During the Spring and Autumn period, Confucius had a student named Bu Shang, also known as Zixia, who was praised for his thoughtful approach to learning. After Confucius died, Zixia settled in Xihe, a region in the state of Wei.
Once, during the sweltering heat of the fifth month, Zi Xia set out by horse-drawn carriage to visit a friend in the state of Jin. The horse was soon drenched in sweat from the long journey. Seeing this, Zi Xia would stop wherever there was water and grass, letting the horse drink and rest before continuing on.
On this day, Zixia arrived at the border of the Wei state, just one day's journey away from reaching the Jin state.
Zi Xia stopped his carriage, led his horse to the riverbank. Just then, under a tree by the river, someone was loudly reciting a book. Zi Xia heard him reading, "The Jin army attacked Qin, three pigs crossed the river." Zi Xia thought, "How could the Jin army have three pigs crossing the river?"
After careful thought, that's it—the characters in the book must have been copied wrong. "Ji" and "San" look similar in form, and "Hai" and "Shi" also resemble each other. It should be "Ji Hai crossed the river" for it to make sense.
The ancients used the ten "Heavenly Stems" and twelve "Earthly Branches" in sequence to mark days, so "Jihai" refers to a specific day. This sentence means: "On the day of Jihai, the Jin army crossed the Yellow River to attack Qin."
Zi Xia thought it over, felt confident in his reasoning, tied his horse to a tree, approached the scholar, and said, "Could you let me take a look at your book?"
Zixia took the book and saw that the two characters were indeed "ji hai," but they looked very much like "san shi." Zixia told the reader, "These two characters should be 'ji hai,' but you read them as 'san shi.' How can that make sense?"
Then Zixia explained the reasoning to him. The man found Zixia's words very reasonable and quickly thanked him, saying, "Thank you for your guidance, sir."
When Zixia arrived in the state of Jin, he met a friend and told him about this encounter on the road. The friend sighed and said, "Reading without using your mind is the most useless. But careless copying can also easily lead to mistakes and cause embarrassment."
Zi Xia said, "That's right. Reading 'jihai' as 'three pigs' is obviously illogical if you think about it, and the error can be corrected immediately. As Confucius said, 'Learning without thought is labor lost.' Diligence and deep thinking are indispensable in the pursuit of knowledge."
Later, the idiom "Three Pigs Cross the River" came to be used as a metaphor for textual or printing errors that are laughably absurd.
Source: *Lüshi Chunqiu*, Chapter "Examining Traditions"
Meaning of the Idiom: Later, the Chinese idiom "三豕涉河" came to describe how textual or printing errors that are laughably absurd.