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

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

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.

Ignorance is the night of the mind, a night without moon or star

The greatest ignorance is to reject something you know nothing about

It takes considerable knowledge just to realize the extent of your own ignorance.