Zhiyi [Chih-i] drew on the Lotus Sūtra’s claim that the Buddha’s various teachings were all his “skillful means,” or teaching devices, preached in accordance with the capacity of different individuals but all ultimately united in the fundamental principle of the one vehicle.

What was that fundamental principle? Zhiyi described it as the “threefold truth,” or “threefold discernment,” of emptiness, conventional existence, and the middle. Discerning all phenomena as “empty,” lacking self-essence or independent existence, frees the practitioner from attachment to desires and intellectual constructs. It collapses all categories, hierarchies, and boundaries to reveal an absolute equality and nondifferentiation. This insight corresponds to the wisdom of persons of the two vehicles of the “Hinayāna,” those who seek the goal of nirvāṇa, stopping the wheel of birth and death, as well as the wisdom of novice bodhisattvas. However, from a Mahāyāna perspective, it is one-sided. Though empty of fixed substance, all things nonetheless exist conventionally in dependence upon causes and conditions. The discernment of “conventional existence” reestablishes discrete entities and conceptual distinctions as features of commonsense experience but without false essentializing or clinging; it frees the practitioner to act in the world without bondage to it. This corresponds to the wisdom of more advanced bodhisattvas. Finally, phenomena are neither one-sidedly empty nor conventionally existing but exhibit both aspects simultaneously: at each moment, every existent, without losing its individual character, permeates and contains all others. This insight, termed “the middle,” encompasses both poles of understanding – emptiness and conventional existence – without dissolving the tension between them. The bodhisattva path culminates in the simultaneous discernment of all three truths as integrated in one. Page 16-17