The saying 'good medicine is bitter but cures the illness' comes from the School Sayings of Confucius.
Tips
history
It was long thought to be a forgery by 王肃 of the 3rd century, who first published it with his commentary. Texts on bamboo slips found in Han tombs in the 1970s match parts of it, and scholars now think much of it is early material that Wang Su edited.
culture
Its chapters are cited by name, as in 孔子家语·观周, where Confucius praises Laozi as a man versed in antiquity who understands the present, the source of the idiom 博古通今.