Universal set

This article is about the largest set in some set theories. For a proper class, see universe (mathematics). For the meaning in probability theory, see sample space.

In set theory, a universal set is a set which contains all objects, including itself. In set theory as usually formulated, the conception of a set of all sets leads to a paradox. The reason for this lies with Zermelo's axiom of comprehension: for any formula $\varphi(x)$ and set A, there exists a set

$\{x \in A \mid \varphi(x)\}$

which contains exactly those elements x of A that satisfy $\varphi$ exists. If the universal set V existed and the axiom of separation applied to it, then Russell's paradox would arise from

$\{x \in V\mid x\not\in x\}$.

More generally, for any set A we can prove that

$\{x \in A\mid x\not\in x\}$

is not an element of A.

A second difficulty is that the power set of the set of all sets would be a subset of the set of all sets, provided that both exist. This conflicts with Cantor's theorem that the power set of any set (whether infinite or not) always has strictly higher cardinality than the set itself.

The idea of a universal set seems intuitively desirable in the Zermelo–Fraenkel set theory, particularly because most versions of this theory do allow the use of quantifiers over all sets (see universal quantifier). This is handled by allowing carefully circumscribed mention of V and similar large collections as proper classes. In theories in which the universe is a proper class, $V \in V$ is not true because proper classes cannot be elements.

Set theories with a universal set

There are set theories known to be consistent (if the usual set theory is consistent) in which the universal set V does exist (and $V \in V$ is true). In these theories, Zermelo's axiom of separation does not hold in general, and the axiom of comprehension of naive set theory is restricted in a different way.

The most widely studied set theory with a universal set is Willard Van Orman Quine’s New Foundations. Alonzo Church and Arnold Oberschelp also published work on such set theories. Church speculated that his theory might be extended in a manner consistent with Quine’s, but this is not possible for Oberschelp’s, since in it the singleton function is provably a set, which leads immediately to paradox in New Foundations.

Zermelo–Fraenkel set theory and related set theories, which are based on the idea of the cumulative hierarchy, do not allow for the existence of a universal set.

See also

• Category of sets
• Non-well-founded set theory
• Recursion
• Russell's paradox
• Universe (mathematics)

References

• Alonzo Church (1974). “Set Theory with a Universal Set,” Proceedings of the Tarski Symposium. Proceedings of Symposia in Pure Mathematics XXV, ed. L. Henkin, American Mathematical Society, pp. 297–308.
• T. E. Forster (1995). Set Theory with a Universal Set: Exploring an Untyped Universe (Oxford Logic Guides 31). Oxford University Press. ISBN 0-19-851477-8. 
• T. E. Forster (2001). “Church’s Set Theory with a Universal Set.”
• Bibliography: Set Theory with a Universal Set, originated by T. E. Forster and maintained by Randall Holmes at Boise State University.
• Randall Holmes (1998). Elementary Set theory with a Universal Set, volume 10 of the Cahiers du Centre de Logique, Academia, Louvain-la-Neuve (Belgium).
• Arnold Oberschelp (1973). “Set Theory over Classes,” Dissertationes Mathematicae 106.
• Willard Van Orman Quine (1937) “New Foundations for Mathematical Logic,” American Mathematical Monthly 44, pp. 70–80.

External links

• Weisstein, Eric W., "Universal Set", MathWorld.