Wikipedia
Essence-Function or Essence and Function, or else Substance and Function (體用, Chinese pinyin: tǐ yòng, Korean: che-yong) is a key concept in Chinese philosophy and other Far-Eastern philosophies ( Japanese philosophy, Korean philosophy, Vietnamese philosophy).
The notions appear already in the Zhongyong (Doctrine of the Mean) attributed to Zi Si, the grandson of Confucius. The first philosopher to systematically use the ti yong schema for the analysis and explanation of deep relationships, was Wang Bi (226-249) in his commentary to Daodejing, chapter 22, when he discussed the metaphysical relation between non-being (wu) and being (you). Subsequently the notion has been borrowed from the Neo-Daoist philosophy to other schools of Chinese philosophy, including Hua-yen and other schools of Buddhism, and Neo-Confucianism of Cheng Yi and Zhu Xi, and served as a basic tool of interpretation of various philosophical relations in compound and dynamic events. With these schools it has travelled to Korea, Japan and Vietnam, and has been developed there. Wing-tsit Chan is one of the first scholars to open the discourse in English on this notion within the Chinese Neo-Confucian philosophy (in his Reflections on Things at Hand, 1967).
On the other hand, A. Charles Muller is one of the first scholars to open the discourse of this notion in Korean Buddhism. The Awakening of Mahayana Faith, attributed to Aśvaghoṣa (?80-?150 CE), employs Essence-Function. Essence-Function forms a fundamental syncretic and ecumenical application in the philosophy of Wonhyo (617–686 CE). Chinul (1158–1210) and Kihwa (1376–1433) also employ and develop this idea of Essence-Function in their writings in particular ways. Wonch'uk (613–696) employed the conceptual and analytical tool, Essence-Function, as an exegetical, hermeneutical and syncretic device.