The method of silent illumination is also known in Zen as shikantaza, from the Chinese zhiguan dazuo. In the West this has also become known as "just sitting." If this method is used incorrectly, you may as well make tea by soaking stones in cold water. Misusing it can also amount to escaping into the demon cave to weave dreams of unconcern. The practitioner who does not understand this method can become like an old turtle buried in an ancient well for centuries.
Suddenly one day it is dug up and comes back to life, but this turtle is still a turtle—it hasn't changed into a phoenix! One surmises that all of this time the old turtle was not really cultivating practice, because "just sitting" is not the same as "having nothing to do."
To avoid the fate of the turtle, it is important to understand the proper use of this method. In silent illumination your mind focuses on the awareness of your body sitting in meditation and nothing else. Therefore, correct posture is critical. Do not focus on parts of the body, but be aware of the totality of the body as a unity. Your body parts may have different feelings and sensations, but be aware only of the whole body.
Your awareness of you just sitting there should fill your mind. If your awareness falters, check and correct your posture, then resume being aware of your sitting, and its sensation as a total unity.
While you are practicing just sitting, be clear about everything going on in your mind. Whatever you feel, be aware of it, but never abandon the awareness of your whole body sitting there. Shikantaza is not sitting with nothing to do; it is a very demanding practice, requiring diligence as well as alertness.
If your practice goes well, you will experience the "dropping off" of sensations and thoughts. You need to stay with it and begin to take the whole environment as your body. Whatever enters the door of your senses becomes one totality, extending from your body to the whole environment. This is silent illumination.
From this it should be evident that it is not a matter of having nothing in your mind but, rather, that "you are thinking of what does not think." Your mind is fully alert but not stuck on forms. As Chan master Hongzhi Zhengjue (1092-1157) explained in his Inscription on Silent Illumination:
In silence and serenity, words are forgotten;
In clarity and luminosity, all things manifest...
Only this silence is the supreme speech,
And this illumination, the universal response.
If in illumination silence is lost,
Then distinctions will be perceived...
If in silence illumination is lost,
Then murkiness will lead to wasted teachings.
In the Platform Sutra the Sixth Patriarch said: "While you are in samadha, prajna is in samadhi, and while you are in prajna, samadhi is in prajna." Silence is samadhi, and illumination is prajna. When samadhi and prajna are not two separate things, this is silent illumination.
Let's talk now of another turtle. In the sutras there is a parable of a turtle pulling in its feet, head, and tail when in danger. This metaphor refers to reining in the six sense faculties in order to disentangle the web of false thoughts they engender and to still the mind. The sutra says, "The four elements join to form the body, and the entangling shadows of the six sense objects become the forms of the mind."
The six internal sense faculties take as their object the six external sense objects, engendering the mind of false thought of the six consiousnesses. If you take the six sense faculties and gather them back in from the six sense objects, the false mind has no entangling objects to which it can cling.
This is good Chan practice, but it does not mean abolishing the function of the six sense faculties. Otherwise you are like the turtle buried in the well. In this kind of Chan, when the eyes see a beautiful form, there is no craving, and when the eyes see an ugly form, there is no loathing—the sense faculties acknowledge the corresponding sense objects but without any false thoughts arising. The six sense faculties and the six sense objects are in contact, but discrimination, attachment, and affliction do not manifest. This is also the case when silent illumination functions in the midst of daily life.
Being silent in this sense means not being subject to the entangling web of delusion; illumination means being clearly aware of the contents of the mind. It surely does not mean not using the six sense faculties, and it surely does not mean not using the mind.