Set-theoretic definition of natural numbers

Several ways have been proposed to construct the natural numbers using set theory. These include the representation of via von Neumann ordinals, commonly employed in axiomatic set theory, and a system based on equinumerosity that was proposed by Frege and by Russell.

In Zermelo–Fraenkel (ZF) set theory, the natural numbers are defined recursively by letting 0 = {} be the empty set and n + 1 = n ∪ {n} for each n. In this way n = {0, 1, ..., n − 1} for each natural number n. The first few numbers defined this way are:

The set N of natural numbers is defined in this system as the smallest set containing 0 and closed under the successor function S defined by S(n) = n ∪ {n}. The structure ⟨N,0,S⟩ is a model of the Peano axioms. The existence of the set N follows from the axiom of infinity in ZF set theory.

The set N and its elements, when constructed this way, are an initial part of the von Neumann ordinals.

Frege and Bertrand Russell each proposed defining a natural number n as the collection of all sets with n elements. More formally, a natural number is an equivalence class of finite sets under the equivalence relation of equinumerosity. This definition may appear circular, but it is not, because equinumerosity can be defined in alternate ways, such as by Hume's principle.

This definition works in naive set theory, type theory, and in set theories that grew out of type theory, such as New Foundations and related systems. But it does not work in the axiomatic set theory ZFC and related systems, because in such systems the equivalence classes under equinumerosity are proper classes rather than sets.

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