“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 – there are things we do not know we don’t know” – Source: Donald Rumsfeld
Some argue there are also unknown knowns. Things we know but we don’t know how like riding a bike. However I tend to think of these rather as things we know but have difficulty communicating.
We know that we know how to ride a bicycle, we just struggle to explain it to others through the limitations of language