Editing Russell's paradox (section)

{{Short description|Paradox in set theory}}
{{Infobox Bertrand Russell}}
In [[mathematical logic]], '''Russell's paradox''' (also known as '''Russell's antinomy''') is a [[Paradoxes of set theory|set-theoretic paradox]] published by the [[United Kingdom|British]] [[philosopher]] and [[mathematician]], [[Bertrand Russell]], in 1901.<ref>Russell, Bertrand, "Correspondence with Frege}. In Gottlob Frege ''Philosophical and Mathematical Correspondence''. Translated by Hans Kaal., University of Chicago Press,  Chicago, 1980.</ref><ref>Russell, Bertrand. ''[[The Principles of Mathematics]]''. 2d. ed. Reprint, New York: W. W. Norton & Company, 1996. (First published in 1903.)</ref> Russell's paradox shows that every [[set theory]] that contains an [[unrestricted comprehension principle]] leads to contradictions.<ref>Irvine, A. D., H. Deutsch (2021). "Russell's Paradox". Stanford Encyclopedia of Philosophy (Spring 2021 Edition), E. N. Zalta (ed.), [https://plato.stanford.edu/entries/russell-paradox/]</ref>  

According to the unrestricted comprehension principle, for any sufficiently well-defined [[Property (mathematics)|property]], there is the [[Set (mathematics)|set]] of all and only the objects that have that property. Let ''R'' be the set of all sets that are not members of themselves. (This set is sometimes called "the Russell set".)  If ''R'' is not a member of itself, then its definition entails that it is a member of itself; yet, if it is a member of itself, then it is not a member of itself, since it is the set of all sets that are not members of themselves. The resulting contradiction is Russell's paradox. In symbols:
: Let <math>R = \{ x \mid x \not \in x \}</math>. Then <math>R \in R \iff R \not \in R</math>.

Russell also showed that a version of the paradox could be derived in the [[axiomatic system]] constructed by the German philosopher and mathematician [[Gottlob Frege]], hence undermining Frege's attempt to reduce mathematics to logic and calling into question the [[Logicism|logicist programme]]. Two influential ways of avoiding the paradox were both proposed in 1908: Russell's own [[type theory]] and the [[Zermelo set theory]]. In particular, Zermelo's axioms restricted the unlimited comprehension principle. With the additional contributions of [[Abraham Fraenkel]], Zermelo set theory  developed into the now-standard [[Zermelo–Fraenkel set theory]] (commonly known as ZFC when including the [[axiom of choice]]). The main difference between Russell's and Zermelo's solution to the paradox is that Zermelo modified the axioms of set theory while maintaining a standard logical language, while Russell modified the logical language itself. The language of ZFC, with the [[Axiom schema of replacement#History|help of Thoralf Skolem]], turned out to be that of [[first-order logic]].<ref name="FraenkelBar-Hillel1973">{{cite book|author1=A.A. Fraenkel|author2=Y. Bar-Hillel|author3=A. Levy|title=Foundations of Set Theory|year=1973|publisher=Elsevier|isbn=978-0-08-088705-0|pages=156–157}}</ref>

The paradox had already been discovered independently in 1899 by the German mathematician [[Ernst Zermelo]].<ref>Bernhard Rang, Wolfgang Thomas: Zermelo's Discovery of the "Russell Paradox", Historia Mathematica 8.</ref> However, Zermelo did not publish the idea, which remained known only to [[David Hilbert]], [[Edmund Husserl]], and other academics at the [[University of Göttingen]]. At the end of the 1890s, [[Georg Cantor]] – considered the founder of modern set theory – had already realized that his theory would lead to a contradiction, as he told Hilbert and [[Richard Dedekind]] by letter.<ref>[[Walter Purkert]], Hans J. Ilgauds: ''Vita Mathematica - Georg Cantor'', Birkhäuser, 1986, {{ISBN|3-764-31770-1}}</ref>