Huajian 畫鑒 "The mirror of painting", also called Gujin huajian 古今畫鑒 or Huayin 畫吟, is a critical book on historical paintings written during the Yuan period 元 (1279-1368) by Tang Hou 湯垕 (fl. 1328), courtesy name Junzai 君載, style Dongchu Caizhenzi 東楚采真子, from Shanyang 山陽 (today’s Huai'an 淮安, Jiangsu).
The brief text was finished in 1328. It is divided into nine chapters according to dynastic eras, but the last part includes foreign paintings (Waiguo hua 外國畫, with examples from Gaochang 高昌 and Koryŏ 高麗), and some words on the theory of painting (Hualun 畫論). The "Mirror" discusses the skills and achievement of outstanding painters through the ages and rates their mastering of the interplay of brush and ink. Tang Hou also analyses how to discern forgeries from originals. To a large extent, his book is based on Mi Fu's 米芾 (1051-1107) Huashi 畫史.
The chapter Zalun 雜論 presents twenty-three rules on the collection and storage of paintings. In the last, theoretical part of the book, Tang stresses the importance of "energy and rhythm" (qiyun 氣韻), "the grasping of spirit" (shencai 神采), the "spirit of the influences or styles" (fengshen 風神), "natural authenticity" (tianzhen 天真), the "intentions of the brush" (biyi 筆意) and methods of using the brush (bifa 筆法). The quality of a painting was not in copying the appearance of a real object, but in the visualization of an idea (dang yi yi xie zhi, bu zai xingsi 當以意寫之，不在形似). Depiction of real objects was just a marginal aim of painting (xingsi wei mo jie 形似為末節), while the creation of novel ideas was its main purpose (zi chu xin yi 自出新意). The painter had to "neglect the shape and strive for spirituality" (yi mao qu shen 遺貌取神).
Some ancient editions include the note Gouqu Waishi ti 勾曲外史題, which refers to Zhang Yu 張雨 (1277-1348), also called Zhang Tianyu 張天雨，courtesy name Boyu 伯雨, who might have revised the original text of Tang Hou.
Tang's Huajian is a valuable book on the theory of painting during the Yuan period. Is is therefore included in many series like Shuofu 說郛, Meishu congshu 美術叢書, Siku quanshu 四庫全書, Tang-Song congshu 唐宋叢書, Xuehai leibian 學海類編 or Congshu jicheng chubian 叢書集成初編.
A modern, commented edition was published in 1959 by the Renmin Meishu Chubanshe 人民美術出版社.