In reply to SouthernSteve:
"Anyone who knows, and knows that he knows,
makes the steed of intelligence leap over the vault of heaven.
Anyone who does not know but knows that he does not know,
can bring his lame little donkey to the destination nonetheless.
Anyone who does not know, and does not know that he does not know,
is stuck forever in double ignorance."
13th Century Persian intellectual Nasir al-Din al-Tusi.
The quote has been around a lot longer than Rumsfeld.