Substance-function， essence-function. A way of talking about the internal/external， hidden/manifest， fundamental/superficial aspects of any person， thing， or situation， referring most commonly to the most fundamental nature of the human mind and its manifest activity/appearances. One of the most important and pervasive metaphysical categories in East Asian philosophical thought. （1） Usually considered to be first articulated as such by Wangbi 王弼， but can be seen as a thematic governing principle in the earliest of texts such as the Yijing易经 and Liji礼记， where the same concept is worked out in the form of "roots and branches 本末." （2） The essence-function construction becomes a seminal tool in the sinification of Buddhism， as it is applied to reconcile the dualism present in such Buddhist categories as nirvana/sam!sa^ra， Buddha and sentient being， innate enlightenment/initial enlightenment （本觉?始觉）， the two truths 二谛 theory and so forth. The essence-function concept was first pervasively applied in the Awakening of Faith 大乘起信论 （along with the intermediate concept of attributes 相）， to explain the meaning of the One mind 一心 and its myriad attributes and functions， as well as to explain the problem of innate vs. initial enlightenment. Subsequent to this， the structure was implemented in the composition of Chinese scriptural works such as the Platform Sutra of the Sixth Patriarch 六祖坛经， and the Sutra of Perfect Enlightenment 圆觉经. The essence-function construction also formed the backbone for the Huayan 华严metaphysics of li/shi 理事 and the four realms of reality 四法界. Later East Asian Maha^ya^na philosophers， such as Wonhyo 元晓， Zongmi 宗密， and Chinul 知讷， as well as the famous Chan masters of the Five Houses 五家， utilized essence-function logic extensively in their work.