...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.Introduction to (and explanation of) this quote series can be found here. Read this tag to see all of them.
—Secretary of Defense Donald Rumsfeld (February, 2002)