There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.
When you start talking about the known knowns and the unknown unknowns, you're thrown into a crazy meta-level discussion. Do I know what I know, do I know what I don't know, do I know what I don't know I don't know. It becomes a strange, Lewis Carroll - like nursery rhyme.