Cheng Hao and Cheng Yi were the founding fathers of Neo-Confucianism in the Northern Song Dynasty. They were blood brothers, differing on minor points but highly consistent on major issues, and later generations collectively refer to them as the "Two Chengs."
The Cheng brothers, Cheng Hao and Cheng Yi, were teaching in Luoyang, attracting many students drawn by their reputation.
Once, a student asked them, "Master, your knowledge is vast. The world has countless matters—do you know them all?"
Cheng Yi, though he considered himself a sage, also admitted there were many things he did not know. He said, "From birth, only nursing is innate knowledge. Everyone knows some things and not others, can do some things and not others. But if you all study diligently, you can gain knowledge."
"Everyone should pay attention: not only must you learn knowledge from books, but you must also focus on accumulating knowledge through practice. The former is common knowledge, while the latter is true knowledge." Cheng Hao added from the side.
"The student asked further, 'Master, could you explain in detail what is common knowledge and what is true knowledge?' The terms 'common knowledge' and 'true knowledge' were ones they had never heard before. 'Let me give a few examples to illustrate what is common knowledge and what is true knowledge,' the Cheng brothers said to their disciples."
For example, when it comes to gold, both the poor and the rich know the difference between yellow gold and white gold, and can even tell whether gold is hard or soft. However, only the rich have a genuine, tangible understanding of gold, because they own it at home. The poor, who have no gold and rarely, if ever, come into contact with it, only know the common knowledge that everyone shares.
"Consider another example: 'kuai zhi' (finely minced meat and roasted meat) – everyone knows it's incredibly delicious. But noble families, with their endless feasts of delicacies, can taste 'kuai zhi' firsthand and truly know its flavor; that is genuine knowledge. Common folk, who have never even seen 'kuai zhi,' only know it's delicious from hearsay – that is merely superficial knowledge."
"Let me tell you something I witnessed myself. Once, a farmer working in the mountains encountered a tiger, was mauled, and nearly died. Even now, his wounds ache, and the memory terrifies him. One day during a break in the fields, everyone was discussing tiger attacks, and all turned pale—except the farmer, who looked especially tense. Having felt the tiger's ferocity firsthand, he knew its true power. Only someone who has been bitten by a tiger truly understands how fearsome it is."
"The idiom 'Pale at the Mention of a Tiger' originally meant that only those bitten by a tiger truly know its ferocity. Later, it came to describe someone who tenses up at the mere mention of something frightening."
Source: *Complete Works of the Cheng Brothers*, Chapter "Surviving Writings, Part Two"
Meaning of the Idiom: Later, the Chinese idiom "谈虎色变" came to describe how someone who has experienced a frightening event firsthand becomes extremely tense or fearful at the mere mention of it.