As we know, there are known knowns; there are things that we know that 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. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.
—Donald Rumsfeld, former Secretary of Defense, U.S. Department of Defense, 2002.
Knowing What We Don't Know
In February of 2002, then-Secretary of Defense Donald Rumsfeld issued the above quote in response to a reporter's question on whether or not Iraq was supplying weapons of mass destruction ...