Common Knowledge, Communication, and Public Reason

35 Pages Posted: 27 Apr 2004 Last revised: 11 Jun 2008

See all articles by Bruce Chapman

Bruce Chapman

Faculty of Law, University of Toronto


In this paper I hope to explain why game theory has been so unsuccessful in accounting for the role of language in social interaction. I will begin by exploring some of its most basic difficulties in this respect, in games of pure coordination, and trace these difficulties back to the most fundamental organizing concepts in the theory of games, namely, Nash equilibrium and common knowledge of rationality. Nash thinkers and Nash actors, I shall argue, are doomed to have very impoverished conversations as Nash talkers. The sorts of conversations they will have will leave them paralyzed in games of pure coordination and largely uncooperative in games where their interactions are at least partially characterized by conflicts of interest. These conversations are impoverished, we will see, because they attempt to forge only a causal connection across the verbal exchanges between rational actors, not a conceptual one. What is needed, I shall argue, is the richer sort of conversation that is idealized by law, that is, one where there is an interpenetration of concepts in the use of language between rational actors, the sort of thing we see under a truly shared or public reason.

I have organized the paper as follows. Part I introduces a simple game of coordination and shows how two fundamental concepts of game theory, namely, common knowledge of rationality and the idea of Nash equilibrium, combine to limit the players in these games to some very unsatisfactory results. It is shown that neither cheap talk nor costly signaling provides an obvious way around these problems; Nash talkers seem destined to replicate the difficulties of Nash thinkers in their only slightly more public interactions. Part II introduces the law's special sense of an objectively reasonable interaction. Law's reasonable thinkers, I argue, are more capable of coordinating, and law's reasonable talkers more capable of cooperating, than their Nash counterparts because, under reasonableness, they are committed to a more public conception of their conduct shaping what they do together. I finish with some brief concluding comments.

Keywords: game theory, coordination, cooperation, communication, common knowledge of rationality, cheap talk, signaling, reasonableness, public reason, rational commitment, rational interaction

Suggested Citation

Chapman, Bruce, Common Knowledge, Communication, and Public Reason. Chicago-Kent Law Review, Vol. 79, p. 1151, 2004. Available at SSRN:

Bruce Chapman (Contact Author)

Faculty of Law, University of Toronto ( email )

78 and 84 Queen's Park
Toronto, Ontario M5S 2C5
416-978-6911 (Phone)
416 978 2648 (Fax)

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