I don’t know who said it first, but it’s worth repeating.

The world is divided into two groups.
 
There are those who know, and those who don’t know.
Those who know, they’re no problem.

Those who don’t know are also divided into two groups.
One is those who don’t know, and know they don’t know. Well, they can learn!

But then, there are those who don’t know, and don’t know they don’t know.
They become supervisors.