and, more generally, the union of countably many (i.e., possibly infinitely many) sets in β also belongs to β. For sets of a Borel space the axioms imply immediately that ∅ ϵ β. Furthermore, for A, B ϵβ we also have A ∩ B ϵ β, since A ∩ B =A ̅∪B ̅.
Of course, a Borel space should be defined in such a way that all possible outcomes of an experiment are really contained in this space.