"There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know."
(1932 - )
Secretary of Defense from 1975 to 1977 under Gerald Ford, and again from January 2001 to December 2006 under George W. Bush.
(Surprisingly the above quote makes a lot of sense when you think about it.)