But it seems to me one can still define this in terms of frequencies using a more general definition of what is meant by repeating an experiment. For example, when you use a probability P as a degree of belief or certainty about whether a patient X with symptoms W has a particular disease Y, one would define the universe as containing all possible realities in which X has the same symptoms W but with different underlying causal factors that lead to the same symptoms. An observation is a uniform sample from this universe and the belief is that a fraction of P of these realities has the cause Y.