 In this context, the two mentioned views about probability (actualism versus possibilism) do not exclude each other. That probabilities quantify ignorance about actual values of observables (as in actualism) does not entail that this ignorance is due to a lack of knowledge about underlying deterministic processes. Possibilism adds that quantum probabilities represent propensities and that our ignorance is a necessary consequence of the indeterministic ontology of the physical world, in which propensities constitute a fundamental category.