Probability as a theory dependent concept

It is argued that probability should be defined implicitly by the distribut ions of possible measurement values characteristic of a theory. These distr ibutions are tested by, but not defined in terms of, relative frequencies o f occurrences of events of a specified kind. The adoption of an a priori pr obability in an empirical investigation constitutes part of the formulation of a theory. In particular, an assumption of equiprobability in a given si tuation is merely one hypothesis inter alia, which can be tested, like any other assumption. Probability in relation to some theories - for example qu antum mechanics - need not satisfy the Kolmogorov axioms. To illustrate how two theories about the same system can generate quite different probabilit y concepts, and not just different probabilistic predictions, a team game f or three players is described. If only classical methods are allowed, a 75% success rate at best can be achieved. Nevertheless, a quantum strategy exi sts that gives a 100% probability of winning.