Known knowns

"The message is:

That there are known knowns; there are things we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know. And each year we discover a few more of those unknown unknowns".

Donald Rumsfeld Feb. 12, 2002, Department of Defense news briefing