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

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

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

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.

Where knowledge ends, religion begins.