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  1. Explicating Logical Independence.Lloyd Humberstone - 2020 - Journal of Philosophical Logic 49 (1):135-218.
    Accounts of logical independence which coincide when applied in the case of classical logic diverge elsewhere, raising the question of what a satisfactory all-purpose account of logical independence might look like. ‘All-purpose’ here means: working satisfactorily as applied across different logics, taken as consequence relations. Principal candidate characterizations of independence relative to a consequence relation are that there the consequence relation concerned is determined by only by classes of valuations providing for all possible truth-value combinations for the formulas whose independence (...)
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  • Variations on a Theme of Curry.Lloyd Humberstone - 2006 - Notre Dame Journal of Formal Logic 47 (1):101-131.
    After an introduction to set the stage, we consider some variations on the reasoning behind Curry's Paradox arising against the background of classical propositional logic and of BCI logic and one of its extensions, in the latter case treating the "paradoxicality" as a matter of nonconservative extension rather than outright inconsistency. A question about the relation of this extension and a differently described (though possibly identical) logic intermediate between BCI and BCK is raised in a final section, which closes with (...)
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  • A Henkin-style completeness proof for the modal logic S5.Bruno Bentzen - 2021 - In Pietro Baroni, Christoph Benzmüller & Yì N. Wáng (eds.), Logic and Argumentation: Fourth International Conference, CLAR 2021, Hangzhou, China, October 20–22. Springer. pp. 459-467.
    This paper presents a recent formalization of a Henkin-style completeness proof for the propositional modal logic S5 using the Lean theorem prover. The proof formalized is close to that of Hughes and Cresswell, but the system, based on a different choice of axioms, is better described as a Mendelson system augmented with axiom schemes for K, T, S4, and B, and the necessitation rule as a rule of inference. The language has the false and implication as the only primitive logical (...)
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  • Replacement in Logic.Lloyd Humberstone - 2013 - Journal of Philosophical Logic 42 (1):49-89.
    We study a range of issues connected with the idea of replacing one formula by another in a fixed context. The replacement core of a consequence relation ⊢ is the relation holding between a set of formulas {A1,..., Am,...} and a formula B when for every context C, we have C,..., C,... ⊢ C. Section 1 looks at some differences between which inferences are lost on passing to the replacement cores of the classical and intuitionistic consequence relations. For example, we (...)
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  • Are relevant logics deviant?Robert G. Wolf - 1978 - Philosophia 7 (2):327-340.
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  • Modal Logics That Are Both Monotone and Antitone: Makinson’s Extension Results and Affinities between Logics.Lloyd Humberstone & Steven T. Kuhn - 2022 - Notre Dame Journal of Formal Logic 63 (4):515-550.
    A notable early result of David Makinson establishes that every monotone modal logic can be extended to LI, LV, or LF, and every antitone logic can be extended to LN, LV, or LF, where LI, LN, LV, and LF are logics axiomatized, respectively, by the schemas □α↔α, □α↔¬α, □α↔⊤, and □α↔⊥. We investigate logics that are both monotone and antitone (hereafter amphitone). There are exactly three: LV, LF, and the minimum amphitone logic AM axiomatized by the schema □α→□β. These logics, (...)
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  • Modal Logic for Other-World Agnostics: Neutrality and Halldén Incompleteness.Lloyd Humberstone - 2007 - Journal of Philosophical Logic 36 (1):1-32.
    The logic of 'elsewhere,' i.e., of a sentence operator interpretable as attaching to a formula to yield a formula true at a point in a Kripke model just in case the first formula is true at all other points in the model, has been applied in settings in which the points in question represent spatial positions, as well as in the case in which they represent moments of time. This logic is applied here to the alethic modal case, in which (...)
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  • Why does Halldén-completeness matter?George F. Schumm - 1993 - Theoria 59 (1-3):192-206.
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  • Note on Extending Congruential Modal Logics.Lloyd Humberstone - 2016 - Notre Dame Journal of Formal Logic 57 (1):95-103.
    It is observed that a consistent congruential modal logic is not guaranteed to have a consistent extension in which the Box operator becomes a truth-functional connective for one of the four one-place truth functions.
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  • Expressive power and semantic completeness: Boolean connectives in modal logic.I. L. Humberstone - 1990 - Studia Logica 49 (2):197 - 214.
    We illustrate, with three examples, the interaction between boolean and modal connectives by looking at the role of truth-functional reasoning in the provision of completeness proofs for normal modal logics. The first example (§ 1) is of a logic (more accurately: range of logics) which is incomplete in the sense of being determined by no class of Kripke frames, where the incompleteness is entirely due to the lack of boolean negation amongst the underlying non-modal connectives. The second example (§ 2) (...)
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  • Zero-place operations and functional completeness, and the definition of new connectives.I. L. Humberstone - 1993 - History and Philosophy of Logic 14 (1):39-66.
    Tarski 1968 makes a move in the course of providing an account of ?definitionally equivalent? classes of algebras with a businesslike lack of fanfare and commentary, the significance of which may accordingly be lost on the casual reader. In ?1 we present this move as a response to a certain difficulty in the received account of what it is to define a function symbol (or ?operation symbol?). This difficulty, which presents itself as a minor technicality needing to be got around (...)
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  • Homeomorphism and the Equivalence of Logical Systems.Stephen Pollard - 1998 - Notre Dame Journal of Formal Logic 39 (3):422-435.
    Say that a property is topological if and only if it is invariant under homeomorphism. Homeomorphism would be a successful criterion for the equivalence of logical systems only if every logically significant property of every logical system were topological. Alas, homeomorphisms are sometimes insensitive to distinctions that logicians value: properties such as functional completeness are not topological. So logics are not just devices for exploring closure topologies. One still wonders, though, how much of logic is topological. This essay examines some (...)
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  • Post Completeness and Ultrafilters.David Makinson & Krister Segerberg - 1974 - Zeitschrift fur mathematische Logik und Grundlagen der Mathematik 20 (25-27):385-388.
    A cardinality result in modal propositional logic.
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