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.
Bur there are also unknown unknowns.
The ones we don’t know we don’t know.

February 12, 2002, Department of Defense news briefing

—Donald Rumsfeld, as versified by Hart Seely in Pieces of Intelligence: The Existential Poetry of Donald H. Rumsfeld, (New York: Free Press, 2003), 2.