Full Idea
The expressive power of second-order logic is too great to admit a proof procedure, but is adequate to express set-theoretical statements, and open questions such as the continuum hypothesis or the existence of big cardinals are easily stated.
Gist of Idea
Second-order logic isn't provable, but will express set-theory and classic problems
Source
Leslie H. Tharp (Which Logic is the Right Logic? [1975], §2)
Book Reference
'Philosophy of Logic: an anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.38