Urelement

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In set theory, a branch of mathematics, an urelement or ur-element (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. Ur-elements are also not identical with the empty set.

Theory

There are several different but essentially equivalent ways to treat urelements in a

first-order theory
.

One way is to work in a first-order theory with two sorts, sets and urelements, with ab 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

unary relation used to distinguish sets and urelements. As non-empty 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

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

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

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 lowest-level 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

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