Chance

Abstract

This chapter is concerned with the ontological dimension of probability, or physical chance. It first reviews the history of probability, and locates the birth of the contemporary form of objective probability in the works of Christian Huygens (1629-1695) and Pierre Simon Laplace (1749-1827). It then broaches issues of interpretation, by assessing frequency and propensity interpretations of objective probability. On the frequency interpretation, probabilities are frequency ratios; and it is very hard to see how these can be explanatory. While the propensity interpretation of probability overcomes this objection, it nonetheless has problems of its own. Finally, the chapter reviews the role of objective probability and chance in current physical theories, particularly classical and quantum mechanics. It finds the sharpest or purest application in so-called "collapse" interpretations of quantum mechanics, which postulate an openly stochastic evolution of the state in order to account for the fact that measurements of any dynamical property on quantum systems routinely obtain definite results. However, it is argued that physical chance persists also in other versions of the theory, endowed with supposedly deterministic or quasi-classical dynamics. And indeed there is a role for chance in classical mechanics itself.