 Since the set of all projector operators on a given Hilbert space does not have a Boolean structure, the Born probability (which is defined over these projectors) does not satisfy the definition of probability of Kolmogorov (which applies to a Boolean algebra of events). For this reason, some authors define a generalized non-Kolmogorovian probability function over the ortho-algebra of quantum events (Hughes 1989, Cohen 1989). Modal interpretations do not follow this path: they conceive probabilities as represented by a Kolmogorovian measure on the Boolean algebra representing the definite-valued quantities, generated by mutually commuting projectors. As we have seen, the various modal interpretations differ from each other in their definitions of the preferred context on which the Kolmogorovian probability is defined.