Month: November 2021

On sequent calculi for Classical Logic where Cut is admissible

Damian Szmuc

The aim of this talk is to examine the class of Gentzen-style sequent calculi where Cut is admissible but not derivable that prove all the (finite) inferences that are usually taken to characterize Classical Logic—conceived with conjunctively-read multiple premises and disjunctively-read multiple conclusions. We’ll do this starting from two different calculi, both counting with Identity and the Weakening rules in unrestricted form. First, we’ll start with the usual introduction rules and consider what expansions thereof are appropriate. Second, we’ll start with the usual elimination or inverted rules and consider what expansions thereof are appropriate. Expansions, in each case, may or may not consist of additional standard or non-standard introduction or elimination rules, as well as of restricted forms of Cut.


Julien Murzi

Murzi & Rossi (2020) put forward a recipe for generating revenge arguments against any non-classical theory of semantic notions that can recapture classical logic for a set of sentences X provided X is closed under certain classical-recapturing principles. More precisely, Murzi & Rossi show that no such theory can be non-trivially closed under natural principles for paradoxicality and unparadoxicality.

In a recent paper, Lucas Rosenblatt objects that Murzi & Rossi’s principles are not so natural, and that non-classical theories can express perfectly adequate, and yet unparadoxical, notions of paradoxicality.

I argue that Rosenblatt’s strategy effectively amounts to fragmenting the notion of paradoxicality, much in the way Tarski’s treatment of the paradoxes fragments the notion of truth. Along the way, I discuss a different way of resisting Murzi & Rossi’s revenge argument, due to Luca Incurvati and Julian Schlöder, that doesn’t fragment the notion of paradoxicality, but that effectively bans paradoxical instances of semantic rules within subproofs, on the assumption that they are not evidence-preserving.

Semantics of First-order Logic: The Early Years

Richard Zach

The model and proof theory of classical first-order logic are a staple of introductory logic courses: we have nice proof systems, well-understood notions of models, validity, and consequence, and a proof of completeness. The story of how these were developed in the 1920s, 30s, and even 40s usually consists in simply a list of results and who obtained them when. What happened behind the scenes is much less well known. The talk will fill in some of that back story and show how philosophical, methodological, and practical considerations shaped the development of the conceptual framework and the direction of research in these formative decades. Specifically, I’ll discuss how the work of Hilbert and his students (Behmann, Schönfinkel, Bernays, and Ackermann) on the decision problem in the 1920s led from an almost entirely syntactic approach to logic to the development of first-order semantics that made the completeness theorem possible.

The Logic of Speech Acts: Sentential Force vs Utterance Force

Sarah Murray

Across languages, sentences are marked for sentence type, or sentential mood, e.g., declarative and interrogative. These sentence types are associated with speech acts: assertions and questions, respectively. However, sentential mood does not determine the force of an utterance of a sentence. We argue that the semantic contribution of sentential mood is a relation that constrains utterance force. This relation takes a proposition as an argument and uses it to affect a component of the context. The semantic constraint together with additional pragmatic factors produce utterance force.

This logic for speech acts involves a semantics for the three main sentence types found cross-linguistically (declarative, interrogative, imperative) as well as a distinction between speaker commitment and discourse reference. In addition to a semantics for sentential mood, this approach provides a framework for a range of phenomena, including evidentials, parentheticals, hedges, and “speech act modifiers”. We conclude by discussing the Linguistic Modification Thesis, the idea that linguistic material can only influence utterance force by influencing sentential force.

This talk is based on joint work with William Starr