The identity of emptiness, thusness and Buddha-nature

Almost every teacher-student conversation and koan reported in the chan literature between the 7th and 13th centuries can be read as a representation of one of these three fundamental Mahayana doctrines, expressed in sign, symbol and metaphor. (By separating the original Buddhist teaching of the non-inherent existence of the personal self, later termed sunyam svabhavam in the Madhyamika formulation, from the more inclusive Mahayana concept of the emptiness of all phenomena, one could then speak of four rather than three fundamental philosophic doctrines articulated in the chan discourse.)
But there exists also an underlying assumption that all three of these positions are identical. Implicit in the chan discourse is the understanding that emptiness, thusness and Buddha-nature, as seen from the standpoint of ultimate or absolute truth, are not only inseparable, but also the same. Therefore the chan master tries to awaken the baffled student by jumping unexpectedly from one position to another. To give two examples from our texts: in the dialogue with Dadian Baotong, Shitou corrects Dadian's false definition of emptiness -"Originally nothing exists" - by showing that if nothing existed, the original face (Buddha-nature) wouldn't exist either. And when Yaoshan tells Yunyan that "six is one and one is six," he means that if all things empty, they are also characterized by thusness (non-duality)."
The entire chan discourse can be seen as an apparently endless proliferation of signs pointing to that which can never be signified to begin with, so that for that very reason new signs are continually generated. (This proliferation of signs is called "supplementarity" by the semioticist Jacques Derrida: the compensation through proliferation on the part of a signifier for a signified which can never be made present. One signified gives way to another, functioning in turn as a signifier.) But a part of the tension which stimulates this ongoing process of symbolic articulation is the switching back and forth from the positions representing emptiness, thusness and Buddha-nature. Paradigmatic in this respect and standing at the very beginning of the chan literature is the famous enlightenment story of Huineng, which led to his recognition as the Sixth Patriarch by Hongren, the Fifth Patriarch in the lineage back to Bodhidharma. Hongren calls upon his students to write a verse expressing his experience of enlightenment. Shenxiu, considered the master's first disciple, writes a poems which expresses tathagata-garbha or Buddha-nature theory, employing the mirror metaphor:
The body is the Bodhi-tree,
The mind is like a clear mirror.
At all times we must strive to polish it,
And must not let the dust collect.
Huineng however responds with a verse composed from the standpoint of emptiness-theory:
Originally there is no tree of enlightenment,
Nor is there a stand with a clear mirror.
From the beginning not one thing exists;
Where, then, is a grain of dust to cling?
(Both poems are quoted here from Heinrich Dumoulin, A History of Zen Buddhism, p. 132. But the Dunhuang manuscripts give a different version of Huineng's famous poems: cf. Yampolsky, The Platform Sutra of the Sixth Patriarch , p. 132, footnote #38.]
Significantly, Hongren says that Huineng's understanding is still not complete - suggesting perhaps that a higher position would be that which equates both theories - but he nonetheless confers the robe of succession to Huineng. It is as if Hongren, faced with the necessity of deciding between sunyata and tathagata-garbha, accepted the former as a superior teaching, but left open the possibility of a position higher than both, expressible however only by symbol and metaphor, and unapproachable through discursive thought. The alternation between these two positions, along with the further position of thusness*, and the subsequent symbolic elaboration which assumes or incorporates both or all three as the substance of reality, inspire chan discourse through the next six centuries, and the writings of Dogen Zenji in 13th-century Japan as well.

* Several texts from the early Tang-period speak of the fusion of thusness and Buddha-nature. A representative example is the Dunhuang manuscript entitled Bodhidharma's Treatise on Contemplating Mind, translated by J.C. Cleary in Zen Dawn, pp. 79-102. The origin of this work is unknown, but it could have been composed in Jiangxi or Hunan during Shitou's lifetime. However, equating the Buddhist absolutes goes far back into the Indian tradition, as for example in the Srimaladevi Sutra (The True Lion's Roar of Queen Srimala): "World-Honored One, the Tathagata-embryo is the Tathagata's knowledge of emptiness," trans. Garma Chang, A Treasury of the Mahayana Sutras, p. 378.