The 2019 Lindström Lectures was delivered by Johan van Benthem, University Professor of Logic, emeritus (University of Amsterdam), Henry Waldgrave Stuart Professor of Philosophy (Stanford University), Jin Yuelin Chair (Tsinghua University Beijing).
Van Benthem obtained his PhD from University of Amsterdam in 1977. His work since then has contributed to a remarkably wide range of topics within logic, including modal and temporal logic, logic and epistemology/philosophy of science, logical semantics and syntax of natural language, logics of space, and dynamic logics of information, computation and communication. Van Benthem has published over 500 articles and 10 edited books in logic, and has supervised over 80 PhD students in the field. He is founding director of the Institute for Logic, Language and Computation (ILLC, University of Amsterdam), first chair of the European Foundation for Logic, Language and Information (FoLLI), chair of the first Dutch national research program in cognitive science, and is co-director of the Amsterdam–Tsinghua Research Center for Logic.
One face of logic is turned toward truth and eternal consequence, but another face is dynamic, looking toward activities of reasoning and information handling by agents. In this lecture, I will develop the dynamic perspective, showing how key aspects of rational agency fit in the agenda of logic, such as handling and integrating information from various sources, revising erroneous beliefs, and balancing information with preferences and goals. In this lecture, the vehicle for achieving this will be dynamic-epistemic logics for various sorts of update. These mesh eventually with richer logics of games and strategic interaction. After all, much of reasoning is a social multi-agent process: ‘intelligence seldom comes alone’. I end this part by noting how the two faces of logic share the same methodology, and are in fact complementary.
However, ‘more logic’ is just one way to go in studying agency. I briefly discuss current challenges from the ‘less logic’ camp, where successful behavior is explained by dynamical systems with very simple agents, or from learning systems that may not have a logical formulation at all. I hope to show that logic retains a valuable role even in that stormy setting.
Information update and real-world action suggest a universe of changing models, where finding the valid dynamic laws involves logical languages with modalities for model change. I will discuss two families of such dynamic modal logics, stating some typical results, as well as open problems.
Dynamic-epistemic logics add dynamic superstructure to existing static logics, and tend to not increase complexity of satisfiability and model checking. Logics of graph change, arising e.g. in the study of games or of languages that change their own models under evaluation, tend to jump to higher complexities, becoming undecidable or worse. I will report some recent results that zoom in on the border line between the two kinds of system, and end with some issues about redesign and restoring decidability for more complex logics of action and information dynamics.
The 2019 Lindström Lectures was co-located with the conference Circularity in Syntax and Semantics.