On February 12, 2002, US Secretary of Defense Donald Rumsfeld offered this pithy summary of learning issues (as used in Problem-Based Learning) as the "known unknowns".
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.
US Sec. of Defense Donald Rumsfeld, Department of Defense news briefing, Feb. 12, 2002
Formatted as poetry by reporter Hart Seely in "Pieces of Intelligence, The Existential Poetry of Donald H. Rumsfeld".