Common Knowledge, Sailboats, and Publicity

TL;DR

The paper argues that Lewisian common knowledge offers a plausible explanation for what it means for an event to be public, reconciling pre-theoretical intuition with formal epistemic models.

Contribution

It introduces a formal account of Lewisian common knowledge within epistemic-plausibility models, clarifying its role in understanding public events.

Findings

Lewisian common knowledge aligns with pre-theoretical notions of publicity.

Formal models capture the intuition that some facts are public without being classical common knowledge.

The approach provides a plausible philosophical and formal account of public events.

Abstract

We revisit a recent puzzle about common knowledge, the ``sailboat" case (Lederman, 2018), and argue that Lewisian common knowledge allows us to reconcile the pre-theoretical intuition that certain facts are ``public" in such situations, while these facts cannot be common knowledge in the classical, iterative sense. The crux of the argument is to understand Lewisian common knowledge as an account of what it means for an event to be public. We first formulate this argument informally to clarify its philosophical commitment and then propose one way to capture it formally in epistemic-plausibility models. Taken together, we take the philosophical and the formal arguments as providing evidence that Lewisian common knowledge is a plausible account of what it means for an event to be public.