At the beginning of a question my book is saying: "Let the universe of discourse be the set ℝ of real numbers, and let [itex]A[/itex] be the empty family of subsets of ℝ.." How on earth can an empty family contain sets which are subsets of something? A family is a set whose elements are sets. If the family is empty, it cannot contain any elements. Thus it cannot contain sets which are subsets of ℝ! Am I right?