The Unknown
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.
— Donald Rumsfeld, Secretary of Defense
Feb. 12, 2002, Department of Defense news briefing