What do we know that we know that we can know?

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.

D. Rumsfeld, February 12, 2002, Department of Defense news briefing