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.
Authentication Score 2
Rumsfeld, Donald. Department of Defense news briefing. 12 Feb. 2002, Pentagon, Washington, DC, USA.