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

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

3. Raising questions is more important than solving them. ——Einstein

4. There is no doubt that there will be doubts in learning. If there is doubt, there will be progress. If there is little doubt, there will be small progress. If there is great doubt, there will be great progress. ——Lu Jiuyuan Song

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

6. Scholars must first doubt. ——Cheng Yi