These three paragraphs by Yanagada Seizan may be the most concise, clearest, balanced view of the complex dialectic involved in early Chan and its later development that I've yet come across, and in turn, sharp pointed guidance to the inherent errors involved in following any concept of the non-conceptual too far in any way.
from: Yanagida, Seizan. "The Li-tai fa-pao chi and the Ch'an Doctrine of Sudden Awakening." Transl. Carl Bielefeldt. in Early Chan in China and Tibet
from: Yanagida, Seizan. "The Li-tai fa-pao chi and the Ch'an Doctrine of Sudden Awakening." Transl. Carl Bielefeldt. in Early Chan in China and Tibet
The teaching of sudden awakening, which split the Ch'an movement in the eighth century, finds its most concrete expression in Shen-hui's central doctrine of "no-thought" (wu-nien). This doctrine was originally put forward as a criticism of the Northern school's teaching of "detachment from thought" (li-nien). The notion of li-nien is later described by Shen-hui's follower Tsung-mi (780-841) as the practice of "wiping away defilements and viewing purity" (fu-ch 'en k'an-ching), a characterization based on the famous verse attributed to Hui-neng's Northern school rival Shen-hsiu (606?-706) in the Liu tsu t'an ching. The Northern school believed in the original purity of the mind—that is, in an original self free from all defilements; its practice was to maintain and to magnify this self. This teaching is common to the Ta sheng ch'i hsin lun [*Mahayana-sraddhotpada, T.1666], the Vimalakirti-sutra [T.475], and many other basic Mahayana texts. But the belief in the original purity of one's own mind can itself become a form of attachment, in which one becomes bound by the notion of purity. The danger of such attachment is recognized in the Vimalakirti-sutra itself, and the Northern school was by no means unaware of it. In fact, despite the criticism leveled against it by the followers of Hui-neng, the Northern school taught a sophisticated and coherent system of Buddhist philosophy. Nevertheless, by emphasizing purity it seemed to be in danger of falling into a form of quietism. It was this point which Shen-hui attacked.
Shen-hui summarized what he considered the mistaken practice of the Northern school masters in a four-line maxim, which was later used by Lin-chi (d. 866) as well: "To concentrate the mind and enter samadhi, to settle the mind and view stillness, to arouse the mind and illumine the outside, to control the mind and verify the inside" (ning-hsin ju-ting chu-hsin k'an-ching ch'i-hsin wai-chao she-hsin nei-cheng) According to Shen-hui, the Northern school's doctrine of li-nien, taught on the basis of the Ta sheng ch'i hsin lun and the Vimalakirti-sutra, involves a bondage to purity: it is the practice of intentionally attempting to look at one's own pure mind. Shen-hui's summary of the Northern school teaching is, in fact, a criticism of a form of Ch'an sickness, in which one is attached to the detachment from thought and the contemplation of the pure original mind. Historically it does not seem that the masters of the Northern school were particularly attached to the practice of li-nien, but it must be admitted that philosophically their thought left them open to such criticism. At least Shen-hui saw the school as attached to li-nien, and taught his own doctrine of wu-nien in opposition to it.
Shen-hui's wu-nien doctrine is based on the notion of "natural knowledge" (tzu-jan chih) or "original knowledge" (pen-chih). The emphasis on such knowledge is the key issue separating the Northern and Southern schools. For Shen-hui, no-thought is itself sudden awakening, and as such there must be knowledge at work within it. The centrality of original knowledge for the Southern school's wu-nien dotrine was later recognized by Tsung-mi, who held that "the single term knowledge is the gateway to all mysteries" (chih chih i-tzu chung-miao chih men) Tsung-mi's emphasis on original knowledge was an attempt to defend Shen-hui's concept of wu-nien from misinterpretation by his contemporaries. On the one hand he was warning against the rise of the Ma-tsu school of Ch'an, which had transformed the teaching of no-thought into an emphasis on vital activity within everyday life. Such was the thrust of the school's famous sayings, "The ordinary mind is the Way" (p'ing-ch'ang hsin shih tao) or "This very mind is the Buddha" (chi hsin chi fo). For Tsung-mi this emphasis on the concrete function of the mind suggested blind activity devoid of original knowledge. At the same time, as we shall see, Tsung-mi's teaching was a criticism of the followers of the LTFPC [Li tai fa pao chi] who, while adopting Shen-hui's doctrine of no-thought, had in Tsung-mi's eyes forgotten that wu-nien is based on the functioning of knowledge.