  The standard way of understanding this is that something—the process of measurement, on the Copenhagen interpretation, or spontaneous collapse on the GRW theory—induces a non-deterministic state transition, called collapse, into a state in which the system really is in a determinate state with respect to a given quality (though it was not previously). These transition probabilities are dictated entirely by the state and the process of collapse, which allows these probabilities to meet the stable trial principle. The models of standard quantum mechanics explicitly permit two systems prepared in identical states to evolve via collapse into any state which has a non-zero prior probability in the original state, which permits these probabilities to meet the BCP. And the no-hidden variables theorems strongly suggest that there is no better information about the system to guide credence in future states than the chances, which makes these probabilities play the right role in the PP. These basic quantum probabilities governing state transitions seem to be strong candidates to be called chances.