| − | The '''empty set''' is the unique set that contains no element and is a subset of every other set. Its existence is postulated by the [[Axiom of empty set]] in the axioms of [[Zermelo–Fraenkel set theory]]. Note that the empty set is not a subset of the empty set in Zermelo–Fraenkel [[set theory]]. | + | The '''empty set''' is the unique set that contains no element and is a subset of every other set. Its existence is postulated by the [[Axiom of empty set]] in the axioms of [[Zermelo–Fraenkel set theory]]. Though this seems counterintuitive, the empty set is a subset of the empty set, and the empty set is [[disjoint]] with itself. |