Year after year students tumble along like the waters of a river. They flow away, and only the teacher is left behind, like some deeply buried rock at the bottom of the current.
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.
- United States Secretary of Defense, Donald Rumsfeld (2002)