Synthese (2007) 156:513–535 DOI 10.1007/s11229-006-9136-7 ORIGINAL PAPER The kinematics of belief and desire Richard Bradley Published online: 10 May 2007 © Springer Science+Business Media B.V. 2007 Abstract Richard Jeffrey regarded the version of Bayesian decision theory he floated 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 Jeffrey’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 first section Jeffrey’s probability kinematics is motivated and extended to the treatment of changes in conditional belief. In the second, analogous kinematical models are devel- oped 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. Keywords Preference revision · Belief revision · Kinematics · Bayesian conditioning 1 Introduction Richard Jeffrey’s writings spanned epistemology, the logic and philosophy of science, and both individual and social decision theory. Of his many new ideas he regarded two to be the most important. The first is the version of Bayesian decision theory This paper is one of a pair dedicated to Richard Jeffrey 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. R. Bradley (B ) Department of Philosophy, Logic and Scientific Method, London School of Economics, Houghton Street, London WC2A 2AE, UK e-mail: r.bradley@lse.ac.uk