Special Session LIRa Seminar on Reasons, Defaults and Formal Epistemology

I will talk about about my recent work aimed at developing default logic as a theory of reasons and their interaction, with applications in ethics and in epistemology.  I’ll try to go quickly through some of the main ideas, so that we can concentrate on new issues and open problems.

In this talk I will look at the notion of interactive rationality from the perspective of philosophical theories of normativity.  I’ll  argue that interaction bring in a new dimension to such theories. The foundation of the argument will be a reformulation of contemporary epistemic game theory in terms that are familiar to  philosophy of action and meta-ethics: normativity, reasons and responsiveness. I will consider choice rules and solution concepts put forward in the decision- and game-theoretical literature, and show that they can be seen as potential “normative sources”, issuing ought statements and singling out what facts count as reasons for action in interactive situations. I’ll then analyze responsiveness to such reasons in terms of the dynamics of the context of the game, viz. in terms of changes in the factual and higher-order information.

Epistemic game theory has shown the importance of informational contexts to understand strategic interaction. We propose a general framework to analyze how such contexts may arise. The idea is to view informational contexts as the fixed-points of iterated, “rational responses” to incoming information about the agents’ possible choices. We show general conditions for stabilization of such sequences of rational responses, in terms of structural properties of both the decision rule and the information update policy. In the process, we generalize existing rules for information updates used in the dynamic-epistemic logic literature. We then apply this framework to admissibility. We give a dynamic analysis of a well-known “paradox” arising from this choice rule, characterize stabilization of iterated rational response to admissibility under two different information update rules, and argue that these embody two different ways to respond to reasons in games.(joint work with Olivier Roy)

In non-monotonic reasoning conflicts between default rules abound. This paper contains a principled account to deal with them. It does so in two ways: semantically within a circumscriptive theory, and syntactilly by supplying an algorithm for inheritance networks. The latter is sound and complete with respect to the first. (joint work with Harald Bastiaanse)