Unknown unknowns

We have been reading Plato's Theatetus in class for the past few weeks.  There is a part of it which rambles about knowing that you know, knowing what you don't know... and all that stuff.

One strange fact that we all agreed upon in class is that we 'cannot know that you don't know'. We know something and there are things we don't know at all.  But there cannot be the case that you do know, that what you do not know.  The only thing you know is that you don't know!

There was mentioned in the class the name of Donald Rumsfield and his quote about this topic.  Interesting and evidently confusing...

There are known knowns; these 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. 

Read more of it here.