DOI 10.1007/s11016-011-9536-2 BOOK REVIEW Jon Williamson: In defence of objective Bayesianism. Oxford: Oxford University Press, 2010, vi+185pp, £44.95 HB Darrell P. Rowbottom Published online: 6 April 2011 Ó Springer Science+Business Media B.V. 2011 Objective Bayesianism is the view that an agent's degrees of belief should satisfy three constraints in order to be rational. First, they should satisfy the probability calculus. Second, they should be sensitive to the agent's evidence (e.g., of physical chances, frequencies or correlations). Third, and finally, they should otherwise be maximally non-committal (or `equivocate between basic outcomes' (iii)). Williamson calls these the probability, calibration and equivocation norms. To illustrate, imagine that an agent knows (or believes) of a die only that it is a regular tetrahedron, with sides labelled `i', `ii', `iii' and `iv'.1 On the probability norm, her degree of belief that it lands on one of these numbers when rolled should be unity, on her background information, i.e., P(i, b) ? P(ii, b) ? P(iii, b) ? P(iv, b) = 1; one of the possible outcomes must occur, and the probability of something that must occur is unity. Then on the equivocation norm, she should remain maximally non-committal about which outcome will occur.
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