Abstracts

Huimin Dong: Logics of Defeasible Permission and its Dynamics

This work develops a deontic logic for defeasible permission and studies norms changing in two updated semantics. When one admits that 𝜑 or 𝜓 is permitted, normally , it goes together with the conjunctions of a permission of 𝜑 and that of 𝜓 . In the monotonic reasoning on this permission, a permission of 𝜑 leads to a permission of 𝜑 and 𝜓 ; while a prohibition of 𝜓 is introduced, we get into trouble. This is the paradox of free choice permission or strong permission. Many solutions have been proposed, but a systematic account of handling with norm change is still needed. This paper first introduces the notion of normality to develop a sound and complete deontic logic for defeasible permission, which can be used to analyze some notions in Chinese language. Further, following Lewis’ idea of norm changing, a systematic way is proposed to capture two dynamics for updating permission and obligation.

Davide Grossi: Logics of Preference When There Is No Best

Well-behaved preferences (e.g., total pre-orders) are a fundamental modelling tool in philosophical logic, artificial intelligence, as well as game and decision theories. Yet, ‘ill-behaved’ (e.g., cyclical) preference structures have proven important in a number of areas, from argumentation theory to tournaments and social choice theory. In this talk I will provide logical foundations, in dyadic modal logic, for reasoning about this type of preference structures where no obvious best elements may exist. I will compare and axiomatize a number of ways in which the concepts of maximality and optimality can be lifted to this general class of preferences. In doing so I try to expand the scope of the long-standing tradition of the logical analysis of preference.

This is joint work with Wiebe van der Hoek and Louwe Kuijer (University of Liverpool).

Ilaria Canavotto: Choice-Driven Counterfactuals

In this talk, I present a STIT semantics for choice driven counterfactuals, that is, counterfactuals whose evaluation relies on auxiliary premises about how agents are expected to act (their default choice behavior). I will discuss the philosophical implications and logical properties of two candidate semantics for choice driven counterfactuals, one called rewind models inspired by Lewis [1979] and the other called independence models motivated by well-known counterexamples to Lewis’ proposal [Slote, 1978]. Taking a cue from the literature on epistemic game theory, in the last part of the talk I will consider how to evaluate choice driven counterfactuals at moments arrived at by some agents not following their default choice behavior.

This is joint work with Eric Pacuit (University of Maryland).

Olivier Roy: Deliberation, Single-Peakedness, and Anchoring

In this talk I will present a number of results stemming from a computational model of collective attitude formation through a combination of group deliberation and aggregation. In this model the participants repeatedly exchange and update their preferences over small sets of alternatives, until they reach a stable preference profile. When they do so the collective attitude is computed by pairwise majority voting. The model shows, on the one hand, that rational preference change can fill an existing gap in known mechanisms purported to explain how deliberation can help avoiding incoherent group preferences. On the other hand, the model also reveals that when the participants are sufficiently biased towards their own opinion, deliberation can actually create incoherent group rankings, against the received view. The model shows furthermore that even rational deliberation can exhibit high levels of path dependencies or "anchoring", where the group opinion is strongly dependent on the order in which the participants contribute to the discussion. I will finish by discussing possible trade-offs between such positive and negative features of group deliberation.

This is joint work with Soroush Rafiee Rad (University of Amsterdam).

Johan van Benthem: Counterfactuals, Geometry and Dynamics

I will pursue two somewhat unusual angles, showing that conditional semantics can still hold surprises. The first is counterfactuals as a geometric logic of 'harmony of the spheres'. The second is a new dynamic construal of the interplay between conditionals, information update and monotonicity inference. The two views also go together as spatial dynamics.

John Horty: Open Texture and Defeasible Semantic Constraint

I will discuss some problems presented by open textured predicates for the semantics of natural language, as well as in legal theory. I will then (i) sketch an account of constraint in common law, (ii) suggest that this account can be adapted to help us understand open textured predicates as well, (iii) talk a bit about the reasoning involved in reaching decisions that satisfy this account of constraint, and (iv) show how this reasoning can be modeled in a simple defeasible logic.