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  1. Existence hedges, neutral free logic and truth.Jan Heylen - forthcoming - Inquiry: An Interdisciplinary Journal of Philosophy.
    Semantic externalism in the style of McDowell and Evans faces a puzzle formulated by Pryor: to explain that a sentence such as 'Jack exists' is only a posteriori knowable, despite being logically entailed by the seemingly logical truth 'Jack is self-identical', and hence being itself a logical truth and therefore a priori knowable. Free logics can dissolve the puzzle. Moreover, Pryor has argued that the existentially hedged 'If Jack exists, then Jack is self-identical', when properly formalised, is a logical truth (...)
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  2. States of Affairs as Structured Extensions in Free Logic.Hans-Peter Leeb - forthcoming - Logic and Logical Philosophy:1.
    The search for the extensions of sentences can be guided by Frege’s “principle of compositionality of extension”, according to which the extension of a composed expression depends only on its logical form and the extensions of its parts capable of having extensions. By means of this principle, a strict criterion for the admissibility of objects as extensions of sentences can be derived: every object is admissible as the extension of a sentence that is preserved under the substitution of co-extensional expressions. (...)
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  3. A Binary Quantifier for Definite Descriptions for Cut Free Free Logics.Nils Kürbis - 2021 - Studia Logica 110 (1):219-239.
    This paper presents rules in sequent calculus for a binary quantifier I to formalise definite descriptions: Ix[F, G] means ‘The F is G’. The rules are suitable to be added to a system of positive free logic. The paper extends the proof of a cut elimination theorem for this system by Indrzejczak by proving the cases for the rules of I. There are also brief comparisons of the present approach to the more common one that formalises definite descriptions with a (...)
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  4. Proof Theory and Semantics for a Theory of Definite Descriptions.Nils Kürbis - 2021 - In Anupam Das & Sara Negri (eds.), TABLEAUX 2021, LNAI 12842.
    This paper presents a sequent calculus and a dual domain semantics for a theory of definite descriptions in which these expressions are formalised in the context of complete sentences by a binary quantifier I. I forms a formula from two formulas. Ix[F, G] means ‘The F is G’. This approach has the advantage of incorporating scope distinctions directly into the notation. Cut elimination is proved for a system of classical positive free logic with I and it is shown to be (...)
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  5. Definite Descriptions in Intuitionist Positive Free Logic.Nils Kürbis - 2020 - Logic and Logical Philosophy 30:1.
    This paper presents rules of inference for a binary quantifier I for the formalisation of sentences containing definite descriptions within intuitionist positive free logic. I binds one variable and forms a formula from two formulas. Ix[F, G] means ‘The F is G’. The system is shown to have desirable proof-theoretic properties: it is proved that deductions in it can be brought into normal form. The discussion is rounded up by comparisons between the approach to the formalisation of definite descriptions recommended (...)
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  6. A State-of-Affairs-Semantic Solution to the Problem of Extensionality in Free Logic.Hans-Peter Leeb - 2020 - Journal of Philosophical Logic 49 (6):1091-1109.
    If one takes seriously the idea that a scientific language must be extensional, and accepts Quine’s notion of truth-value-related extensionality, and also recognizes that a scientific language must allow for singular terms that do not refer to existing objects, then there is a problem, since this combination of assumptions must be inconsistent. I will argue for a particular solution to the problem, namely, changing what is meant by the word ‘extensionality’, so that it would not be the truth-value that had (...)
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  7. A Binary Quantifier for Definite Descriptions in Intuitionist Negative Free Logic: Natural Deduction and Normalisation.Nils Kürbis - 2019 - Bulletin of the Section of Logic 48 (2):81-97.
    This paper presents a way of formalising definite descriptions with a binary quantifier ι, where ιx[F, G] is read as ‘The F is G’. Introduction and elimination rules for ι in a system of intuitionist negative free logic are formulated. Procedures for removing maximal formulas of the form ιx[F, G] are given, and it is shown that deductions in the system can be brought into normal form.
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  8. Two Treatments of Definite Descriptions in Intuitionist Negative Free Logic.Nils Kürbis - 2019 - Bulletin of the Section of Logic 48 (4):299-317.
    Sentences containing definite descriptions, expressions of the form ‘The F’, can be formalised using a binary quantifier ι that forms a formula out of two predicates, where ιx[F, G] is read as ‘The F is G’. This is an innovation over the usual formalisation of definite descriptions with a term forming operator. The present paper compares the two approaches. After a brief overview of the system INFι of intuitionist negative free logic extended by such a quantifier, which was presented in (...)
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  9. Anselm's God in Isabelle/HOL.Ben Blumson - 2017 - Archive of Formal Proofs:9.
    Paul Oppenheimer and Edward Zalta's formalisation of Anselm's ontological argument for the existence of God is automated by embedding a free logic for definite descriptions within Isabelle/HOL.
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  10. Russell's Revenge: A Problem for Bivalent Fregean Theories of Descriptions.Jan Heylen - 2017 - Pacific Philosophical Quarterly 98 (4):636-652.
    Fregean theories of descriptions as terms have to deal with improper descriptions. To save bivalence various proposals have been made that involve assigning referents to improper descriptions. While bivalence is indeed saved, there is a price to be paid. Instantiations of the same general scheme, viz. the one and only individual that is F and G is G, are not only allowed but even required to have different truth values.
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  11. Truth and Existence.Jan Heylen & Leon Horsten - 2017 - Thought: A Journal of Philosophy 6 (1):106-114.
    Halbach has argued that Tarski biconditionals are not ontologically conservative over classical logic, but his argument is undermined by the fact that he cannot include a theory of arithmetic, which functions as a theory of syntax. This article is an improvement on Halbach's argument. By adding the Tarski biconditionals to inclusive negative free logic and the universal closure of minimal arithmetic, which is by itself an ontologically neutral combination, one can prove that at least one thing exists. The result can (...)
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  12. Gödel's "slingshot" argument and his onto-theological system.Srećko Kovač & Kordula Świętorzecka - 2015 - In Kordula Świętorzecka (ed.), Gödel's Ontological Argument: History, Modifications, and Controversies. Semper. pp. 123-162.
    The paper shows that it is possible to obtain a "slingshot" result in Gödel's theory of positiveness in the presence of the theorem of the necessary existence of God. In the context of the reconstruction of Gödel's original "slingshot" argument on the suppositions of non-Fregean logic, this is a natural result. The "slingshot" result occurs in sufficiently strong non-Fregean theories accepting the necessary existence of some entities. However, this feature of a Gödelian theory may be considered not as a trivialisation, (...)
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  13. Quantificational Logic and Empty Names.Andrew Bacon - 2013 - Philosophers' Imprint 13.
    The result of combining classical quantificational logic with modal logic proves necessitism – the claim that necessarily everything is necessarily identical to something. This problem is reflected in the purely quantificational theory by theorems such as ∃x t=x; it is a theorem, for example, that something is identical to Timothy Williamson. The standard way to avoid these consequences is to weaken the theory of quantification to a certain kind of free logic. However, it has often been noted that in order (...)
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  14. State-of-affairs Semantics for Positive Free Logic.Hans-Peter Leeb - 2006 - Journal of Philosophical Logic 35 (2):183-208.
    In the following the details of a state-of-affairs semantics for positive free logic are worked out, based on the models of common inner domain - outer domain semantics. Lambert's PFL system is proven to be weakly adequate (i.e., sound and complete) with respect to that semantics by demonstrating that the concept of logical truth definable therein coincides with that one of common truth-value semantics for PFL. Furthermore, this state-of-affairs semantics resists the challenges stemming from the slingshot argument since logically equivalent (...)
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  15. Sachverhalte und Extensionalität in der freien Logik.Hans-Peter Leeb - 2006 - Sankt Augustin: Academia Verlag.
    Empty individual expressions are needed to reconstruct the actual use of scientific language as well as to make logic free from existence assumptions. According to Quine, a language must be extensional to be adequate for the purposes of science. By means of Lambert's non-extensionality argument it can be demonstrated that a language containing empty individual expressions cannot be extensional as long as truth-values are the extensions of sentences. This book investigates the soundness of Lambert's argument and examines the question of (...)
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  16. Karel Lambert, Free Logics: Their Foundations, Character, and Some Applications Thereof (Prophil Projekte zur Philosophie Bd. 1. Eine Schriftenreihe des Forschungsinstituts Philosophie/Technik/Wirtschaft an der Universität Salzburg). Sankt Augustin: Academia-Verlag, 1997. 156 pp. Asch. 239. ISBN 3-89665-000-9. [REVIEW]Hans-Peter Leeb - 2001 - History and Philosophy of Logic 22:233-236.
    Free logics aim at freeing logic from existence assumptions by making them explicit, e.g., by adding an existence premisse to the antecedence of the classical axiom-schema of Universal Instantiation. Their historical development was motivated by the problem of empty singular terms, and that one of simple statements containing at least one such singular term: what is the referential status of such singular terms and what truth-value, if any, do such statemants have? Free logics can be classified with regard to their (...)
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  17. Comment on Mark Textor: Brentano's Positing Theory of Existence.Nils Kürbis - manuscript
    This article is the text of a commentary on a talk delivered by Mark Textor entitled 'Brentano's Positing Theory of Existence' at King's College London in December 2015. It contains ideas on implementing Textor's Neo-Brentanian theory of existence in a natural deduction proof system for negative free logic.
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