[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