Donald Rumsfeld was the United States Secretary of Defense from 2001 to 2006 under President Bush. He is known for many things, but will always be remembered for his statement of an old truism, as quoted -
… 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. 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.
© 2014, Christopher Ambler