"No one person can know everything," the master said to the student, "but one person can think that he knows everything."
"But Master," the student implored, "How can I separate what I know from what I do not?"
The master stared into the fire for a minute infinity, and slowly replied, "You take what you know, and subtract it from everything else. Whatever you have left is what you don't know."
The student nodded, "But the man who thinks he knows everything, what will he have left?"