But There Are Also Unknown Unknowns

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
February 2, 2002 (Department of Defense news briefing)