This paper introduces new logical systems which axiomatize a formal representation of inconsistency (here taken to be equivalent to contradictoriness) in classical logic. We start from an intuitive semantical account of inconsistent data, fixing some basic requirements, and provide two distinct sound and complete axiomatics for such semantics, LFI1 and LFI2, as well as their first-order extensions, LFI1* and LFI2*, depending on which additional requirements are considered. These formal systems are examples of what we dub Logics of Formal Inconsistency (LFI) (...) and form part of a much larger family of similar logics. We also show that there are translations from classical and paraconsistent first-order logics into LFI1* and LFI2*, and back. Hence, despite their status as subsystems of classical logic, LFI1* and LFI2* can codify any classical or paraconsistent reasoning. (shrink)

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It is well known that Lukasiewicz’s three-valued-logic L3 admits – unlike classical logic – the definition of two non trivial, truth-functional modal operators and ∆. We address the question of finding a convenient syntactic characterization of the “modal content” of L3. To this aim, we consider Wajsberg’s axiomatization of L3 and prove its equivalence with a modal calculus W_ which, essentially, includes: the BCK+double negation schemas, the characteristic modal schemas of S5 , full contraction for boxed formulas and the “partial (...) collapse” schema α → . As applications, we obtain a simple and natural completeness proof à la Lindenbaum for W, as well as a considerable simplification of Wajsberg’s original, ingenious completeness proof. (shrink)

We propose an extension of answer sets, that we call safe beliefs, that can be used to study several properties and notions of answer sets and logic programming from a more general point of view. Our definition, based on intuitionistic logic and following ideas from D. Pearce [Stable inference as intuitionistic validity, Logic Programming 38 79–91], also provides a general approach to define several semantics based on different logics or inference systems. We prove that, in particular, intuitionistic logic can be (...) replaced with any other proper intermediate logic without modifying the resulting semantics. We also show that the answer set semantics satisfies an important property, the “extension by definition”, that can be used to construct program translations. As a result we are able to provide a polynomial translation from propositional theories into the class of disjunctive programs. (shrink)