US History 1973 - 2014 Commonplace Book: Lecture 25, The Iraq War

...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.

—Secretary of Defense Donald Rumsfeld (February, 2002)

Introduction to (and explanation of) this quote series can be found here.  Read this tag to see all of them.