In axiomatic set theory and the branches of mathematics and philosophy that use it, the axiom of infinity is one of the axioms of Zermelo–Fraenkel set theory. It guarantees the existence of at least one infinite set, namely a set containing the natural numbers. It was first published by Ernst Zermelo as part of his set theory in 1908. Formal statement In the formal language of the Zermelo–Fraenkel axioms, the axiom reads: In words, there is a set I (the set which is postulated to be infinite), such that the empty set is in I, and such that whenever any x is a member of I, the set formed by taking the union of x with its singleton {x} is also a member of I. Such a set is sometimes called an inductive set. Interpretation and consequences This axiom is closely related to the von Neumann construction of the natural numbers in set theory, in which the successor of x is defined as x ∪ {x}. If x is a set, then it follows from the other axioms of set theory that this successor is also a uniquely defined set. Successors are used to define the usual set-theoretic encoding of the natural numbers. In this encoding, zero is the empty set: 0 = {}. The number 1 is the successor of 0: 1 = 0 ∪ {0} = {} ∪ {0} = {0} = {{}}. Likewise, 2 is the successor of 1: 2 = 1 ∪ {1} = {0} ∪ {1} = {0,1} = { {}, {{}} }, and so on: 3 = {0,1,2} = { {}, {{}}, {{}, {{}}} }; 4 = {0,1,2,3} = { {}, {{}}, { {}, {{}} }, { {}, {{}}, {{}, {{}}} } }. A consequence of this definition is that every natural number is equal to the set of all preceding natural numbers. The count of elements in each set, at the top level, is the same as the represented natural number, and the nesting depth of the most deeply nested empty set {}, including its nesting in the set that represents the number of which it is a part, is also equal to the natural number that the set represents. This construction forms the natural numbers. However, the other axioms are insufficient to prove the existence of the set of all natural numbers, . Therefore, its existence is taken as an axiom –