Fashulun 法書論 is a theory of calligraphy written during the late Tang period by Cai Xizong 蔡希綜 (the last character might also be written 悰, 琮 or 宗; c. 750), from Puyang 濟陽 or Qu’a 曲阿 (today’s Danyang 丹陽, Jiangsu). He hailed from a family of famous calligraphers. His calligraphic interpretation of the text Zhi Puqiao ji 治浦橋記 has survived. His „theory of calligraphy“ has a length of one fascicle and is divided into five chapters. The first expounds of the history of 19 generations the Cai family with their outstanding calligraphers, beginning in the Later Han period with the ancestor Cai Yong 蔡邕, who mastered the small seal script 篆, the large seal script 籀, and the bati style八體 of the chancery script, and ending with the author’s generation, mainly his older brothers Cai Xiyi 蔡希逸 and Cai Xiji 蔡希寂. The second chapter is dedicated to other calligrapher families, from the early Zhou period to the then-present time. Cai mainly explains which calligraphic styles the individual proponents mastered. Examples for family traditions are the master Cui Yuan 崔瑗and Cui Shi 崔寔 father and son, the brothers Zhang Zhi 張芝 and Zhang Chang 張昶, Wei Guan 衛瓘 and Wei Huan衛恒father and son, Zhong Yao 鍾繇 and Zhong Hui 鍾會 father and son, or Wang Xizhi 王羲之 and Wang Xianzhi 王獻之 father and son. The third chapter examines the correct use of the brush based on the practice of Wang Xizhi and Cai Yong. The imitation of the ancient masters was essential to learn how to handle the brush correctly. The student of calligraphy had first to adjust his mindset and follow his emotions (xian shi yi ren qing 先適意任情) before starting with actual works of the art. Then, the practitioner had to sit upright, calm the mind, and let the brush follow one’s intent (sui yi suo ni 隨意所擬). The words to write should not be uttered, and the breath should not be interrupted. Next, one must "sit upright, calm the mind, and let the brush follow one's intent. Words should not be spoken aloud, and the breath should not be interrupted. In this way, there will be no imperfection." The fourth section recounts Zhong Yao’s experiences in mastering calligraphy. Zhong Yao’s learning reached a profound level. Zhong said, “Whenever I see the myriad writing styles, I replicate them all.” His style especially emphasised the spirit and intention behind the writing (yi zai bi qian, ranhou zuo zi 意在筆前,然後作字). He said, “If you want to write, first, grind the ink, concentrate, and meditate. Anticipate the brush’s size, orientation, and movement, ensuring the strokes connect.” Each piece of writing should possess its own character. Even if characters are repeated, each must convey a distinct meaning (xu zi zi yi shu 須字字意殊). The fifth section discusses the method of putting brush to paper. Cai Xiyong explains that when starting to write, one must conceal the tip of the brush and rotate the wrist, with a slow start followed by a swift finish. The forms of the characters should connect like intertwined insects or snakes, and the intention must not be interrupted. It should also be concise and not overly elaborate. For instance, the strokes should rise and fall according to the natural flow. The text is quoted in the book Shuyuan jinghua 書苑菁華 and included in the series Lidai shufa lunwen xuan 歷代書法論文選.