In set theory, a universal set is a set which contains all objects, including itself.Forster 1995 p.
1. In set theory as usually formulated, the conception of a universal set leads to Russell's paradox and is consequently not allowed.
However, some non-standard variants of set theory include a universal set.
Notation
There is no standard notation for the universal set of a given set theory.
Common symbols include , ,  and .
Reasons for nonexistence
Many set theories do not allow for the existence of a universal set.
For example, it is directly contradicted by the axioms such as the axiom of regularity and its existence would imply inconsistencies.
The standard Zermelo–Fraenkel set theory is instead based on the cumulative hierarchy.
Russell's paradox
Russell's paradox prevents the existence of a universal set in Zermelo–Fraenkel set theory and other set theories that include Zermelo's axiom of comprehension.
This axiom states that, for any formula \varphi(x) and any set , there exists a set
\{x \in A \mid \varphi(x)\}
that contains exactly those elements  of  that satisfy \varphi.
As a consequence, for every set A we can find a set that it does not contain,The proof is inspired by Russell's paradox: Given A, choose B=\{x \in A\mid x\not\in x\}, that is, let B contain exactly those elements of A that are not a member of itself.
Then B is a subset of A by construction.
Assume for contradiction that B was a member of A.
Then, in which case is B also a member of the smaller set B?
By definition of B, we have B \in B if, and only if, B \not\in B.
But this is a contradiction; hence B cannot be a member of A. hence there is no set of all sets.
This indeed holds even with predicative comprehension and over Intuitionistic logic.
Cantor's theorem
A second difficulty with the idea of a universal set concerns the power set of the set of all sets.
Because this power set is a set of sets, it would necessarily be a subset of the set of all sets, provided that both exist.
However, 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.
Theories of universality
The difficulties associated with a universal set can be avoided either by using a variant of set theory in which the axiom of comprehension is restricted in some way, or by using a universal object that is not considered to be a set.
Restricted comprehension
There are set theories known to be consistent (if the usual set theory is consistent) in which the universal set  does exist (and V \in V is true).
In these theories, Zermelo's axiom of comprehension does not hold in general, and the axiom of comprehension of naive set theory is restricted in a different way.
A set theory containing a universal set is necessarily a non-well-founded set theory.
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,Church 1974 p. 308.
See also Forster 1995 p. 136 or 2001 p. 17.  but this is not possible for Oberschelp's, since in it the singleton function is provably a set,Oberschelp 1973 p. 40. which leads immediately to paradox in New Foundations.Holmes 1998 p. 110.
Another example is positive set theory, where the axiom of comprehension is restricted to hold only for the positive formulas (formulas that do not contain negations).
Such set theories are motivated by notions of closure in topology.
Universal objects that are not sets
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).
One way of allowing an object that behaves similarly to a universal set, without creating paradoxes, is to describe  and similar large collections as proper classes rather than as sets.
One difference between a universal set and a universal class is that the universal class does not contain itself, because proper classes cannot be elements of other classes.
Russell's paradox does not apply in these theories because the axiom of comprehension operates on sets, not on classes.
The category of sets can also be considered to be a universal object that is, again, not itself a set.
It has all sets as elements, and also includes arrows for all functions from one set to another.
Again, it does not contain itself, because it is not itself a set.
See also
Universe (mathematics)
Grothendieck universe
Domain of discourse
Von Neumann–Bernays–Gödel set theory — an extension of ZFC that admits the class of all sets
Notes
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 (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.
Willard Van Orman Quine (1937) “New Foundations for Mathematical Logic,” American Mathematical Monthly 44, pp.
70–80.
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