Agreeing to Disagree Politically

Agreeing to Disagree Politically

Robert Aumann's agreement theorem and subsequent work shows that people who are rational in a certain Bayesian sense cannot agree to disagree on matters of fact, as long as there is common knowledge of this common rationality. This result hinges on a type of epistemic impartiality: a rational person will not give extra weight to a piece of evidence simply because they themselves discovered it rather than someone else. Of course, 'Good Bayesians' who agree on matters of fact can nevertheless disagree due to differences of preference and value. We question the possibility of reasonable political disagreement by adding a 'public reason' constraint derived from the political liberalism of John Rawls and interpreted more precisely in light of work by John Harsanyi. On our reading, a 'Good Liberal' must not give extra weight in public deliberations to their own preferences or values simply because they are their own. This political impartiality mirrors the epistemic impartiality of Aumann's theorem and we argue that disagreement on policy is impossible in a world of 'Good Liberal Bayesians', assuming common knowledge of both Bayesian rationality and Liberal reasonableness.

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