Chih-i defines the three aspects of threefold contemplation as:

To enter [an insight] into emptiness from [the viewpoint of] conventional existence: this is called the contemplation of the two truths. To enter [an insight] into conventional existence from [the viewpoint of] emptiness: this is called the contemplation of equality. These two contemplations are the path of expedient means for attaining entry to the Middle Path, wherein both of the two truths are illumined. The thoughts of the mind are extinguished and put to rest, and one spontaneously enters the sea of universal wisdom [sarvajña]. This is called the contemplation of the Middle Path and the truth of supreme meaning [T. 46, 24b5-8)]

Entering emptiness from conventional existence

At this first level of contemplation, “conventional existence” refers to the ordinary, mistaken perception of phenomena as existing substantially (as svabhāba), and “entering emptiness” means to negate the existence of independent substantial Being in these phenomena. Thus, as Chih-i says, “When one encounters emptiness, one perceives not only emptiness but also knows [the true nature of] conventional existence” (24b10—11).

Entering conventional existence from emptiness

At this second level of contemplation, “conventional existence” refers to a correct understanding and positive acceptance of objective phenomena as interdependently and conditionally co-arisen. Emptiness here refers to a mistaken attachment to the concept of emptiness, or a misunderstanding of emptiness as merely a nihilistic nothingness. As Chih-i says:

If one understands [“enters”] emptiness, [one understands that] there is no “emptiness.” Thus one must “re-enter” conventional existence. One should know that this contemplation is done for the sake of saving sentient beings and know that true reality is not [substantial] true reality but an expedient means which appears conventionally. Therefore it is called “from emptiness.” One differentiates the medicine according to the disease without making conceptual discriminations. Therefore it is called “entering conventional existence.”[T 46, 240-11]

This insight is compared to blind men who regain their sight. They can then perceive both space and forms and colors, and can differentiate between various grasses and trees, roots and stalks, branches and leaves, medicine and poison. At the first stage of “entering emptiness from conventional existence,” one perceives the two truths but is one-sidedly concerned with emptiness and cannot utilize or see the reality of conventional existence. If one’s eyes are opened concerning the validity of objective conventional reality, one perceives not only emptiness (“space”), but also the visible forms of conventional existence. One can then understand clearly the minute, conditionally co-arisen phenomena of everyday life and use this knowledge to benefit others (24cl 1-19).

The contemplation of the Middle Path of supreme meaning

This refers to the highest level of contemplation wherein one simultaneously and correctly perceives the validity of both emptiness and conventional existence. As Chih-i says:

First, to contemplate [and attain insight concerning] the emptiness of conventional existence is to empty saṃsāra [of substantial Being]. Next, to contemplate [and attain insight concerning] the emptiness of emptiness is to empty nirvāṇa. Thus both extremes are negated. This is called the contemplation of two [sides of] emptiness as a way of expedient means in order to attain encounter with the Middle Path. . . . The first contemplation utilizes emptiness, and the later contemplation utilizes conventional existence. This is an expedient means recognizing the reality of both [in an extreme way], but when one enters the Middle Path, both of the two truths are illumined [simultaneously and as identical and synonymous]. [T. 46, 24c21-26]