Penultimate draft, forthcoming in The Journal of PhilosophyHow slack are requirements of rationality? Given a body of evidence, is there just one perfectly rational doxastic state to be in given that evidence? Defenders of Uniqueness answer 'yes' while defenders of Permissivism answer 'no.' For Permissivists, rationality gives you a certain amount of leeway in what to believe, with the result that rational people with the same evidence can nonetheless disagree with each other. By contrast, Uniqueness theorists hold that if two agents share the same total evidence and are rational in evaluating that evidence, they must have the same beliefs in response to that evidence.
1The debate over Uniqueness and Permissivism is important in its own right, but it also has implications for a number of other epistemological disputes. First, it has taken on a central role in debates about the epistemology of disagreement and the role of higher-order evidence.2 Second, it bears on whether requirements of rationality are diachronic, concerning how your beliefs at different times ought to be related to each other, or only synchronic, concerning only how your beliefs are at particular times, and 1 Uniqueness and Permissivism are compatible with any number of views about how to conceive of rational doxastic states -they could be sets of binary (on/off) beliefs, or credence functions, or pairs consisting of a credence function and a set of binary beliefs, or all manner of other possibilities. What Uniqueness says is that given a body of total evidence, there's a unique doxastic state (whatever structure that might have) that it's rational to be in, while Permissivism denies this.
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