If the sets and have the same members, then they are the same set.[7][1]
In pure set theory, all members of sets are themselves sets, but not in set theory with urelements. The axiom's usefulness can be seen from the fact that, if one accepts that , where is a set and is a formula that occurs free in but doesn't, then the axiom assures that there is a unique set whose members are precisely whatever objects (urelements or sets, as the case may be) satisfy the formula .
Quine's New Foundations (NF) set theory, in Quine's original presentations of it, treats the symbol for equality or identity as shorthand either for "if a set contains the left side of the equals sign as a member, then it also contains the right side of the equals sign as a member" (as defined in 1937), or for "an object is an element of the set on the left side of the equals sign if, and only if, it is also an element of the set on the right side of the equals sign" (as defined in 1951). That is, is treated as shorthand either for , as in the original 1937 paper, or for , as in Quine's Mathematical Logic (1951). The second version of the definition is exactly equivalent to the antecedent of the ZF axiom of extensionality, and the first version of the definition is still very similar to it. By contrast, however, the ZF set theory takes the symbol for identity or equality as a primitive symbol of the formal language, and defines the axiom of extensionality in terms of it. (In this paragraph, the statements of both versions of the definition were paraphrases, and quotation marks were only used to set the statements apart.)
In Quine's New Foundations for Mathematical Logic (1937), the original paper of NF, the name "principle of extensionality" is given to the postulate P1, ,[10] which, for readability, may be restated as . The definition D8, which defines the symbol for identity or equality, defines as shorthand for .[10] In his Mathematical Logic (1951), having already developed quasi-quotation, Quine defines as shorthand for (definition D10), and does not define an axiom or principle "of extensionality" at all.[11]
Thomas Forster, however, ignores these fine distinctions, and considers NF to accept the axiom of extensionality in its ZF form.[12]
In ZU set theory
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In the Scott–Potter (ZU) set theory, the "extensionality principle" is given as a theorem rather than an axiom, which is proved from the definition of a "collection".[13]
In set theory with ur-elements
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An ur-element is a member of a set that is not itself a set.
In the Zermelo–Fraenkel axioms, there are no ur-elements, but they are included in some alternative axiomatisations of set theory.
Ur-elements can be treated as a different logical type from sets; in this case, makes no sense if is an ur-element, so the axiom of extensionality simply applies only to sets.
Alternatively, in untyped logic, we can require to be false whenever is an ur-element.
In this case, the usual axiom of extensionality would then imply that every ur-element is equal to the empty set.
To avoid this consequence, we can modify the axiom of extensionality to apply only to nonempty sets, so that it reads:
That is:
Given any set A and any set B, if A is a nonempty set (that is, if there exists a member X of A), then if A and B have precisely the same members, then they are equal.
Yet another alternative in untyped logic is to define itself to be the only element of
whenever is an ur-element. While this approach can serve to preserve the axiom of extensionality, the axiom of regularity will need an adjustment instead.
Paul Halmos, Naive set theory. Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by Springer-Verlag, New York, 1974. ISBN 0-387-90092-6 (Springer-Verlag edition).
Jech, Thomas, 2003. Set Theory: The Third Millennium Edition, Revised and Expanded. Springer. ISBN 3-540-44085-2.
Kunen, Kenneth, 1980. Set Theory: An Introduction to Independence Proofs. Elsevier. ISBN 0-444-86839-9.
^"Naive Set Theory". sites.pitt.edu. Retrieved 2024-08-20.
^Bourbaki, N. (2013-12-01). Theory of Sets. Springer Science & Business Media. p. 67. ISBN 978-3-642-59309-3.
^Deskins, W. E. (2012-05-24). Abstract Algebra. Courier Corporation. p. 2. ISBN 978-0-486-15846-4.
^"Zermelo-Fraenkel Set Theory". www.cs.odu.edu. Retrieved 2024-08-20.
^"Intro to Axiomatic (ZF) Set Theory". www.andrew.cmu.edu. Retrieved 2024-08-20.
^ ab"Set Theory > Zermelo-Fraenkel Set Theory (ZF) (Stanford Encyclopedia of Philosophy)". plato.stanford.edu. Retrieved 2024-11-24.
^"Zermelo-Fraenkel Set Theory". www.cs.odu.edu. Retrieved 2024-11-24.
^"Naive Set Theory". sites.pitt.edu. Retrieved 2024-11-24.
^ abQuine, W. V. (1937). "New Foundations for Mathematical Logic". The American Mathematical Monthly. 44 (2): 74, 77. doi:10.2307/2300564. ISSN 0002-9890.
^Quine, W. V. (1951-12-31). "Mathematical Logic". DeGruyter: 134–136. doi:10.4159/9780674042469.
^Forster, Thomas (2019), Zalta, Edward N. (ed.), "Quine's New Foundations", The Stanford Encyclopedia of Philosophy (Summer 2019 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-11-24
^Potter, Michael D. (2004). Set theory and its philosophy: a critical introduction. Oxford ; New York: Oxford University Press. p. 31. ISBN 978-0-19-926973-0. OCLC 53392572.