Full Idea
It has turned out that the solution of certain arithmetical problems requires the use of assumptions essentially transcending arithmetic.
Gist of Idea
Some arithmetical problems require assumptions which transcend arithmetic
Source
Kurt Gödel (Russell's Mathematical Logic [1944], p.449)
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.449
A Reaction
A nice statement of the famous result, from the great man himself, in the plainest possible English.