A set is an idea from mathematics. A set has "members" (also called "elements"). A set is fixed by its members, so any two sets with the same members must be same (e.g., if set "X" and set "Y" have the same members, then "X = Y"). A set cannot have the same member more than once. Membership is the only thing that means anything. For example, there is no order or other distinction among the members. One particular set is the "empty set" (also called the "null set" and represented by the symbol formula_1). The empty set has "no" members. "Anything" can be a member of a set, including sets themselves (though if a set is a member of itself, paradoxes such as Russell's paradox can happen).