知之为知之，不知为不知
1. When you understand a thing, say that you know it, and when you do not understand a thing, say that you do not know it.
2. 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.
3. to know it as you know; and to know it not, as you don't.
4. One speaks only what one really knows and is frank about it when one does not know it.
