Nordic Online Logic Seminar
An online seminar for logicians and logic aficionados worldwide.
The Nordic Online Logic Seminar (NOL Seminar) is a monthly seminar series initiated in 2021 presenting expository talks by logicians on topics of interest for the broader logic community. Initially the series focused on activities of the Nordic logic groups, but has since expanded to offer a variety of talks from logicians around the world. The seminar is open to professional or aspiring logicians and logic aficionados worldwide.
The tentative time slot is Monday, 16.0017.30 (Stockholm/Sweden time). If you wish to receive the Zoom ID and password for it, as well as regular announcements, please subscribe to the NOL Seminar mailing list.
NOL seminar organisers
Valentin Goranko and Graham Leigh

Nina Gierasimczuk (Danish Technical University)
Nina Gierasimczuk

Eric Pacuit (University of Maryland)
From paradox to principles: Splitting cycles and breaking ties
Voting on two alternatives appears unproblematic compared to voting on three (or more). When faced with only two alternatives, many arguments show that Majority Rule distinguishes itself from all other ways of making a group decision. For three or more alternatives, one faces the socalled “Paradox of Voting”: there may be elections with a majority cycle in which a majority of voters prefer A to B, a majority of voters prefer B to C, and a majority of voters prefer C to A. In this talk, I will explain a series of results that axiomatically characterize rules for resolving majority cycles in elections. These rules avoid the “Strong No Show Paradox” by responding properly to the addition of new voters to an election and mitigate spoiler effects by responding properly to the addition of new candidates to an election.
This talk is based on joint work with Wes Holliday.

Ivano Ciardelli (University of Padua)
Inquisitive modal logic: an overview
Inquisitive modal logic is a generalization of standard modal logic where the language also contains questions, and modal operators that can be applied to them. In this talk, I will provide an introductory overview of inquisitive modal logic. I will review some motivations for the approach, present some prominent examples of inquisitive modal logics, mention some results about them, and outline directions for future work.

Ana María MoraMárquez (University of Gothenburg)
Medieval Aristotelian Logic is Scientific Method
This presentation aims to show that medieval Aristotelian logic can be generally characterized as scientific method. To be sure, this method includes formal logic as one of its parts, but formal logic is by no means the crucial part. In fact, if, as I intend to show, the main aim of medieval Aristotelian logic is to provide methods for knowledge production and distribution, so its crucial parts are the methods for scientific proof provided in commentaries on Aristotle’s Posterior Analytics and Topics.
In the first part of the presentation, I argue for the possibility of talking of medieval ‘science’, ‘scientific knowledge’, and ‘scientific method’ from a modern perspective, and discuss how the modern perspective relates to the Latin ‘scientia’ in its different senses. In the second part, I show the progression from Nicholas of Paris (1240s) and Albert the Great (1250s), who see Aristotelian logic as a systematic scientific method where syllogistic argument is fundamental, but who struggle to coherently organize it around syllogistic argument, to Radulphus Brito (1290s) who, still seeing Aristotelian logic as scientific method, uses the notion of ‘second intention’ in order to coherently structure it around syllogistic argument.

Albert Visser (Utrecht University)
Restricted Sequential Theories
Sequential theories form a fundamental class of theories in logic. They have full coding possibilities and allow us to build partial satisfaction predicates for formulas of bounded depthofquantifieralernations. In many respects, they are the proper domain of Gödelian metamathematics. We explain the notion of sequential theory.
A theory is restricted if it can be axiomatised by axioms of bounded depthofquantifieralernations. All finitely axiomatised theories are restriced, but, for example, also Primitive Recursive Arithmetic. We explain the smallisverysmall principle for restricted sequential theories which says that, whenever the given theory shows that a definable property has a small witness, i.e., a witness in a sufficiently small definable cut, then it shows that the property has a very small witness, i.e., a witness below a given standard number.
We sketch two proofs that restricted theories are incomplete (however complex the axiom set). One uses the smallisverysmall principle and the other a direct Rosser argument. (The second argument was developed in collaboration with Ali Enayat.)

Lauri Hella (Tampere University)
Game characterizations for the number of quantifiers
A game that characterizes definability of classes of structures by firstorder sentences containing a given number of quantifiers was introduced by Immerman in 1981. In this talk I describe two other games that are equivalent with the Immerman game in the sense that they characterize definability by a given number of quantifiers.
In the Immerman game, Duplicator has a canonical optimal strategy, and hence Duplicator can be completely removed from the game by replacing her moves with default moves given by this optimal strategy. On the other hand, in the other two games there is no such optimal strategy for Duplicator. Thus, the Immerman game can be regarded as a oneplayer game, but the other two games are genuine twoplayer games.
The talk is based on joint work with Kerkko Luosto.

Peter Pagin (Stockholm University)
Switcher Semantics and quantification
Switcher Semantics is a semantic framework that is basically characterised by allowing switching: when recursively applying a semantic function \(\mu\) to a complex term \(t\), the semantic function applying to an immediate subterm \(t'\) of \(t\) may be a function \(\mu'\), distinct from \(\mu\). An operatorargumentposition pair is called a switcher if it induces such a switch. Switcher semantic systems do not satisfy the standard form of compositionality, but a generalized form, which allows greater flexibility. In earlier work (mostly published), some together with Kathrin Glüer, some with Dag Westerståhl, it has been applied to natural language constructions like proper names in modal contexts, general terms in modal contexts, indexicals in temporal contexts, quotation, and belief contexts. This talk will focus on quantifiers and quantification. Firstorder quantifiers can be regarded as switchers, switching from truth conditions to satisfaction conditions. The larger topic is quantification into switched contexts. I shall begin by giving an introduction to the framework.

Göran Sundholm (Leiden University)
CurryHoward: a meaning explanation or just another realizability interpretation?
Around 1930 a major paradigm shift occurred in the foundations of mathematics; we may call it the METAMATHEMATICAL TURN. Until then the task of a logician had been to design and explain a fullscale formal language that was adequate for the practice of mathematical analysis in such a way that the axioms and rules of inference of the theory were rendered evident by the explanations.
The metamathematical turn changed the status of the formal languages: now they became (meta)mathematical objects of study. We no longer communicate with the aid of the formal systems – we communicate about them. Kleene’s realizability (JSL 1945) gave a metamathematical (re)interpretation of arithmetic inside arithmetic. Heyting and Kolmogorov (19312), on the other hand, had used “proofs” of propositions, respectively “solutions” to problems, in order to explain the meaning of the mathematical language, rather than reinterpret it internally.
We now have the choice to view the CurryHoward isomorphism, say, as a variant of realizability, when it will be an internal mathematical reinterpretation, or to adopt an atavistic, Fregelike, viewpoint and look at the language as being rendered meaningful. This perspective will be used to discuss another paradigm shift, namely that of distinguishing constructivism and intuitionism. The hesitant attitude of Gödel, Kreisel, and Michael Dummett, will be spelled out, and, at the hand of unpublished source material, a likely reason given.

Sonja Smets (Institute for Logic, Language and Computation, University of Amsterdam)
Reasoning about Epistemic Superiority and Data Exchange
In this presentation I focus on a framework that generalizes dynamic epistemic logic in order to model a wider range of scenarios including those in which agents read or communicate (or somehow gain access to) all the information stored at specific sources, or possessed by some other agents (including information of a nonpropositional nature, such as data, passwords, secrets etc). The resulting framework allows one to reason about the state of affairs in which one agent (or group of agents) has ‘epistemic superiority’ over another agent (or group). I will present different examples of epistemic superiority and I will draw a connection to the logic of functional dependence by A. Baltag and J. van Benthem. At the level of group attitudes, I will further introduce the new concept of ‘common distributed knowledge’, which combines features of both common knowledge and distributed knowledge. This presentation is based on joint work with A. Baltag in [1].
[1] A. Baltag and S. Smets, Learning what others know, in L. Kovacs and E. Albert (eds.), LPAR23 proceedings of the International Conference on Logic for Programming, AI and Reasoning, EPiC Series in Computing, 73:90110, 2020. https://doi.org/10.29007/plm4

Dag Westerståhl (Stockholm University, Tsinghua University)
From consequence to meaning: the case of intuitionistic propositional logic (IPL)
One quarter of the talk presents background on how facts about entailments and nonentailments can single out the constants in a language, and in particular on an idea originating with Carnap that the standard relation of logical consequence in a formal language should fix the (modeltheoretic) meaning of its logical constants. Carnap’s focus was classical propositional logic (CPL), but his question can be asked for any logical language. The rest of the talk gives a very general positive answer to this question for IPL: the usual IPL consequence relation does indeed determine the standard intuitionistic meaning of the propositional connectives, according to most wellknown semantics for IPL, such as Kripke semantics, Beth semantics, Dragalin semantics, topological semantics, and algebraic semantics.