Cantor's paradox

In <a href="/facts/Set_theory/1ptfsvUP">set theory</a>, Cantor's paradox states that there is no <a href="/facts/Set_(mathematics)/BfucfHMq">set</a> of all <a href="/facts/Cardinalities/m6VPojwT">cardinalities</a>. This is derived from the <a href="/facts/Theorem/f2Pe1FMD">theorem</a> that there is no greatest <a href="/facts/Cardinal_number/ccoKwU4R">cardinal number</a>. In informal terms, the paradox is that the collection of all possible "infinite sizes" is not only infinite, but so infinitely large that its own infinite size cannot be any of the infinite sizes in the collection. The difficulty is handled in <a href="/facts/Axiomatic_set_theory/1ptfsvUP">axiomatic set theory</a> by declaring that this collection is not a set but a <a href="/facts/Proper_class/F5EmDd9b">proper class</a>; in <a href="/facts/Von_Neumann%E2%80%93Bernays%E2%80%93G%C3%B6del_set_theory/5wLhAIMz">von Neumann–Bernays–Gödel set theory</a> it follows from this and the <a href="/facts/Axiom_of_limitation_of_size/CMylopzq">axiom of limitation of size</a> that this proper class must be in <a href="/facts/Bijection/j4gTuTmW">bijection</a> with the class of all sets. Thus, not only are there infinitely many infinities, but this infinity is larger than any of the infinities it enumerates.
This <a href="/facts/Paradox/3VLGjuWf">paradox</a> is named for <a href="/facts/Georg_Cantor/k5v1DvW4">Georg Cantor</a>, who is often credited with first identifying it in 1899 (or between 1895 and 1897). Like a number of "paradoxes" it is not actually contradictory but merely indicative of a mistaken intuition, in this case about the nature of infinity and the notion of a set. Put another way, it is paradoxical within the confines of <a href="/facts/Na%C3%AFve_set_theory/hYIC5xPf">naïve set theory</a> and therefore demonstrates that a careless axiomatization of this theory is inconsistent.

Cantor's paradox open-in-new

Cantor's paradox