Full Idea
The consistency of set theory cannot be established without assumptions transcending set theory.
Gist of Idea
To prove the consistency of set theory, we must go beyond set theory
Source
Volker Halbach (Axiomatic Theories of Truth (2005 ver) [2005], 2.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.5