Full Idea
Cantor first stated the Union Axiom in a letter to Dedekind in 1899. It is nearly too obvious to deserve comment from most commentators. Justifications usually rest on 'limitation of size' or on the 'iterative conception'.
Gist of Idea
The Axiom of Union dates from 1899, and seems fairly obvious
Source
report of George Cantor (works [1880]) by Penelope Maddy - Believing the Axioms I §1.3
Book Reference
-: 'Journal of Symbolic Logic' [-], p.484
A Reaction
Surely someone can think of some way to challenge it! An opportunity to become notorious, and get invited to conferences.