Every day in the morning, Zen master Ruiyan would slap himself in the face and say, "Don't be fooled by anything today!" To which he responded, "No I won't."
What does this mean?
It means he was entering the last phase of self-mastery. He was not accepting any concept. In fact, ANY concept that arose, he would cut it down.
After one has attained a sense of completeness, you realize that contentment comes from not accepting anyone's "truth" except your own. And, you yourself have no truth at all, no concepts, no understanding, and as soon as anyone throws a concept at you, you knock it away, leaving you just walking forward, straight ahead, not fooled by any "truth."
This was said even more cogently by the Sixth Patriarch, Hui Neng, who said, "The only truth is that there is no truth; beware even of this truth."
So many cannot imagine living in freedom where the only "book" they read is their own intuition in the moment, an intuition derived from throwing all books, teachings and teachers away.