A set theory wITh the following set of axioms: ExtensionalITy: two sets are equal if and only if they have the same elements. Union: If U is a set, so is the union of all ITs elements. Pair-set: If a and b are sets, so is a, b. Foundation: Every set contains a set disjoint from ITself. Comprehension (or Restriction): If P is a formula wITh one free variable and X a set then x: x is in X and P. is a set. InfinITy: There exists an infinITe set. Power-set: If X is a set, so is ITs power set. Zermelo set theory avoids Russell' s paradox by excluding sets of elements wITh arbITrary properties - the Comprehension axiom only allows a property to be used to select elements of an existing set. Zermelo Fränkel set theory adds the Replacement axiom. [Other axioms?] (1995-03-30)