The Fragility of Common Knowledge

Erkenntnis 82 (3):451-472 (2017)
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Abstract

Ordinary common knowledge is formally expressed by strong probabilistic common belief. How strong exactly? The question can be answered by drawing from the similar equivalence, recently explored, between plain and probabilistic individual beliefs. I argue that such a move entails that common knowledge displays a double fragility: as a description of a collective state and as a phenomenon, because it can respectively disappear as group size increases, or more worryingly as the epistemic context changes. I argue that despite this latter fragility, the effects of common knowledge on action are robust. Unfortunately, this in turn leads to a third fragility, that of the concept of common knowledge, which threatens to collapse on probabilistic common belief. This also reveals a disanalogy between the individual and the collective cases. I finally pinpoint the subtle difference entailed by the two concepts, expressed in terms of the attitude towards contrary evidence or of the agents’ awareness. As a result, common knowledge can be defended as a concept, which refers to a fragile yet distinct collective attitude.

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Cedric Paternotte
Université Paris-Sorbonne

Citations of this work

Common knowledge.Peter Vanderschraaf - 2008 - Stanford Encyclopedia of Philosophy.
Who's afraid of common knowledge?Giorgio Sbardolini - forthcoming - Philosophical Studies.
Common Knowledge.Peter Vanderschraaf - unknown - Stanford Encyclopedia of Philosophy.
Common Knowledge, Common Attitudes and Social Reasoning.Richmond H. Thomason - 2021 - Bulletin of the Section of Logic 50 (2):229-247.
Reasoning with reasons: Lewis on common knowledge.Huub Vromen - forthcoming - Economics and Philosophy:1-22.

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Convention: A Philosophical Study.David Kellogg Lewis - 1969 - Cambridge, MA, USA: Wiley-Blackwell.
Change in View: Principles of Reasoning.Gilbert Harman - 1986 - Cambridge, MA, USA: MIT Press.
Convention: A Philosophical Study.David Lewis - 1969 - Synthese 26 (1):153-157.

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