In set theory, a branch of mathematics, an urelement or urelement (from the German prefix ur, 'primordial') is an object that is not a set (has no elements), but that may be an element of a set. It is also referred to as an atom or individual. Urelements are also not identical with the empty set.
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Set Theory  Introduction

Bertrand Russell, Set Theory and Russell's Paradox  Professor Tony Mann

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Transcription
Theory
There are several different but essentially equivalent ways to treat urelements in a firstorder theory.
One way is to work in a firstorder theory with two sorts, sets and urelements, with a ∈ b only defined when b is a set. In this case, if U is an urelement, it makes no sense to say , although is perfectly legitimate.
Another way is to work in a onesorted theory with a unary relation used to distinguish sets and urelements. As nonempty sets contain members while urelements do not, the unary relation is only needed to distinguish the empty set from urelements. Note that in this case, the axiom of extensionality must be formulated to apply only to objects that are not urelements.
This situation is analogous to the treatments of theories of sets and classes. Indeed, urelements are in some sense dual to proper classes: urelements cannot have members whereas proper classes cannot be members. Put differently, urelements are minimal objects while proper classes are maximal objects by the membership relation (which, of course, is not an order relation, so this analogy is not to be taken literally).
Urelements in set theory
The Zermelo set theory of 1908 included urelements, and hence is a version now called ZFA or ZFCA (i.e. ZFA with axiom of choice).^{[1]} It was soon realized that in the context of this and closely related axiomatic set theories, the urelements were not needed because they can easily be modeled in a set theory without urelements.^{[2]} Thus, standard expositions of the canonical axiomatic set theories ZF and ZFC do not mention urelements (for an exception, see Suppes^{[3]}). Axiomatizations of set theory that do invoke urelements include Kripke–Platek set theory with urelements and the variant of Von Neumann–Bernays–Gödel set theory described by Mendelson.^{[4]} In type theory, an object of type 0 can be called an urelement; hence the name "atom".
Adding urelements to the system New Foundations (NF) to produce NFU has surprising consequences. In particular, Jensen proved^{[5]} the consistency of NFU relative to Peano arithmetic; meanwhile, the consistency of NF relative to anything remains an open problem, pending verification of Holmes's proof of its consistency relative to ZF. Moreover, NFU remains relatively consistent when augmented with an axiom of infinity and the axiom of choice. Meanwhile, the negation of the axiom of choice is, curiously, an NF theorem. Holmes (1998) takes these facts as evidence that NFU is a more successful foundation for mathematics than NF. Holmes further argues that set theory is more natural with than without urelements, since we may take as urelements the objects of any theory or of the physical universe.^{[6]} In finitist set theory, urelements are mapped to the lowestlevel components of the target phenomenon, such as atomic constituents of a physical object or members of an organisation.
Quine atoms
An alternative approach to urelements is to consider them, instead of as a type of object other than sets, as a particular type of set. Quine atoms (named after Willard Van Orman Quine) are sets that only contain themselves, that is, sets that satisfy the formula x = {x}.^{[7]}
Quine atoms cannot exist in systems of set theory that include the axiom of regularity, but they can exist in nonwellfounded set theory. ZF set theory with the axiom of regularity removed cannot prove that any nonwellfounded sets exist (unless it is inconsistent, in which case it will prove any arbitrary statement), but it is compatible with the existence of Quine atoms. Aczel's antifoundation axiom implies that there is a unique Quine atom. Other nonwellfounded theories may admit many distinct Quine atoms; at the opposite end of the spectrum lies Boffa's axiom of superuniversality, which implies that the distinct Quine atoms form a proper class.^{[8]}
Quine atoms also appear in Quine's New Foundations, which allows more than one such set to exist.^{[9]}
Quine atoms are the only sets called reflexive sets by Peter Aczel,^{[8]} although other authors, e.g. Jon Barwise and Lawrence Moss, use the latter term to denote the larger class of sets with the property x ∈ x.^{[10]}
References
 ^ Dexter Chua et al.: ZFA: Zermelo–Fraenkel set theory with atoms, on: ncatlab.org: nLab, revised on July 16, 2016.
 ^ Jech, Thomas J. (1973). The Axiom of Choice. Mineola, New York: Dover Publ. p. 45. ISBN 0486466248.
 ^ Suppes, Patrick (1972). Axiomatic Set Theory ([Éd. corr. et augm. du texte paru en 1960] ed.). New York: Dover Publ. ISBN 0486616304. Retrieved 17 September 2012.
 ^ Mendelson, Elliott (1997). Introduction to Mathematical Logic (4th ed.). London: Chapman & Hall. pp. 297–304. ISBN 9780412808302. Retrieved 17 September 2012.
 ^ Jensen, Ronald Björn (December 1968). "On the Consistency of a Slight (?) Modification of Quine's 'New Foundations'". Synthese. 19 (1/2). Springer: 250–264. doi:10.1007/bf00568059. ISSN 00397857. JSTOR 20114640. S2CID 46960777.
 ^ Holmes, Randall, 1998. Elementary Set Theory with a Universal Set. AcademiaBruylant.
 ^ Thomas Forster (2003). Logic, Induction and Sets. Cambridge University Press. p. 199. ISBN 9780521533614.
 ^ ^{a} ^{b} Aczel, Peter (1988), Nonwellfounded sets, CSLI Lecture Notes, vol. 14, Stanford University, Center for the Study of Language and Information, p. 57, ISBN 0937073229, MR 0940014, retrieved 20161017.
 ^ Barwise, Jon; Moss, Lawrence S. (1996), Vicious circles. On the mathematics of nonwellfounded phenomena, CSLI Lecture Notes, vol. 60, CSLI Publications, p. 306, ISBN 1575860090.
 ^ Barwise, Jon; Moss, Lawrence S. (1996), Vicious circles. On the mathematics of nonwellfounded phenomena, CSLI Lecture Notes, vol. 60, CSLI Publications, p. 57, ISBN 1575860090.