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 the are also unknown unknowns,
The ones we don't know we don't know. 

February 12, 2002, Department of Defense news briefing

-Donald Rumsfeld