The kinematics of belief and desire

Richard Je¤rey regarded the version of Bayesian decision theory he ‡oated in ‘The Logic of Decision’and the idea of a probability kinematics a generalisation of Bayesian conditioning to contexts in which the evidence is ‘uncertain’as his two most important contributions to philosophy. This paper aims to connect them by developing kinematical models for the study of preference change and practical deliberation. Preference change is treated in a manner analogous to Je¤rey’s handling of belief change: not as mechanical outputs of combinations of intrinsic desires plus information, but as a matter of judgement and of making up one’s mind. In the …rst section Je¤rey’s probability kinematics is motivated and extended to the treatment of changes in conditional belief. In the second, analogous kinematical models are developed for preference change and in particular belief-induced change that depends on an invariance condition for conditional preference. The two are the brought together in the last section in a tentative model of pratical deliberation. This paper is one of pair dedicated to Richard Je¤rey and prepared for a workshop held in his memory at the 26th International Wittgenstein Symposium. My thanks to the organisers of, and the participants in, this workshop and to two anonymous referees for their comments.