The background of Chinese philosophy
Rather much has been made by Western scholars of the apparent ability of the Chinese to syncretize the traditional doctrines of Buddhism, thus reconciling doctrinal viewpoints and differences which had remained very distinct in the minds of many teachers in India. But in fact the desire to harmonize and unify the basic beliefs of Mahayana doctrine was also a constant tendency in Indian scholasticism. Tathagata-garbha theory never became the basis for a separate doctrinal school, but it was easily incorporated into the Yogacara system, as was also the theory of thusness or non-duality. Then came the sudden rise to popularity of tantra or esoteric Buddhism in the 6th and 7th centuries, the explication and codification of which must have required a large number of scholars, leaving the further development of scholastic philosophy in India more or less suspended.The influence of tantra was so pervasive that we must look to Tibetan rather than to Chinese Buddhism to gain some impression of what mainstream Buddhism was probably like in India before its extinction.
Nor should we think that Buddhism somehow fit peacefully and without controversy into the native Chinese philosophical landscape. Far from it. Taoists, Confucianists and Buddhists attacked each other constantly and vied with one another to obtain imperial favor for their sects, seldom resisting the desire to denigrate the philosophical substance of the opposing sects in the process. Monk Zhiban's Fo zu tung ji, a chronicle history of Buddhism from 581-960 written from a Tiantai standpoint, reports of the endless controversies among the sects, and it quotes for example the great 7th-century translator Xuanzang as saying, "There are great differences between the teachings of Buddha and those of Lao-tse. It is impossible to explain the meaning of Lao-tse in Buddhistic terms. Moreover, the fundamental meaning of Lao-tse is very shallow." [From Jan Yuen-Hua, A Chronicle History of Buddhism in China 581-960 AD: Translations from Monk Chih-p'an's Fo-tsu T'ung-chi, p.34.]
But there was one element of native Chinese thought which was never distant from Buddhist literature in the Tang period, and which had exceptional influence on the Cao-Dong ancestors, namely the correlative cosmological concept of yin and yang. As categories of cosmic order underlying all phenomena, yin and yang provide a conceptual basis for perceiving reality in a dualistic framework. All phenomena are caused by the interaction of two polar opposite forces, which permeate each other in limitless configurations, and account for our experience of life as a vastly variable world of changing circumstances and situations.
One of the Five Classics, I Jing (Book of Changes) is a system of divination based on the permutations of yin and yang, examining present tendencies toward change as represented through the use of six-line combinations of broken and unbroken lines, called hexagrams. Dongshan Liangjie refers expressly to this work in his famous poem, Baojing sanmei ke (Song of the Jewel Mirror Samadhi), a core-text of Cao-Dong: "It is like the six lines of the double split hexagram; the relative and absolute integrate—piled up, they make the three; the complete transformation makes five." Indeed, Dongshan's teaching of the Five Ranks can also be understood as a diagrammatic explanation of the interaction between yin and yang, transposed into a Buddhist context.
Shitou Xiqian also refers repeatedly to the interplay of opposing forces: principle and phenomena, light and dark, forward and backward, near and far, sage and commoner, etc. Any of these pairs of opposites can be seem as deriving from the Chinese metaphysical notion of two correlative opposing forces underlying the dynamic of creation. Chan Buddhism attempts to present, experientially through the practice of meditation and symbolically through its recorded teachings, a reconciliation of all antinomies or opposites, and the instructions of the chan masters of all epics continually thematicize this process of resolution. Posed as a problem of logic, the opposites are resolved through the realization of thusness, or non-duality. As a metaphysical topic, they are all united by the insight that emptiness is constituted by means of form, and form itself is an expression of the same emptiness. Much of the koan literature is generated by the sudden awakening of a student to these perceptions, and the centuries-old Chinese traditional teaching of yin and yang serves as an insistent subtext. It is a tacit, consensual substruction which unites Confucianism, Taoism and Buddhism, just as the acceptance of reincarnation and the practice of yoga as a means towards liberation (moksa) characterize the diverse Hindu, Jain and Buddhist traditions of India.
It should be remembered too that the early Cao-Dong masters were very familiar with the Chinese classical literature. They were well-read in the Buddhist sutras and commentary, would have studied such basic texts as Sengzhao and The Awakening of Faith in the Mahayana, and must have thought earnestly and often about the new Huayan ideas popular in the early Tang. Achieving this degree of proficiency in Buddhist theory must have necessitated some knowledge of the Confucianist Five Classics, the study of which served as a curriculum for the education of young scholars, as well as for any common definition of literacy in Tang times. It seems obvious as well that both Shitou and Dongshan knew their way around Taoist literature—it is thought by some that Shitou's poem The Agreement of Difference and Unity is named after Zhao Lun, a classic Taoist work. At the same time, both teachers were certainly very practical men, who were clearly convinced of the importance of intuitive realization through sitting meditation as the best means to approach Buddhist truth, requiring neither scholarship nor philosophic study, or for that matter even the ability to read or write, and therefore accessible to everyone.