Chih-i in the [Great Concentration and Insight] and [ Profound Meaning of the Lotus Sutra ] explicitly identifies the four phrases of [the Mūlamadhyamakakārika] verse as corresponding to the four ways of interpreting the four noble truths. The four noble truths are, briefly, the four basic Buddhist truths of all is suffering, the cause of suffering, the extinction of suffering, and the path, which are common to all Buddhist systems. The four ways of interpreting the four noble truths are as arising-and-perishing, as neither arising nor perishing, as immeasurable, and as spontaneous.

Chih-i writes:

In the Mūlamadhyamakakārika verse [24: 18], “All things which arise through conditioned co-arising” refers to [the viewpoint of] “arising and perishing.” “I explain as emptiness” refers to “neither arising nor perishing.” “Again, it is a conventional designation” refers to “the immeasurable.” “Again, it is the meaning of the Middle Path” refers to “the spontaneous.”

This categorization of four ways of interpreting the four noble truths is original with Chih-i, though Chih-i claims that it is based on the “Chapter on Noble Activity” in the Mahāparinirvāṇa Sūtra. The Mahāparinirvāṇa Sūtra does discuss the four noble truths in detail, expounding on the eight kinds of suffering, various causes of suffering, and so forth, but this four-fold classification should be attributed to Chih-i. This fourfold classification does not posit four kinds of four noble truths, because there is only one “four noble truths,” but refers to four ways of viewing, or interpreting, the four noble truths:

1. The four noble truths as arising and perishing. This is the standpoint which emphasizes the constant flux of phenomena. All things are constantly arising and instantly perishing in an interdependent web of causes and conditions. From this point of view, as Chih-i says, “Suffering consists of passing through the three stages [of birth, change, and annihilation], the cause of suffering consists of flowing through the four [defiled] mental states, the path consists of conquering and eliminating [the defilements], and extinction consists of extinguishing Being and returning to non-Being” (T. 46, 5b15-16). In Chih-i’s words, this is the realm of “change” (T. 46, 5b18). This is the viewpoint expressed in the first phrase of the verse: “All things which arise through conditioned co-arising.”
2. The four noble truths as neither arising nor perishing. This is the standpoint which emphasizes that all is empty (T. 46, 5b19). There is no real coming into Being nor the extinguishing of Being, because there is no substantial Being. Suffering has no real existence, and by extension there is no real cause of suffering. There are no real defilements to extinguish nor to eliminate on the path. All conditioned things, by definition, lack an eternal, unchanging, self-existent Being. What, then, can ever truly arise or perish? This is the viewpoint expressed in the second phrase of the verse: “I explain as emptiness.”
3. The four noble truths as immeasurable . This is the standpoint which emphasizes that, although all things lack substantial Being, there are immeasurable aspects to temporary conventional existence. As Chih-i points out, there are immeasurable sufferings in only one realm of existence (such as that of man), how much more so in all of the different realms of existence (from hell to Buddha) together. These immeasurable sufferings have innumerable causes, including “greed, anger, ignorance, and the various [defiled activities of] mind, body, and speech” (T. 46, 5c3). So also there must be innumerable features to the path, such as scholastic analysis, mystical insight, clumsy and skillful means, ways which are crooked or straight, long or short, teachings which are provisional or complete. Finally, there are immeasurable features of extinction, because there are innumerable delusions and defilements to extinguish.

   Chih-i is quick to point out that all this is from the standpoint of “conventional speech,” and that ultimately there are no distinctions because ultimately all are empty of substantial Being. Nevertheless, “it is not a mistake nor a confusion to make these [innumerable] distinctions” (T. 46, 5c9-10), as long as one realizes that one is speaking conventionally. This is the viewpoint expressed in the third phrase of the verse: “Again, it is a conventional designation.”

4. The four noble truths as spontaneous. This is the expression of ultimate reality which is beyond conceptualization and verbal distinctions. There is no difference between suffering, its cause, its extinction, and the path. All is One. This is the viewpoint expressed in the fourth phrase of the verse: “Again, it is the meaning of the Middle Path.”

The objects of our experience have a temporary reality. We do experience something. Nevertheless, the world which we experience is empty of an eternal, unchanging, svabhāba-like substance. Lest one lapse into a mistaken nihilism, one must realize the Middle Path. One must realize the emptiness of phenomenal reality simultaneously with the temporal, provisional reality of these empty objects. This Middle Path, however, must not be grasped as an eternal, transcendental Reality; it is, rather, manifested in and through and is identical with temporal phenomenal reality, which is again in turn empty of an unchanging substance. The circle is complete in itself, what Chih-i calls “a perfectly integrated threefold truth.”

This concept is summarized by Chih-i in his Fa hua hsüan I [ Profound Meaning of the Lotus Sutra ]:

The “perfect threefold truth” means that it is not only the Middle Path which completely includes the Buddha-Dharma, but also the real and the mundane [truths]. This threefold truth is perfectly integrated; one-in-three and three-in-one.

In other words, the real truth, the mundane truth, and the Middle Path are three ways of expressing the threefold aspects of a single integrated reality. This concept of the threefold truth plays a central role in Chih-i’s Tien-t’ai philosophy and provides the structure for his interpretation of the Buddha-dharma.