원효는 『대승기신론별기』에서 진여문(眞如門) 가운데 ‘사(事)’를 생략해 놓았다고 밝힌다. 이 문장을 기반으로 『대승기신론』에서 진여문(眞如門)과 생멸문(生滅門)을 각각 ‘이(理)’와 ‘사(事)’로 나눠서, 진여문과 생멸문의 이(理)를 모두 ‘여체(如體)’로 상정하고, 진여문과 생멸문의 사(事)를 모두 ‘여용(如用)’이라고 상정하였다. 통상적으로 일심(一心)을 체용(體用)으로 구분하면, 체(體)를 적멸로 하고, 용(用)을 여래장(如來藏)으로 할 수 있다. 이(理)의 체는 생멸상(生滅相)이 사라졌지만, 상주성(常住性)이 아니기 때문에, 연을 따라서 자유롭게 생멸한다. 여체여용(如體如用)으로 진여문이 여체(如體)에 한정되나 생멸문이 여용(如用)에 한정되지 않기에, 자유자재한 일여 경지를 이루게 된다. 오도(悟道) 후 원효는 ‘마음이 생하니 모든 법이 생한다.’라고 했다. 그 뒤 최후의 저서로 알려진 『금강삼매경론』에는 생(生)이 바로 멸(滅)이고 멸(滅) 자체가 생(生)이라고 하여 ‘생멸무이경지’를 설명한다. 여(如)의 경지는 적멸하지만 공(空)에도 얽매이지 않는 묘공(妙空)이다. 고정상이 없으므로 진여의 작용에 걸림이 없다. 진여에서 만법이 생하지만 근본이 공하기 때문에, 그 위에 만들어진 상(相)도 공하다. 여체여용(如體如用)의 경지에서는 집착하는 하나가 없어서 둘을 나타내는 것이 가능하고, 생한 둘의 근본도 역시 공(空)이다. 여체여용(如體如用)은 체(體)에 머물러 용(用)이 가능한 경지로 생멸문과 진여문이 각각 분리되지 않은 여(如)의 경지를 의미한다. 이 경지는 유무의 상(相)이 사라지고 이분법의 논리를 벗어난 초지(初地) 보살 이상의 경지이다. 여체여용(如體如用)은 체(體)에도 용(用)이 있고, 용(用)에도 체(體)가 있는 원효의 자유로운 깨달음의 경지이다.
One of the important figures in the history of Korean Buddhism, Wonhyo, is famous for his deep understanding of Buddhist teachings and his original interpretation of scriptures. This paper explains his core insights in terms of the concept of the essence of thusness․the function of thusness(如體如用). Wonhyo is said to have omitted the ‘external activity(事)’ of the true-thusness aspect in the Expository Notes on the Awakening of Faith(大乘起信論別記). Based on this sentence, this paper distinguishes the true-thusness aspect and the production-and-extinction aspect in the Awakening of Faith into fundamental essence(理) and external activity(事) and shed light on them in terms of the essence of thusness․the function of thusness(如體如用) respectively. Generally, given that the one mind can be divided into the essence(體) and function(用), the essence can be splitted into the true-thusness aspect of the calm extinction and the function into the production-and-extinction aspect of the womb of the Tathāgata. At this time, the fundamental essence of the true-thusness aspect(眞如門) and the production-and-extinction aspect(生滅門) is referred to as ‘the essence of thusness(如體)’, and the external activity of the true-thusness aspect and the production-and-extinction aspect is assumed to be ‘the function of thusness(如用)’. The essence of thusness․the function of thusness(如體如用) is the state of enlightenment of Wonhyo, where the function of thusness(如用) is within the essence(體), and the essence of thusness(如體) is also within the function(用). The essence of fundamental essence(理) is something out of the form of arising and ceasing aspect of the mind, but because it doesn't guard the permanent self-nature itself, it freely arises and ceases(生滅) according to causes and conditions(緣) of avidyā. Through the essence of thusness․the function of thusness(如體如用), one achieves a free and versatile state of oneness(一如境地), the true-thusness aspect(眞如門) is not limited to the essence or the production-and-extinction aspect is not restricted to the function, After awakening, Wonhyo said, “Because of the arising of thought, various phenomena arise, since thought ceases, a cave and a grave are not two.”(心生故種種法生 心滅故龕墳不二).
In the Exposition of the Vajrasamādhi Sūtras, his thought develops into the Dharma of nonduality, which is `extinction is in fact production and production is in fact extinction' and extinction and production are not two. The essence of thusness․the function of thusness(如體如用) represents the state of his free enlightenment.
There's no proper way to account the stage of thusness in terms of existence and non-existence. Indeed, thusness is a marvelous emptiness(妙空) that is calm extinction, but not bound by emptiness(空).
In addition, it doesn't have obstacle in making use of it because it dosen’t possess any fixed form. Although it gives rise to all dharmas, its root is void, and the form established upon it is also void. Since it has nothing to cling to, it is possible to manifest two, and the root of the two that arise is the void. Furthermore, The essence(體) has the potential for function(用), and the function(用) contains the essence(體). The essence of thusness․the function of thusnes(如體如用) represents the state of Wonhyo, who stays in the essence(體) of the thusness(眞如) and performs the function(用), and it means that the true-thusness aspect and the production-and- extinction aspect do not separate from each other. It is the state of enlightenment of Wonhyo, who has transcended the logic of dichotomy, with the form of existence and nonexistence disappearing and contains the core of his Buddhist teachings.