Our knowledge is like a circle. Inside the circle is the thing you know about, outside the circle is the thing you don’t know about. A the edge of the circle is the thing you know you don’t know about.
The longer this circumference is, the more knowledge you have, the more you realize you don’t know about.
A person have a large circle knows a lot he/she don’t know, so one have to be humble. A person have a small circle knows a little, so one think the whole world is in one’s hands.