Studies in History and Philosophy of Modern Physics 38 (2007) 275–291 What could be objective about probabilities? $ Tim Maudlin Department of Philosophy, Rutgers University, New Brunswick, NJ 08901-2882, USA Abstract The basic notion of an objective probability is that of a probability determined by the physical structure of the world. On this understanding, there are subjective credences that do not correspond to objective probabilities, such as credences concerning rival physical theories. The main question for objective probabilities is how they are determined by the physical structure. In this paper, I survey three ways of understanding objective probability: stochastic dynamics, humean chances, and deterministic chances (typicality). The first is the obvious way to understand the probabilities of quantum mechanics via a collapse theory such as GRW, the last is the way to understand the probabilities in the context of a deterministic theory such as Bohmian mechanics. Humean chances provide a more abstract and general account of chances locutions that are independent of dynamical considerations. r 2007 Elsevier Ltd. All rights reserved. Keywords: Probability; Typicality; Stochastic dynamics; Bohmian mechanics 1. Objective probability 1: Stochastic dynamics Systems evolving in time are governed by a dynamics: Laws concerning how the state changes with time. In a deterministic system, specification of the state of the system at one time together with the dynamics determines the state at later times. 1 In an indeterministic system, the state of a system at one time and the laws are jointly compatible with different ARTICLE IN PRESS www.elsevier.com/locate/shpsb 1355-2198/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.shpsb.2006.04.006 $ An extended version of this paper (with the same title) will appear in Probabilities in Physics (Ed. C. Beisbart and S. Hartmann forthcoming). E-mail address: maudlin@rci.rutgers.edu. 1 There are all sorts of subtleties here that will not much concern us. The locus classicus for a discussion is John Earman’s A Primer on Determinism (1986).