MARC LANGE IS JEFFREY CONDITIONALIZATION DEFECTIVE BY VIRTUE OF BEING NON-COMMUTATIVE? REMARKS ON THE SAMENESS OF SENSORY EXPERIENCES 1. INTRODUCTION It has often been held (e.g., by Domotor 1980; Skyrms 1986; van Fraassen 1989; Döring 1999) that Jeffrey’s rule for updating degrees of belief on the basis of uncertain evidence (“Jeffrey conditionalization”) is defective by virtue of being non-commutative: if we switch the order in which a pair of observations is taken into account by the rule, then we may change the degrees of belief ultimately generated by the rule. I shall argue that although Jeffrey’s rule is formally noncommutative, this does not represent a defect in the rule. On the contrary, this kind of non-commutativity is exactly right. The key point will be that in switching the order in which numbers are plugged into Jeffrey’s rule, we are not really switching the order in which the same two sensory experiences are taken into account. Rather, we are dealing with entirely different pairs of observations. That is why they should generally yield different final degrees of belief. 2. WHY JEFFREY CONDITIONALIZATION HAS BEEN CONSIDERED NON- COMMUTATIVE Bayesian conditionalization is a rule for revising our subjective probability judgments in light of experience. If the agent learns that e is the case, and e captures all that the agent has learned from her experience, and pr old (.) is her probability distribution before discovering e, and pr new (.) is her probability distribution after discovering e, and 0 < pr old (e) < 1 and 0 < pr old (h) < 1, then according to Bayesian conditionalization, the agent’s pr new (h) should equal her pr old (h|e). In his influential (1965; 1983) , Richard Jeffrey proposed a means of extending Bayesian conditionalization to a case where there is “no propos- ition e in the agent’s preference ranking of which it can correctly be said Synthese 123: 393–403, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.