In the Profound Meaning of the Lotus Sutra, Chih-i reminds his audience that the “mind” in no way differs from all other dharmas and is singled out as the object of contemplation only because it is closest to us. (p.197)

Every dharma, contemplated in its ultimate truth, reveals itself as pervading all phenomena. Most significantly, the ordinary moment of consciousness encompasses all dharmas: “All one can say is that the mind is all dharmas and that all dharmas are the mind.” We are now in a better position to understand how the image of Śākyamuni in the Lotus Sutra represents the ultimate truth of the phenomenal realm, the Buddha-nature. From Chih-i’s standpoint, the depiction of the all-pervading Śākyamuni does not figure a “pure” consciousness that is devoid of conceptuality and its objects and simultaneously the ground of phenomenal existence; it is rather an image of the “true aspect” of the phenomena themselves, the ordinary dharmas that are at once empty and provisionally existent, each encompassing the totality and therefore both “one” and “many.” This ultimate truth of the phenomenal realm is the “Middle Way” that Chih-i refers to as the resplendent realm of the Buddha which is “eternal, blissful, selfhood and pure.” The ultimate truth may be identified as the cause of Buddhahood or the Buddha-nature (tathāgata-garbha) because it is the identity of one’s own mind with this truth which makes the realization of the truth and the attainment of Buddhahood possible. However, it must be stressed that the Buddha-nature for Chih-i is not the transcendent pure mind of the Tathāgata-garbha sutras, but simply the “true aspect” of the moment of ordinary consciousness and of all phenomena.
A Buddhist Kaleidoscope; Susan Mattis, Chih-i and the Subtle Dharma of the Lotus Sutra: Emptiness or Buddha-nature?, Page 255-256

Chih-i expresses his conception of the ultimate truth of phenomenal entities as the simultaneous affirmation of “three truths”: That which originates dependent on causes and conditions is (1) empty (k’ung), (2) provisionally (i.e., dependently) existent (chia), and (3) the middle way (chung). The simultaneous affirmation of emptiness and dependent existence is obviously in accord with Mādhyamika philosophy. The assertion of the third, middle truth, however, has reinforced the view that Chih-i introduces to his conception of the ultimate truth notions from the tathāgata-garbha tradition. While some interpreters believe that the third, middle, truth is nothing more than the explicit affirmation of the identity of the truth of emptiness and the truth of provisional existence, others hold that the middle truth is a reference to the real, absolute pure mind of the tathāgata-garbha tradition.

Indeed, Chih-i’s characterizations of the ultimate truth are ambiguous and often appear to affirm monistic conceptions like that found in the Tathāgata-garbha sutras. In addition to defining ultimate truth as the simultaneous affirmation of emptiness and dependent existence, he defines it on some occasions as the negation of both emptiness and dependent existence and on others as the simultaneous affirmation and negation of emptiness and dependent existence. The impression created by these descriptions of an absolute reality transcending the realm of empty, dependently originating phenomena is further reinforced by Chih-i’s consistent use of the term tathāgata-garbha to refer to the ultimate truth.

Nonetheless, the central importance in Chih-i’s thought of the identity of all phenomena with ultimate truth militates against the idea that Chih-i understands the ultimate truth as a pure mind. When ultimate truth is conceived as a pure consciousness, in itself devoid of conceptuality and its objects, there is a fundamental dichotomy between the ultimate truth and the phenomenal realm. Even when, as in the Hua-yen tradition, the pure consciousness is understood as the ground for the appearance of the phenomenal realm, a true oneness of the phenomena and ultimate truth is not obtained; the phenomena are one with ultimate truth only insofar as phenomenal particularity is transcended and one discovers the unitary pure consciousness within all.
A Buddhist Kaleidoscope; Susan Mattis, Chih-i and the Subtle Dharma of the Lotus Sutra: Emptiness or Buddha-nature?, Page 253

[W]hat Chih-i and Nāgārjuna share in common should not be underestimated. Both unqualifiedly affirm that there is no distinction between the realm of ultimate truth and the realm of dependent origination. As Nāgārjuna states in the Mūlamadhyamakakārikās, “There is nothing whatever which differentiates the existence-in-flux (samsara) from nirvāṇa; / And there is nothing whatever which differentiates nirvāṇa from existence-in-flux.” Chih-i also maintains that the “Middle Path” or ultimate truth is not a transcendent reality but simply the realm of dependent origination, realized as empty and dependently existent. In The Great Calming and Contemplation he states this unequivocally: “The dharma-nature and totality of dharmas are not two, not separate. … The dharmas of the ordinary are themselves the dharma of ultimate reality.”

For Chih-i as for Nāgārjuna there is no reality or truth to be realized beyond the play of the ephemeral, conditioned elements of the realm of dependent origination; the ultimate, middle truth is nothing other than the realization of the true aspect of the phenomenal realm, that is, its empty, conditioned existence. This identity of ultimate truth and phenomena is for Chih-i the central and unequivocal teaching of the Lotus Sutra, the message embodied in the image of the Buddha pervading all realms of existence.
A Buddhist Kaleidoscope; Susan Mattis, Chih-i and the Subtle Dharma of the Lotus Sutra: Emptiness or Buddha-nature?, Page 252-253

In Chih-i’s view there can be no doubt that the ultimate truth revealed in the Lotus Sutra is the inherent Buddha-nature. Chih-i felt that the Lotus Sutra surpasses all others in its articulation, demonstration, and explanation of the promise that all sentient beings can become Buddhas. The doctrine of universal Buddhahood is proclaimed in the second chapter, where Śākyamuni explains that instead of three “vehicles,” or ultimate goals for three different kinds of beings, the śrāvakas, pratyekabuddhas, and bodhisattvas, there is only one goal, Buddhahood, for all. The sutra also demonstrates the universality of Buddhahood by depicting the dragon king’s daughter (possessing the disadvantages of being both female and a reptile) instantaneously attaining enlightenment, and by predicting the future enlightenment of the Buddha’s cousin, Devadatta, who had committed the most heinous evils — attempting to kill the Buddha and to disrupt the Buddhist Saṃgha. Chih-i believed that only a doctrine of universal Buddha-nature could justify the sutra’s unqualified promise that all sentient beings can become Buddhas. The sutra’s visionary representation of the identity of the Buddha Śākyamuni with all reality may therefore be taken as a revelation of the Buddha-nature inherent in all things.
A Buddhist Kaleidoscope; Susan Mattis, Chih-i and the Subtle Dharma of the Lotus Sutra: Emptiness or Buddha-nature?, Page 246-247

[I]n China the Buddha of the Lotus Sutra was traditionally understood as a representation of the eternal dharmakāya, his unimaginably long life span being seen as a metaphor for the “beginningless” truth realized by the Buddha. From this standpoint, the mythic images of Śākyamuni’s pores emitting light that pervades the universe and of his body splitting into innumerable forms that fill the ten directions are metaphorical representations of the pervasive and unchanging ultimate truth and its salvific function. When Śākyamuni of the Lotus Sutra is understood as the dharmakāya, and the dharmakāya or ultimate truth is identified as emptiness, the difference between the Lotus Sutra and the Prajn͂āpāramitā sutras appears to be one of style, not substance. As a representation of the dharmakāya, however, the concrete and dynamic image of the supramundane Śākyamuni may suggest a somewhat different conception of ultimate truth than that found in the Prajn͂āpāramitā sutras.
A Buddhist Kaleidoscope; Susan Mattis, Chih-i and the Subtle Dharma of the Lotus Sutra: Emptiness or Buddha-nature?, Page 245-246

The fundamental question regarding the Lotus Sutra’s vision of ultimate truth is its relation to the essential teaching of the Prajn͂āpāramitā sutra, the doctrine of emptiness (śūnyāta). While the sutra does not expound the principle of emptiness at great length, it refers to emptiness on several occasions, and many of its principles and teachings appear to presuppose the concept of emptiness. One of the most extensive direct references to it appears in chapter 5:

Those grasses and trees, shrubs and forests, and medicinal herbs do not know themselves whether their nature is superior, intermediate or inferior; but the Thus Come One knows this Dharma of a single mark and a single flavor, namely, the mark of deliverance, the mark of disenchantment, the mark of extinction, the mark of ultimate nirvāṇa of eternally quiescent nirvāṇa, finally reducing itself to Emptiness. (p. 103)

In this passage the sutra declares that despite their apparent diversity, the ultimate truth of all beings is the single mark of emptiness. Later, in chapter 10, the sutra further expounds that it is through the realization of this emptiness of the dharmas that one gains entrance into Buddhahood:

The room of the Thus Come One is the thought of great compassion toward all living beings. The cloak of the Thus Come One is the thought of tender forbearance and the bearing of insult with equanimity. The throne of the Thus Come One is the emptiness of all dharmas. It is only by dwelling securely among these that he or she can with unabating thought broadly preach this Scripture of the Dharma Blossom to the bodhisattvas and the fourfold assembly. (p. 180)

In addition to statements such as these directly referring to emptiness, passages which proclaim a singular ultimate truth of all dharmas or which deny any distinction between the phenomenal realm and ultimate truth may be viewed as expressions of this doctrine. Thus, a passage in chapter 2 which states that the “reality” of all aspects of all dharmas is their “suchness” (tathatā) appears to be a reference to the emptiness of the dharmas:

Concerning the prime, rare and hard-to-understand dharmas, which the Buddha has perfected, only a Buddha and a Buddha can exhaust their reality, namely, the suchness of the dharmas, the suchness of their marks, the suchness of their nature, the suchness of their substance, the suchness of their powers, the suchness of their functions, the suchness of their causes, the suchness of their conditions, the suchness of their effects, the suchness of their retributions, and the absolute identity of their beginning and end. (pp. 22-23)

In chapter 16 the Lotus Sutra asserts the identity of the world of samsara, the cycle of birth and death, and the realm of the Buddha, a central theme of the exposition of emptiness in the Prajn͂āpāramitā sutras. Using vivid language the Lotus Sutra explains that the transient phenomenal world, which to the unenlightened is a place of torment, is itself the “pure land” of the Buddha:

When the beings see the kalpa ending
And being consumed by a great fire,
This land of mine is perfectly safe,
Ever full of gods and men;
In it are gardens and groves, halls and towers,
Variously adorned with gems,
As well as jeweled trees with many blossoms and fruits, Wherein the beings play and amuse themselves;

My Pure Land is not destroyed,
Yet the multitude, seeing it consumed with flame,
Are worried, and fear the torment of pain;
The likes of these are everywhere. (p. 243)

Another motif of the Lotus Sutra that associates it with the tradition’s expositions of emptiness is the distinction made between the revelation of the Buddha Dharma in this sutra and the “expedient devices” the Buddha has previously used to lead practitioners to this truth. The Lotus Sutra is famous (or infamous) for denouncing the doctrines taught to the Śrāvakas (voice-hearers) as “expedient devices” intended only to prepare the practitioner to grasp the truth revealed in the sutra. In one passage, the “nirvāṇa” taught to the voice-hearers is repudiated explicitly on the grounds of the emptiness of all dharmas:

Though I preach nirvāṇa, This is no true extinction.
The dharmas from their very origin
Are themselves eternally characterized by the marks of quiet extinction. (p. 37)

The Lotus Sutra does not discuss a universal buddha-nature but often speaks of a universal buddha-seed. Chih-i re-uses the image of the seed to formulate a temporal succession in the process of enlightenment, which reflects also the way the Buddha acts: the seed is first sown, then left to sprout and grow, and finally the plant ripens. … [I]n Chih-i’s exegesis of the enlightenment of Śākyamuni the time between the sowing (the original enlightenment of Śākyamuni) and the ripening (the recent enlightenment of Śākyamuni) is an upāya, because the deeds Śākyamuni performs during this period are according to teachings other than the Lotus Sutra. The Lotus Sutra represents the world of the original enlightenment of Śākyamuni and that of his present enlightenment. The world in between is denoted by the other sutras Śākyamuni preached during his lifetime according to people’s capacity.

In Nichiren’s interpretation the upāya no longer has a function, the seed becomes equivalent to enlightenment, and the planting of the seed amounts to the attainment of buddhahood. The temporal interval between the primordial time and the present of Śākyamuni loses significance, and so does the difference between the original time and original land and the present of human beings. The Lotus Sutra is the buddha seed planted in people, the only means to realize the human potential for buddhahood. At any moment this scripture is read and diffused, the seed of buddhahood is again planted in everybody who chooses to listen and keep it, and the primordial relation with the Buddha is reestablished. If nobody “uses” the sutra, the seed disappears and no one is aware of their tie with the Buddha.

The seed is thus the necessary and sufficient cause of buddhahood. Yet, compared with the idea of buddha-nature, unchangeable by definition, the seed gives the idea of something belonging to the phenomenal world, subject to disappearance. “If people do not believe in this sutra and vilify it, then they cut off all the buddha-seeds in the world,” the sutra says. It is thus necessary to sow the seed again. If buddha-seeds occur “according to circumstances and conditioned cause,” as suggested in the Lotus Sutra itself, both the infinite action of the Buddha and one’s own activity are necessary. The image of the seed also conveys a more individual nuance than the universality of the buddha-nature: “Human beings defiled by evil encounter the bodhisattvas of the honmon, and the buddha-seeds are planted.”
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 234-235

The presentation of the relationship Buddha-humanity in terms of father and son also deserves attention. The term “the Buddha’s children” appears often in the sutra to indicate the Buddha’s disciples, bodhisattvas, and all those who practice according to the teaching of the Buddha. They are called “children born of the Buddha’s mouth.” Nichiren underlines that only a Buddha coming from this Sahā world can be considered father of the beings living in this world. Other Buddhas who abide in different parts of the universe are not qualified because they do not have this bond, thus their existence is almost irrelevant for people, or at best an upāya.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 233-234