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.
Madness string 81577
Yes, this is common sense, but common sense is not as common as it is touted to be. (As we all know, or do we?)