Ecumenical logic aims to peacefully join classical and intuitionistic logic systems, allowing for reasoning about both classical and intuitionistic statements. This paper presents a semantic tableau for propositional ecumenical logic and proves its soundness and completeness concerning Ecumenical Kripke models. We introduce the Ecumenical Propositional Tableau ( $$E_T$$ ) and demonstrate its effectiveness in handling mixed statements.

Ecumenical views in metaethics hold that normative utterances express hybrid mental states, states which include both a cognitive and a conative component. The ecumenicist can have her cake and eat it too: the view reaps the benefits of both cognitivist and non-cognitivist theories of normative judgement. The conative component of normative judgements accounts for their necessary link with motivation and rational action. The cognitive component makes it possible for the ecumenicist to endorse expressivism without facing the most difficult Frege-Geach (...) challenges. Ridge (2014) provides a defence of ecumenical expressivism about practical normativity that is both ambitious and compelling. I discuss and try to challenge two (unrelated) parts of the view. In the first section, I’ll raise a question for Ridge’s strategy for addressing the Frege-Geach problem, and in particular his proposed recipe for how to interpret normative assertions of arbitrary logical complexity. Then I discuss Ridge’s defence of cognitivism about rationality. I try to highlight some considerations that make expressivism about theoretical and practical rationality attractive. (shrink)

Much has been said about intuitionistic and classical logical systems since Gentzen’s seminal work. Recently, Prawitz and others have been discussing how to put together Gentzen’s systems for classical and intuitionistic logic in a single unified system. We call Prawitz’ proposal the Ecumenical System, following the terminology introduced by Pereira and Rodriguez. In this work we present an Ecumenical sequent calculus, as opposed to the original natural deduction version, and state some proof theoretical properties of the system. We (...) reason that sequent calculi are more amenable to extensive investigation using the tools of proof theory, such as cut-elimination and rule invertibility, hence allowing a full analysis of the notion of Ecumenical entailment. We then present some extensions of the Ecumenical sequent system and show that interesting systems arise when restricting such calculi to specific fragments. This approach of a unified system enabling both classical and intuitionistic features sheds some light not only on the logics themselves, but also on their semantical interpretations as well as on the proof theoretical properties that can arise from combining logical systems. (shrink)

Ridge defends a form of hybrid expressivism where normative judgements are constituted by two elements, normative perspectives and representational beliefs that invoke standards our normative perspectives determine. He thinks this view will enable him to ‘offload logical complexity’ onto the latter, representational components of our judgements, thereby taming the Frege-Geach Problem and conferring a dialectical advantage over non-hybrid, ‘pure’ forms of expressivism. But this will only work if our normative perspectives are themselves consistent in ways that are resistant to such (...) offloading. There are, moreover, serious problems with building normative perspectives into what we understand by moral judgement that appear best tamed by thinning out our understanding of them in ways that effectively collapse the ecumenical form of expressivism back into its purer cousin. The supposed dialectical advantage proves, I argue, chimerical. (shrink)

In 2015 Dag Prawitz proposed an Ecumenical system where classical and intuitionistic logic could coexist in peace. The classical logician and the intuitionistic logician would share the universal quantifier, conjunction, negation and the constant for the absurd, but they would each have their own existential quantifier, disjunction and implication, with different meanings. Prawitz’ main idea is that these different meanings are given by a semantical framework that can be accepted by both parties. The aim of the present paper is (...) [1] to prove the normalization theorem for the propositional fragment NEp of Prawitz’ ecumenical system, and [2] to show that NEp is sound and complete with respect to a Kripke-style semantics for the language of NEp. (shrink)

How does vagueness interact with metaphysical modality and with restrictions of it, such as nomological modality? In particular, how do definiteness, necessity (understood as restricted in some way or not), and actuality interact? This paper proposes a model-theoretic framework for investigating the logic and semantics of that interaction. The framework is put forward in an ecumenical spirit: it is intended to be applicable to all theories of vagueness that express vagueness using a definiteness (or: determinacy) operator. We will show (...) how epistemicists, supervaluationists, and theorists of metaphysical vagueness like Barnes and Williams (2010) can interpret the framework.  We will also present a complete axiomatization of the logic we recommend to both epistemicists and local supervaluationists. . (shrink)

In 2015 Dag Prawitz proposed a natural deduction ecumenical system, where classical logic and intuitionistic logic are codified in the same system. In his ecumenical system, Prawitz recovers the harmony of rules, but the rules for the classical operators do not satisfy separability. In fact, the classical rules are not pure, in the sense that negation is used in the definition of the introduction and elimination rules for the classical operators. In this work we propose an ecumenical (...) system adapting, to the natural deduction framework, Girard’s mechanism of stoup. This allows for the definition of a pure harmonic natural deduction system for the propositional fragment of Prawitz’s ecumenical logic. We conclude by presenting an extension proposal to the first-order setting and a different approach to purity based on some ideas proposed by Julien Murzi. (shrink)

Empiricism has been a pivotal philosophical topic for more than two millennia. Several Sophists, Aristotle, the Epicureans, Sextus Empiricus, Francis Bacon, Locke, Hume, Mill, Mach and the Logical Empiricists represent a long line of historically influential empiricists who share a prioritising of the sensory over all other forms of knowledge. The latest influential incarnation, Bas van Fraassen’s constructive empiricism, takes science to aim at empirically adequate theories, i.e. theories that save all and only the observable phenomena. Roughly put, an object (...) or phenomenon is observable in van Fraassen’s view if a properly functioning member of the human epistemic community can detect it via their unaided senses. Scientific realists and other critics have contested this notion citing among other reasons that most knowledge in natural science concerns objects that can only be detected with instruments, i.e. those that are unobservable and hence unknowable by van Fraassen’s lights. Beg-the-question accusations fly back and forth. As a consequence a stalemate has ensued in the scientific realism debate. (shrink)

This paper articulates and defends a conception of logical form as semantic form revealed by a compositional meaning theory. On this conception, the logical form of a sentence is determined by the semantic types of its primitive terms and their mode of combination as it relates to determining under what conditions it is true. We develop this idea in the framework of truth-theoretic semantics. We argue that the semantic form of a declarative sentence in a language L is revealed by (...) a (canonical) proof of its T- sentence in an interpretive truth theory for L. We give a precise characterization of sameness of logical form between any two declarative sentences in any two languages in terms of the notion of corresponding proofs in interpretive truth theories for the languages. We illustrate the utility of this approach with a number of examples. We then extend the characterization to non-declaratives in a generalization of truth-theoretic semantics that appeals to fulfillment conditions, of which truth conditions are one variety. On this approach, logical forms are not reified, and the notion of sameness of logical form is treated as conceptually basic. We discuss the relation of this conception of logical form to the project of identifying logical constants, reviewing two approaches, one of which takes topic neutrality as central, the other recursion. We argue that the project of identifying logical constants for the purposes of classifying together valid arguments is largely independent of that identifying logical form of sentences, and urge an ecumenical approach to extending talk of logical constants beyond where it is currently well-grounded. (shrink)

In recent years, mathematical logic has developed in many directions, the initial unity of its subject matter giving way to a myriad of seemingly unrelated areas. The articles collected here, which range from historical scholarship to recent research in geometric model theory, squarely address this development. These articles also connect to the diverse work of Väänänen, whose ecumenical approach to logic reflects the unity of the discipline.

It is a privilege to be invited to contribute to the Festschrift dedicated to Professor Johan Buitendag, Emeritus Dean, Faculty of Theology and Religion at the University of Pretoria, South Africa. While his own work often examined the relationship between theology and natural science, he was also passionate about ecumenism and, in that spirit, the present essay utilised what might be described as an inductive approach to an important ecumenical question, the unity between Methodists and Catholics. Ecumenical dialogue (...) is often undertaken from a theological and dogmatic perspective. This essay took a different approach in that it sought to inductively examine what might be described as the inner logic of two ecclesial systems (Methodist and Catholic). In doing so, it was premised upon an Aristotelian philosophical principle, epistemic fit, in that it presupposed that both these ecclesial bodies possess an intrinsic logic that can be brought to the surface in order to exhibit parallel contours in the wider ecclesial terrain.Contribution: This article fits well within the scope of HTS Teologiese Studies/Theological Studies in that it contributed to a new subdiscipline in epistemology, which explores the conceptual and epistemological dimensions of ecclesiology. Using the concept of epistemic fit, the article offered an inductive approach to some of the practical issues that emerge in ecumenical dialogue. (shrink)

Tarski's general conception of logic placed it at the center of all rational thought, and he took its aim to be the creation of a unified conceptual apparatus. In pursuit of this conviction, from his base at the University of California in Berkeley in the post-war years he campaigned vigorously on behalf of logic, locally, nationally and internationally. Though Tarski was ecumenical in his efforts to establish the importance of logic in these various ways, in his own work—even that (...) part of it which is considered to be philosophically significant—he was first and foremost a mathematician, and, even more so, a set-theoretical one. The paper traces this particular conception of logic back to the influence of his teachers in set theory and topology in Warsaw, and then details how it affected his explication of the notions of satisfaction, truth, logical consequence and logical term. (shrink)

I read Stefan Collini’s What are Universities For? last week with very mixed feelings. In the past, I’ve much admired his polemical essays on the REF, “impact”, the Browne Report, etc. in the London Review of Books and elsewhere: they speak to my heart. If you don’t know those essays, you can get some of their flavour from his latest article in the Guardian yesterday. But I found the book a disappointment. Perhaps the trouble is that Collini is too decent, (...) too high-minded, has too unrealistically exalted a view about what actually happens in universities which is too coloured by attitudes appropriate to the traditional humanities. And he is optimistic to the point of fantasy if he thinks that people are so susceptible to “the romance of ideas and the power of beauty” that they will want, or can be brought to want, lots and lots of universities in order to promote the ideas (as if they would suppose that the task of “conserving, understanding, extending and handing on to subsequent generations the intellectual, scientific, and artistic heritage of mankind” was clearly ill-served when there were only forty universities in England, as opposed to a hundred and whatever). (shrink)

A common reaction to Hegel’s suggestion that we collapse Kant’s distinction between form and content is that, since such a move would also deprive us of any way of distinguishing the merely logical from the real possibility of our concepts, it is incoherent and ought to be rejected. It is true that these two distinctions are intimately related in Kant, such that if one goes, the other does as well. But it is less obvious that giving them up as Kant (...) conceives them is as incoherent a proposal as many of Hegel’s critics think. It has been the point of a recent account of Hegel’s idealism to demonstrate that his critique of Kant’s dichotomy between form and content or concept and intuition does not commit him to the view that human cognition is materially creative of its content in the manner of a God-like or intuitive intellect—does not, in other words, signify a return to a pre-Critical metaphysics. This is the interpretative stance I adopt here as well, in hope of giving defenders of Kant reasons for taking Hegel more seriously. (shrink)

The Editors express their gratitude and appreciation to the indi-viduals listed below who served as referees for Informal Logic for Volumes 31 (2011) and 32 (2012).