The Department of Epistemology will hold a meeting at which Dr. Gergei Bana will deliver a talk on "The subjectivist agent and his model of objective chance: an attempt to be rigorous about Lewis's Principal Principle". The start of the meeting: Thursday, 23 March, 4 p.m., Room 39.

Abstract: David Lewis linked objective chance and subjective degrees of belief with his Principal Principle. He proposed his ideas without mathematical rigor, and that style remained typical in the  literature on the Principal Principle, leading to numerous paradoxical statements. In this talk I aim to be mathematically rigorous, and that forces us to discover a number of structures and properties that are necessary to be present in a possible-world model of objective chance. Although Lewis considered possible-world models, he missed these structures and properties. I take special care to separate the subjectivist agent and his mathematical model of possible worlds, incorporating objective chance in the latter. I talk about what mathematical objects correspond to statements concerning the setup of a chancy experiment, what to the outcomes, what historic evidence is, and what can the notorious "admissible" evidences possibly be. We then use this model to take a new look at the so-called "Big Bad Bug" that worried Lewis. Namely, in my opinion the Big Bad Bug originates from an inconsistent treatment of "present chances", and hence there is in fact no Big Bad Bug.