The famous sayings with this spirit are as follows:

1. Those who do not doubt when there is doubt have never learned, but learning must be doubtful.

2. Question, rather than question, "Real singers can sing out the silence in people's hearts."

3. Those who have no doubts about reading must learn to have doubts, and those who have doubts must have no doubts. This is the only way to make progress.

4. Teachers use doubts, and friends use doubts.