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 isa, 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 thenx: 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)