The origin of the allusion of brush character

Mi Fu is one of the four masters of calligraphy in the Song Dynasty (Su Huang, Mi Cai). Xizhi studied calligraphy, Shi Zhenzong of seal script, and was subordinate to Dharma Master Yiguan. In his later years, there are rules for coming in and out, and people compete for treasures. When he asked Huizong about it, he said, "I am writing brushes on my writings." Also, those who claim to be good at calligraphy only have one stroke, while I only have four sides. Throughout his life, he was proud to be able to write small regular script with a pen. His strokes are neat and precise, his words are like a fly's head, and his position and scale are all like big characters.