Here is a spiritual practice uncomplicated enough for anyone to learn, yet rich enough to be worked with for a lifetime. The traditional Chan (Chinese Zen) practice called Silent Illumination begins with nothing more than putting aside all thoughts except the awareness of oneself ''just sitting.'' It's so simple in execution that it has sometimes been called the ''method of no-method''--yet simple as it is, the practice is subtle and profound, with the potential for ever subtler refinements as the practitioner moves toward mastery of it. When fully penetrated, this radical form of emptying one's busy mind-stream leads to perception of the vast ocean of pure awareness.