Zen
Non-duality

Zen texts also stress the concept of non-duality (Skt: advaya, Ch: 不二), which is an important theme in Zen literature and is explained in various different ways. One set of themes is the non-dual unity of the absolute and the relative truths (which derives from the classic Buddhist theme of the two truths). This can be found in Zen sources like the Five Ranks of Tozan, Faith in Mind, and the Harmony of Difference and Sameness. It is also an important theme in Mahayana sutras which are important to Zen, like the Vimalakīrtinirdeśa and the Laṅkāvatāra Sūtra.

A related explanation of non-duality which is influential in Zen makes use of the Chinese Buddhist discourse of essence-function (Ch: tiyong), which is most famously taught in the influential Awakening of Faith. In this type of discourse, the essence refers to the inner nature of things, the absolute reality, while the functions refer to the more external, relative and secondary characteristics of things. The Platform Sutra compares the essence to a lamp, while the function is its light.

Another application of non-duality in Zen discourse is the idea that mundane reality (which includes the natural world) i.e. samsara (the world of suffering) and nirvana (the ultimate, enlightened reality) are not separate. This is a view found in Indian Mahayana sources like Nagarjuna 's Root Verses on Madhyamaka. As such, Buddhas and sentient beings as well as Buddhahood and the natural world, are also considered to be non-dual in Zen. This idea influenced Zen attitudes on social harmony and harmony (he, 和) with the natural world.

A further meaning of non-duality in Zen is as the absence of a duality between the perceiving subject and the perceived object. This understanding of non-duality is derived from the Indian Yogachara school. The philosophy of the Huayan school also had an influence on Chinese Chan 's conception of the non-dual ultimate truth and its understanding of essence-function. One example is the Huayan doctrine of the interpenetration of phenomena or "perfect interfusion" (yuanrong, 圓融), which also makes use of native Chinese philosophical concepts such as principle (li) and phenomena (shi). The influence of the related Huayan theory of the Fourfold Dharmadhatu can be seen in the Five Ranks of Dongshan Liangjie (806–869), the founder of the Caodong lineage of Chan.
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