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. And if one looks throughout the history of our country and other free countries, it is the latter category that tends to be the difficult ones.[1]
Donald Rumsfeld, United States Secretary of Defense, 2002 February 12
Known unknowns refers to “risks you are aware of, such as cancelled flights”, whereas unknown unknowns are risks that come from situations that are so unexpected that they would not be considered in advance.
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