One of the most important contributions of Master Song-chol's Seon thought in the history of Buddhism lies in the fact that he clarified the supreme principle of both Tien-tai school and Hua-yen school was the Middle Path where both sides of one's viewpoint are blocked up totally and illuminated completely.
The distinguishing points of Master Song-chol's view on the philosophies of Tien-tai and Hua-yen are as follows:
First, according to Master Song-chol, Tien-tai philosophy expounded the truth of the Middle Path by means of the two doctrines; 1) the three thousand realms are contained in one mind, and 2) there is the perfect harmony among the three truths. It is also said that the meaning of the Middle Path is identical with those of conditioned arising (pratītyasamutpāda) and emptiness (śūnyatā), and that the Middle Path in Tien-tai school is expressed in terms of blocking up both sides of one's viewpoint and illuminating the two sides totally, and blocking up and illuminating the two simultaneously. In Tien-tai school, the Middle Path is also called One Mind Dharma realm, Dharma realm without hindrance, non-dual Dharma realm. According to Master Song-chol, on the basis of the doctrines that the six characteristics of a thing influence one another and are perfectly blended, and that the ten characteristics of the world in which phenomena are interdependent, Hua-yen school clarified the doctrine of pratītyasamutpāda of dharma-dhātu on the basis of the principle of the Middle Path.
Second, Master Song-chol pointed out the differences between the two schools, Tien-tai and Hua-yen with regard to their views of human nature; while Tien-tai school asserts that self-nature is inherent in man from the first, Hua-yen school insists that self-nature is developed later. As a result of such a difference, Tien-tai school emphasized the need of practice, whereas Hua-yen school stressed the importance of theory. However, in spite of such differences both schools are equally prominent and we can not make any discrimination between the two because both are on the basis of the Middle Path.
Third, Master Song-chol criticized the two school in respect that they regarded the Āgamas, Source of the Teaching of the original Buddhism, as Hīnayāna or Lesser Vehicle when they classified the various tenets of Buddhism from their standpoints. Furthermore, Master Song-chol insisted that the original Buddhism is both the starting point and foundation of Buddhism.