledge bases by adopting a paraconsistent semantics with the four 'Belnapian' values: exactly true (T), exactly false (F), both (B), and neither (N). In contrast to prior studies on paraconsistent DLs, we allow truth value operators in the query language over concept and role atoms, which can be used to differentiate between answers obtained from contradictory evidence and those based upon only positive evidence. We present a reduction to classical DL query answering that allows us to pinpoint the precise combined and data complexity of answering queries with values in paraconsistent ALCHI with two- and four-valued roles and their sublogics. Notably, we show that tractable data complexity is retained for Horn DLs. We also present a comparison with repair-based inconsistency-tolerant semantics, showing that the two approaches are incomparable: if we consider queries with the T (exactly true) operator, then we neither over-approximate the most cautious repair-based semantics, nor under-approximate the least cautious ones.

In this paper, we explore the problem of explaining observations starting from a classically inconsistent theory by adopting a paraconsistent framework. More precisely, we consider theories formulated in the well-known Belnap–Dunn paraconsistent four-valued logic BD and its implicative expansion BD⊃. Abductive solutions are then given in one of the two further expansions of BD: BD∘, which introduces formulas of the form ∘φ (‘the information on φ is reliable’), and BD△, which augments the language with formulas of the form △φ (‘there is information that φ is true’). We show that explanations in BD∘ and BD△ are not reducible to one another. We analyse the complexity of standard abductive reasoning tasks (solution recognition, solution existence, and relevance/necessity of hypotheses) depending on the language of the solution (BD∘ or BD△) and on the language of the theory (BD or BD⊃). In addition, we consider the complexity of abductive reasoning in the Horn fragment of BD⊃. By showing how to reduce abduction in BD and its expansions to abduction in classical propositional logic, we enable the reuse of existing abductive reasoning procedures.

We explore the problem of explaining observations in contexts involving statements with truth degrees, such as ‘the lift is loaded’, ‘the symptoms are severe’, etc. To formalise these contexts, we consider infinitely-valued Łukasiewicz fuzzy logic. We define and motivate the notions of abduction problems and explanations in the language of Łukasiewicz logic expanded with ‘interval literals’ of the form p≥c, p≤c, and their negations that express the set of values a variable can have. We analyse the complexity of standard abductive reasoning tasks (solution recognition, solution existence, and relevance / necessity of hypotheses) in Łukasiewicz logic for the case of the full language and for the case of theories containing only disjunctive clauses and show that in contrast to classical propositional logic, the abduction in the clausal fragment has lower complexity than in the general case.

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Abstract We present an axiomatization of the fuzzy bi-Gödel modal logic ${\textbf{K}\textsf{biG}}^{\textsf{f}}$ formulated in the language containing $\triangle $ (Baaz Delta operator) and treating $-\!-\!< $ (co-implication) as the defined connective. We also consider two paraconsistent relatives of ${\textbf{K}\textsf{biG}}^{\textsf{f}}$ — $\textbf{K}\textsf{G}^{2\pm \textsf{f}}$ and $\textsf{G}^{2\pm \textsf{f}}_{\blacksquare ,\blacklozenge }$. These logics are defined on fuzzy frames with two valuations $e_{1}$ and $e_{2}$ standing for the support of truth and falsity, respectively, and equipped with two fuzzy relations $R^{+}$ and $R^{-}$ used to determine supports of truth and falsity of modal formulas. We construct embeddings of $\textbf{K}\textsf{G}^{2\pm \textsf{f}}$ and $\textsf{G}^{2\pm \textsf{f}}_{\blacksquare ,\blacklozenge }$ into ${\textbf{K}\textsf{biG}}^{\textsf{f}}$ and use them to obtain the characterization of $\textbf{K}\textsf{G}^{2}$- and $\textsf{G}^{2}_{\blacksquare ,\blacklozenge }$-definable frames. Moreover, we study the transfer of ${\textbf{K}\textsf{biG}}^{\textsf{f}}$ formulas into $\textbf{K}\textsf{G}^{2\pm \textsf{f}}$, i.e., formulas that are ${\textbf{K}\textsf{biG}}^{\textsf{f}}$-valid on mono-relational frames $\mathfrak{F}$ and $\mathfrak{F}^{\prime}$ iff they are $\textbf{K}\textsf{G}^{2\pm \textsf{f}}$-valid on their bi-relational counterparts. Finally, we establish $\textsf{PSpace}$-completeness of all considered logics.

Abstract This paper is an extended version of Bílková et al. ((2023b). Logic, Language, Information, and Computation. WoLLIC 2023, Lecture Notes in Computer Science, vol. 13923, Cham, Springer Nature Switzerland, 101–117.). We discuss two-layered logics formalising reasoning with probabilities and belief functions that combine the Łukasiewicz $[0,1]$ -valued logic with Baaz $\triangle$ operator and the Belnap–Dunn logic. We consider two probabilistic logics – $\mathsf {Pr}^{{\mathsf {\unicode {x0141}}}^2}_\triangle$ (introduced by Bílková et al. 2023d. Annals of Pure and Applied Logic, 103338.) and $\mathbf {4}\mathsf {Pr}^{{\mathsf {\unicode {x0141}}}_\triangle }$ (from Bílková et al. 2023b. Logic, Language, Information, and Computation. WoLLIC 2023, Lecture Notes in Computer Science, vol. 13923, Cham, Springer Nature Switzerland, 101–117.) – that present two perspectives on the probabilities in the Belnap–Dunn logic. In $\mathsf {Pr}^{{\mathsf {\unicode {x0141}}}^2}_\triangle$ , every event $\phi$ has independent positive and negative measures that denote the likelihoods of $\phi$ and $\neg \phi$ , respectively. In $\mathbf {4}\mathsf {Pr}^{{\mathsf {\unicode {x0141}}}_\triangle }$ , the measures of the events are treated as partitions of the sample into four exhaustive and mutually exclusive parts corresponding to pure belief, pure disbelief, conflict and uncertainty of an agent in $\phi$ . In addition to that, we discuss two logics for the paraconsistent reasoning with belief and plausibility functions from Bílková et al. ((2023d). Annals of Pure and Applied Logic, 103338.) – $\mathsf {Bel}^{{\mathsf {\unicode {x0141}}}^2}_\triangle$ and $\mathsf {Bel}^{\mathsf {N}{\mathsf {\unicode {x0141}}}}$ . Both these logics equip events with two measures (positive and negative) with their main difference being that in $\mathsf {Bel}^{{\mathsf {\unicode {x0141}}}^2}_\triangle$ , the negative measure of $\phi$ is defined as the belief in $\neg \phi$ while in $\mathsf {Bel}^{\mathsf {N}{\mathsf {\unicode {x0141}}}}$ , it is treated independently as the plausibility of $\neg \phi$ . We provide a sound and complete Hilbert-style axiomatisation of $\mathbf {4}\mathsf {Pr}^{{\mathsf {\unicode {x0141}}}_\triangle }$ and establish faithful translations between it and $\mathsf {Pr}^{\mathsf {\unicode {x0141}}^2}_\triangle$ . We also show that the validity problem in all the logics is $\mathsf {coNP}$ -complete.

We present a novel approach to querying classical inconsistent description logic (DL) knowledge bases by adopting a paraconsistent semantics with the four ‘Belnapian’ values: exactly true (T), exactly false (F), both (B), and neither (N). In contrast to prior studies on paraconsistent DLs, we allow truth value operators in the query language, which can be used to differentiate between answers having contradictory evidence and those having only positive evidence. We present a reduction to classical DL query answering that allows us to pinpoint the precise combined and data complexity of answering queries with values in paraconsistent ALCHI and its sublogics. Notably, we show that tractable data complexity is retained for Horn DLs. We present a comparison with repair-based inconsistency-tolerant semantics, showing that the two approaches are incomparable.

We explore the problem of explaining observations starting from a classically inconsistent theory by adopting a paraconsistent framework. We consider two expansions of the well-known Belnap-Dunn paraconsistent four-valued logic BD: BD-circ introduces formulas of the form circ phi (‘the information about phi is reliable’), while BD-triangle augments the language with formulas triangle phi (‘there is information that phi is true’). We define and motivate the notions of abduction problems and explanations in BD-circ and BD-triangle and show that they are not reducible to one another. We analyse the complexity of standard abductive reasoning tasks (solution recognition, solution existence, and relevance / necessity of hypotheses) in both logics. Finally, we show how to reduce abduction in BD-circ and BD-triangle to abduction in classical propositional logic, thereby enabling the reuse of existing abductive reasoning procedures.

We introduce two-dimensional logics based on \L{}ukasiewicz and G\"{o}del logics to formalize paraconsistent fuzzy reasoning. The logics are interpreted on matrices, where the common underlying structure is the bi-lattice (twisted) product of the $[0,1]$ interval. The first (resp.\ second) coordinate encodes the positive (resp.\ negative) information one has about a statement. We propose constraint tableaux that provide a modular framework to address their completeness and complexity.