From the "TERMS & CONCEPTS" Series in the ZenBuddhism Electronic forum, 9/2/98, by Charles Muller http://www.human.toyogakuen-u.ac.jp/~acmuller
No matter how well language is initially handled, Mahayana Buddhism teaches that if one has not yet been awakened, the problem of the tendency of the human mind to become entrapped by language has to be dealt with over and over again. If this problem is not continually addressed in a fully conscious manner, even the most profound of the Buddhist teachings will become layered over and deadened. Nagarjuna or, more @ Nagarjuna (?150-?250 C.E.) was a Buddhist thinker who realized at a profound level the difficulties of carrying out Buddhist discourse in the medium of language, and the degree of attachment that could occur with even such subtle concepts as Shunyata, (SA mtshungs pa nyid - sameness [Syn. for shunyata] (stong pa nyid). Therefore he endeavored to prevent people from falling into the error of attaching to emptiness as a "something" or as "non-existence." He made his project an exercise in consciousness that sought to free people from being limited in thought by the linguistic options of "this or that" and "existence or non-existence." He did this by taking Buddhist philosophical terms and putting them into his formula of "neither x nor not-x." According to this formula, existence is "neither empty nor not empty," "neither samsara nor nirvana." Nagarjuna's teachings are not something new ontologically speaking, but were developments toward a more advanced logical form that can be seen in his Madhyamakakarika. In these texts, he strove to stop the reification of the concept of emptiness by: (1) stressing the non-difference between emptiness and dependent origination; (2) by emphasizing the understanding of emptiness as a mental attitude which pays attention to the non-attachment to concepts and theories. That is, emptiness should not be made into a theory to be clung to (as are other philosophical and religious doctrines). According to Nagarjuna, he who does so is like "a customer to whom a merchant has said that he has nothing to sell and the customer now asks to buy this 'nothing' and carry it home."
For Nagarjuna, emptiness should not be interpreted ontologically, but rather in the way of the parable of the raft: The Buddhist teaching (especially shunyata), is like the raft one constructs for the crossing of a river. Once the river is crossed, the purpose of the raft has been served. It may now be discarded. The same is true of emptiness: it should not be held on to; one who does hold on to it will have trouble functioning in life. In this sense, emptiness could also be compared to a laxative: once the obstruction has passed, there is no need to continue taking it. Nagarjuna wrote extensively, and his teachings resulted in the formation of an Indian school called Madhyamika or the "Middle Way School." This school was later transmitted to China under the name "San-lun tsung" (School of the Three Treatises), where, although it died out as an independent sect, had great influence on the formation of Chinese Buddhist philosophy.
Just a bit of reflection on the main points of Nagarjuna's philosophy can easily show the direct influence on Ch'an/Son/Zen, since in these East Asian meditative schools a great deal of emphasis is placed on remaining unattached to intellectual positions. The Zen master's instructional techniques of "pulling the rug out" from the student's established personal views can be seen as having direct correlation to Nagarjuna's nonabiding philosophy of the Two Levels of Truth.