If things far away don’t concern you, you’ll soon mourn things close at hand.

To know what you know and what you do not know, that is true knowledge.

The hardest thing of all is to find a black cat in a dark room, especially if there is no cat.

Real knowledge is to know the extent of one's ignorance.

When you know a thing, to hold that you know it, and when you do not know a thing, to allow that you do not know it - this is knowledge.

Blind is the man who thinks he knows it all. Blinder is the man who follows after this man blindly.