1. Teachers use doubts, and friends use doubts. ——Li Xing

2. Problems are midwives, they can help the birth of new ideas. ——Socrates

3. The spirit of skepticism is an important part of the scientific spirit. ——Zhou Guangzhao

4. Thinking starts with questions and surprises. ——Aristotle

5. Those who have no doubts in reading must learn to have doubts, and those who have doubts must have no doubts. This is the only way to make progress. ——Zhu Xi

6. The ears have no bottom and can be heard from morning to night.

——Africa