On February 12, 2002, “regarding the threats that confront the United States, he [Secretary of Defense Donald Rumsfeld] said, ‘There 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. …Each year, we discover a few more of those unknown unknowns.’ “
– Foreign Affairs, The U.S. vs. al Qaeda, Page 122