From its conception until now, ZFC has had its problems.

   "In 1908, Ernst Zermelo proposed the first axiomatic set theory, Zermelo set theory. However, as first pointed out by Abraham Fraenkel in a 1921 letter to Zermelo, this theory was incapable of proving the existence of certain sets and cardinal numbers whose existence was taken for granted by most set theorists of the time, notably, the cardinal number ℵ ω and, where Z0 is any infinite set and ℘ is the power set operation, the set {Z0, ℘ (Z0), ℘ (℘(Z0)),...} (Ebbinghaus 2007, p. 136). Moreover, one of Zermelo's axioms invoked a concept, that of a 'definite' property, whose operational meaning was not clear. In 1922, Fraenkel and Thoralf Skolem independently proposed operationalizing a 'definite' property as one that could be formulated as a first order theory whose atomic formulas were limited to set membership and identity. They also independently proposed replacing the axiom schema of specification with the axiom schema of replacement. Appending this schema, as well as the axiom of regularity (first proposed by Dimitry Mirimanoff in 1917), to Zermelo set theory yields the theory denoted by ZF. Adding to ZF either the axiom of choice (AC) or a statement that is equivalent to it yields ZFC."20

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