User blog comment:TheKing44/Grand Finale Functions/@comment-35470197-20190611051239/@comment-39605890-20190611120131

> Oh, I did not know the statement. Could you give me a source?

Actually, I seem to be mistaken. The article Rayo's number makes that claim.

Also, I see the ambiguity. Thanks for pointing that out! I have edited the post to clarify that I mean first-order set theory with semantics provided by V. In this sense, MK and NBG are essentially second-order set theories since they can quantify over predicates and functions over V.

Oh, and I imagine the axioms Rayo was thinking of were second-order ZFC or something. It is concerning that he did not clarify it though.