“…there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don’t know…”

DoD News Briefing – Secretary Rumsfeld and Gen. Myers
Presenter: Secretary of Defense Donald H. Rumsfeld February 12, 2002 11:30 AM EDT
http://www.defense.gov/transcripts/transcript.aspx?transcriptid=2636