Zen Buddhism is by definition indefinable, and in the context of its study, nothing--not even nothing--can be defined. In the Diamond Sutra, we are told that there is no formulation of consummate truth. The Buddha himself, herself, or itself cannot be distinguished by any characteristic whatever. Huang Po says:
"This spiritually enlightening nature is without beginning, as ancient as the Void, subject to neither birth nor destruction, neither existing nor not existing, neither impure nor pure, neither clamorous nor silent, neither old nor young, occupying no space, having neither inside nor outside, size nor form, color nor sound. It cannot be looked for or sought, comprehended by wisdom or knowledge, explained in words, contacted materially or reached by meritorious achievement. All the Buddhas and Bodhisattvas, with all wriggling things possessed of life, share in this great Nirvanic nature."
Despite such vivid cautions, some students understand this empty nature conceptually, and risk getting stuck in an undifferentiated place where correct and incorrect are the same, where male and female are the same--where all configurations disappear into a kind of pudding. The great teachers of the past adressed this risk directly:
The venerable Yen-yang asked Chao-chu, "When one has not brought a single thing, what then?"
Chao-chu said, "Put it down."
When you cling to nothing as something, then you yourself are not truly empty, and the emptiness you cherish is no more than an idea. Within this NOTION of emptiness, you can be persuaded that the homeless are an illusion, the rainforests are not being destroyed, there are no traditional peoples who are dying out, there is no one freezing or starving or dying from schrapnel in the former Yugoslavia. When you run over a child in your car, there is no child, after all. Put down that "not a single thing" or your successors will use it to enhance and support brutality and imperialism.
--- Robert Aitken