What is the consciousness of "books are expensive, hard and well-informed" in calligraphy?

A poem by Du Fu, commenting on the calligraphy of the Tang Dynasty. Du Fu put forward that "the book is expensive but hard, and the spirit can be solved" has both artistic aspects: taking the early Tang Dynasty as the standard, advocating style and pursuing ancient style. On the other hand, Du has a reason, which refers to the method of "fable" and "prosperity". Especially after the Anshi Rebellion, Du Fu, who entered his later years, faced the decline of the Tang Dynasty and remembered the past vividly. Du Fu, who used to be a "gentleman who never forgets a meal", failed to realize his political ideal in the end, so his thoughts on ancient love, the rise and fall of the country and his own destiny all belong to the current dynasty. Therefore, the resentment against the emperor, the memory of old friends and the nostalgia for the ancient style are all vented through his poems.

Later, people used this sentence to evaluate calligraphy works with thin and hard fonts, such as the words of Qi Gong and Mao Dun. Not a uniform standard, the word fat also has a fat style.