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Concept
Set-theoretic definition of natural numbers
Summary
In set theory, several ways have been proposed to construct the natural numbers. These include the representation via von Neumann ordinals, commonly employed in axiomatic set theory, and a system based on equinumerosity that was proposed by Gottlob Frege and by Bertrand Russell.
Zermelo ordinals
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. This definition has the property that n is a set with n elements. 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 is equivalent to 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. Ravven and Quine refer to these sets as "counter sets".
Gottlob 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, for instance by saying that two sets are equinumerous if they can be put into one-to-one correspondence—this is sometimes known as Hume's principle.
This definition works in type theory, and in set theories that grew out of type theory, such as New Foundations and related systems. However, it does not work in the axiomatic set theory ZFC nor in certain related systems, because in such systems the equivalence classes under equinumerosity are proper classes rather than sets.
For enabling natural numbers to form a set, equinumerous classes are replaced by special sets, named cardinal.
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