간화선은 화두의 타파를 통해 자신의 본성을 깨닫는 수행법이다. 대부분의 화두는 선지식이 수행자를 지도하면서 가르친 말씀이나 행동의 奇緣語句들로 우리의 ``본래면목``을 체득하기 위한 하나의 과제이다. 선에서는 이런 ``본래면목``을 실상, 본심, 본성, 일념, 일구 등이라 하면서 이는 모양이나 언어문자로 표현하거나 설명할 수 없는 것이라 한다. 형태적인 면에서의 화두는 이 일념, 일구가 안이비설신의와 어우러져起動하기 이전의 세계이다. 이런 점에서 화두는 이미 실상이다. 깨닫지 못했을 때에는 과제이고 도구이자 열쇠이지만 화두는 이미 실상의 세계에 있다. 반면 천태 일념삼천설은 일념을 논리적으로 설명한다. 일념에 십계와 십계호구,백계에의 십여시와 삼종세간 등 삼천세계가 있다 하고, 이 삼천세계가 각각 호구호융하고 있어 삼천이 무궁무한하게 전개됨을 밝힌다. 일념이 삼천이지만 삼천의 각각은 일념 아님이 없다. 여기에 일념의 존재상으로 공가중 삼제가 있고, 이 삼제 각각에 또한 공가중이 있어 원융삼제하다. 선이 이론적 설명을 배제하고 오로지 본래면목이나 본성 등을 체득해야 함을 강조하고 있다면 천태 일념삼천설의 일념은 매우 논리적인 설명 구조를 보인다는 점에서양자의 차이가 있다. 하지만 무한 삼천의 호구호융이 일념이라는 점에서 천태의 일념 역시 궁극적으로는 이론을 떠나 있다. 그리고 화두가 ``실상``을 체득하고자 하고 천태가 ``중도``를 깨닫도록 한다는 점에서 이들 모두 담고 있는 내용이나 표현하는 주체, 지향하는 목표는 동일하다. 한편 이의 체득을 위한 수행법에서는 동이점이 보인다. 간화선은 화두에 대한 강력한 의심을 필수로 한다. 의심이 없으면 간화의 방법이 아니며, 그것도 단지 의심하는 정도가 아니라 온 몸으로 덩어리 되어 지속되는 의단독로의 타성일편을 강조한다. 천태의 경우, 실상을 궁극적으로는 부사의경으로 설명하며 이의 체득에 일심삼관을 말한다. 일심에 공가중 삼관과 이 삼관이 원융하는 원융삼관이어야 하며, 이는 思議로서 살필 수 있는 것이 아닌 不思議一心三觀이라 한다. 간화의 참구법과 천태 관부사의경의 구체적 행법인 일심삼관 모두 비사의적인 방법이라는 점에서는 동일하다. 간화가 화두 하나의 타파로 일체의 타파를 말하고 있는 것처럼 천태의 관부사의경 역시 음입계경을 비롯한 십경의 어느 것에서든 성취된다면 구경으로 보고 있다는 점 역시 같은 구조이다. 그러나 간화가 ``의심``을 핵심 요소로 하고 있음에 비해 천태는 의심을 언급하지 않으며, 천태는 삼제가 원융하는 무진의 세계를 일심삼관으로 관하는 관법을 말하지만 간화는 ``觀``법은 아니다.
The structure of Tiantai is different from that of Seon, and particularly Patriarchal Seon. The Tiantai School analyzes all Buddhist doctrines and practices and systemizes their logical structure in order to explain the direct experience of completeness and immediacy. On the other hand, Seon does not focus on all those but only emphasizes directly entering into true reality. In other words, the emphasis of Seon is direct experience rather than the understanding of reality. Seon masters think that understanding through intellectual discrimination cannot reveal ultimate reality, and so they admonish practitioners to abandon knowledge attained by the intellectual understanding of scripture and doctrine and to experience reality directly and physically. In Ganhwa Seon, a hwadu is a device practitioners use to grasp and awaken to true reality. Ganhwa Seon aims to comprehend all things by penetrating a single hwadu. A hwadu can be called a concentration of mind, the whole of mind, or true reality. When a practitioner cannot reach a state of awakening by other means, a hwadu is instrumental in reaching said awakening. Nevertheless, a hwadu is a true reality. For instance, all beings exist in true reality which is the true reality of the Middle Way, and we can not escape this principle. The realm of hwadu can be said to be the world prior to conceptual thinking, but it pervades the as many dharma realms throughout the universe as there are grains of sand in the Ganges river. It is also the original mind, the original nature, the one thought, the one word, and the one mind which encompasses all distinctions, such as great and small, long and short, circumference and circle, blue, yellow, red, and white, etc. It seems to be both the beginning and the whole, and it seems to be one but cannot be said to be only one. In addition, it is crystal clear both inside and outside and yet seems to be empty. Thus, one thought and one word are true reality and truthful dharma. This one thought and one phrase is the hwadu in which eye, ear, nose, tongue, body, and mind are still. The technique of investigating the hwadu in Ganhwa Seon means to not to discriminate between the one mind, one word, etc. but to lump these all together in the doubt mass ( uidan) and then break through it is. How can a phrase of the Tiantai School explain that which is difficult to verbalize? Tiantai Buddhism explains the principle of the "one thought" in its doctrines of the "Three Thousand Worlds in One Thought" and the "Perfect Harmony of the Threefold Truth." The former illustrates the structure of mind and essentially means that one thought contains three thousand worlds. In other words, one moment of thought equates to ten dharma worlds, ten aspects of "suchness" and three realms. It is well known that the doctrine of "Three Thousand Worlds in One Thought" implies that one thought contains all ten dharma realms. Each of the ten dharma realms comprises one hundred worlds that compose a thousand worlds by ten aspects of suchness. And finally, the thousand worlds can be multiplied by the three realms to become three thousand worlds. However, the concept that one thought contains ten dharma realms does not mean that dharma nature originates in such a way. When one thought arises then an infinite number of dharma worlds also arise, but they do not interrupt or hinder each other. There may seem to be many, but they actually do not exist and to be the one actually cannot be said to be non-existence. In addition, it seems to be many but non-difference, and is the one but not the sameness. Therefore, it is described as "the many are one and the one is many." It means that the ten dharma realms (十界互具), the ten aspects of suchness, and the three realms encompass each other. Thus, the term "three thousand worlds" does not mean it is a finite number, because all worlds are contained in a particle of dust and are unlimited in number, that is, eighty-four thousand, and it is all dharmas. One thought includes all things perfectly unified with each other, thus revealing the true reality of all dharmas. Moreover, the mind of one moment of thought can be explained using three principles: all things are emptiness, all things are phenomenal, and all things are in the middle. In other words, the mind in one thought is emptiness, empirical, and the middle way. It means the threefold truth is not three, but three and the structure of three is at the same time not three. It is neither united nor scattered. It is oneness and sameness, and yet a different structure of oneness. Therefore, the structure of a Ganhwa Seon hawdu is in accord with the Tiantai doctrine of "three thousand worlds in one thought" in which all things are simultaneously the truths o emptiness, phenomenal, and middle and where one moment of thought perfectly unifies the threefold world. In Ganhwa Seon, a hwadu is a realm beyond the expression of language and letters. It is a means to create strong doubt in one`s mind which precedes conceptual thinking and classification which is unified, and then investigated to determine what it is. When doubt is strengthened, the "sensation" of doubt (疑情) emerges. When a sensation of doubt is strong, we do not feel cold even though submerged in ice water. We do not feel hot even though walking through fire. We do not feel pain even though treading on thorns. This state is only concentrated in a sensation of doubt, called the "ball of doubt" (疑團). In other words, one can look at the sky but not see it, or one can look at the ground but not see it. Similarly, one can look at a mountain but not see it, or look at the water but not see it. Other examples can be losing one`s way when one is going somewhere or being seated without being aware of it; or not seeing even one person when in a crowd. The best method for practicing Ganhwa Seon is to concentrate one`s doubt into a single ball of doubt. On the other hand, the major practices of Tiantai Buddhism are the ten objects of or stages of meditation (十境), and meditation in ten "vehicles" or stages (十乘 觀法). The main point among those is the first stage, the realm of the aggregates ( skandhas), sense bases ( ayatana) and elements ( dhatu) (陰入界境), and the contemplation of the ineffable realm (觀不思議境). The terms skandhas, ayatana and dhatu refer respectively to the five aggregates, the twelve sense bases and the eighteen elements. These constitute the real world wherein it is possible to recognize the difference between subjectivity and objectivity. The contemplation of the ineffable realm is meditation on that realm which transcends thoughts and words. There are several differences between the Ganhwa Seon and Tiantai doctrines. Ganhwa emphasizes that a practitioner has to detach himself from all things and only grasp one hwadu and concentrate on it, while Tiantai synthesizes all Buddhist doctrines and practices. Nevertheless, some aspects of Tiantai Buddhism are closely tied to Ganhwa Seon doctrines and practices. Ganhwa Seon practitioners attempt to grasp a hwadu which transcends language and letters and contemplate it without engaging in conceptualization. Similarly, in Tiantai Buddhism, the concept that one thought contains three thousand worlds can not truly be explained with language or letters. In addition, the method of practice requires the contemplation of an ineffable realm, that is, non-conceptualization. The big difference between the two schools is that Ganhwa Seon emphasizes that the sensation of doubt evolves into a great mass of doubt while Tiantai makes no mention of doubt. However, the objects of skandhas, ayatana, and dhatu are closely related to hwadu and understanding its content; in addition, the contemplation of the ineffable realm is related to specific doctrinal aspects of hwadu contemplation.