2002 Donald Rumsfeld Unknown unknowns

Former United States Secretary of Defense, Donald Rumsfeld, Defense Department Briefing on February 12, 2002:

Reports that say that something hasn't happened are always interesting to me, because as we know, 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.