As noted in chapter 1, this mutual encompassing of the mind and all dharmas marks an important difference between Chih-i’s thought and that of Hua-yen and other forms of Mahāyāna in which the dharmas are said to arise from the one mind.

Chih-i begins by noting that the mind comprises “ten dharma realms.” These ten realms (jikkai) refer to the ten categories of living beings: hell-dwellers, hungry ghosts, asura demons, humans, gods, voice-hearers, condition-perceivers, bodhisattvas, and Buddhas. While these ten are ranged hierarchically from the viewpoint of provisional existence, from the viewpoint of emptiness, they lack independent self-nature and therefore co-penetrate, thus making “a hundred dharma realms.” The mutual encompassing or co-penetration of the ten realms (jikkai gogu) collapses any ontological distinction between the Buddha and the beings, implying that the nine realms of unenlightened beings possess the Buddha nature inherently, while the Buddha possesses the nine realms of unenlightened beings. The mutual inclusion of the ten realms represents an important characteristic of Chih-i’s thought: there is no original “pure mind”; good and evil are always nondual and mutually possessed. The most depraved icchantika is endowed the Buddha realm, while the Buddha is still latently endowed with the realms of unenlightened beings. This would eventually give rise to the controversial T’ien-t’ai doctrine of “inherent not being extirpated” even in the mind of the Buddha.

Each of the ten realms further possesses the ten suchnesses (jūnyoze) that constitute the “true aspect of the dharmas” as set forth in the following passage of the Lotus Sūtra:

“Only a Buddha and a Buddha together can fathom the true aspect of the dharmas, that is to say, the suchness of their characteristics, the suchness of their nature, the suchness of their essence, the suchness of their power, the suchness of their activity, the suchness of their causes, the suchness of their conditions, the suchness of their effects, the suchness of their recompenses and the suchness of their ultimate equality from beginning to end.”

Chih-i explains these ten suchnesses as follows:
” ‘Characteristics’ has its point of reference externally. What can be distinguished by being seen is called ‘characteristics.’ Nature has its point of reference internally. That which intrinsically belongs to oneself and does not change is called ‘nature.’ That which is the central quality [of something] is called ‘essence.’ The ability to influence is called ‘power.’ That which constructs is called ‘activity.’ Repetitive causes are called ‘causes’ [i.e., karma]. Auxiliary causes are called ‘conditions.’ Repetitive results are called ‘effects.’ Retributive effects are called ‘recompenses.’ The first, ‘characteristics,’ is called the ‘beginning’; the ninth, ‘recompense,’ is called the ‘end’; and the place to which they belong is ‘ultimately equal.’ ”

Each of the ten realms contains the ten suchnesses. The ten dharma realms, interpenetrating to form a hundred realms that each simultaneously possesses the ten suchnesses, comprise the thousand suchnesses.

Moreover, each of the ten dharma realms may be understood in terms of the three realms (san seken): the realm of the five skandhas or aggregates, the realm of sentient beings, and the realm of the land. The “realm of the five skandhas” represents an analysis of the sentient being in terms of its psychosomatic constituents: forms, perceptions, conceptions, volitions, and consciousness. The “realm of living beings” views the living being as an independent existent that can be said to belong to one or another of the ten dharma realms. The “realm of the land” is the objective realm in which the beings dwell. Because each of the ten dharma realms, which embodies the ten suchnesses, can also be understood in terms of these three categories, Chih-i says, “One realm comprises thirty kinds of realms.” Thus, the ten dharma realms, co-penetrating, yield a hundred realms; multiplied times the ten suchnesses they yield a thousand suchnesses; and multiplied times the three realms they equal three thousand realms.

The number three thousand is itself arbitrary; the point is that “all of reality is an integrated, interdependent unity,” as Paul Swanson puts it. (Page 179-181)