One more key concept of Mahayana Buddhism must be touched on here, a concept that is crucial to Lin-chi’s entire doctrine. This is the concept of shunyata — emptiness, or nondualism. Mahayana Buddhism in its writings manifests a profound distrust of words, insisting that the highest truth or reality can never be formulated or conveyed through verbal teachings, and Ch’an masters will be found repeatedly harping on this theme. When Mahayana texts designate the absolute, or highest truth, as emptiness, they mean that it is empty of any characteristics by which we might describe it. This is because it is a single, undifferentiated whole, and the moment we being applying terms to it, we create dualisms that immediately do violence to that unity. Hence even the term emptiness itself must in the end be rejected, since it implies that there is something outside of emptiness that is not empty.
If reality is a single, all-embracing oneness, with nothing whatsoever outside it, then the entire phenomenal world as we know and perceive it, all time and all space, must be included within that unity. In the end, then, the absolute must be synonymous with the relative or phenomenal world; or, as the Heart Sutra puts it, “Form is emptiness, emptiness is form.”
Burton Watson, The Zen Teachings of Master Lin-chi, pp. xxi-xxii