All mystics should constantly and surely dwell in serenity and insight because all forms of meditational absorption are unified in these two....
Serenity is so called because, having pacified distractions, one focuses always on an inward image joyfully, naturally, and without interruptions while maintaining a perfect ease of mind. Insight is that which examines the nature of that serene state so long as it remains....
Because the mind is restless like water, it cannot settle without the foundation of serenity. The mind that has not settled in serene absorption cannot perceive its [own] inherent purity....
Without serenity, insight distracts a meditator's mind toward objects, rendering it unstable like a butter lamp in the wind and hindering it from attaining a clear perception of original awareness. One should therefore work toward attaining a balance of [serenity and insight]....
When the mental focus on the visualized image of serenity becomes firm, if at that moment one examines it through wisdom, an illumination of pure awareness will emerge, like light clearing the darkness. [Thus] pure awareness arises when the harmony between serenity and insight is achieved. This harmony arises in the same way as ones eyes and light harmonize to produce a visual perception. This absorptive trance is, by its nature, devoid of darkness, its essential characteristic being single-mindedness. By such meditational equipoise, ultimate reality can be perceived as it is.
Excerpts from The Cooperation of Concentration and Insight by Yung-Ming (905 - 976 CE)
One-sided cultivation of concentration is pure yin. If you use accurate insight to illuminate meditation, all things will naturally be clear as a mirror.
One-sided cultivation of insight is pure yang. You should use subtle concentration to help contemplative exercise, like the clear light of the moon removing a film of mist.
I recommend equal cultivation of concentration and insight, not one-sided practice. They are originally one entity, not two things. It is like a bird flying through the sky with two wings, or like a chariot drawn on two wheels.
There is concentration on the concrete, in which everything is accomplished by placing the mind on one point. There is concentration on the abstract, in which one must only look directly into the essential nature of mind.
There is contemplation of the concrete, in which one clarifies the characteristics of things and develops judgment. There is contemplation of the abstract, in which it is suddenly realized there is no One and no Beyond....
The total effect of concentration and insight should never be forgotten -- in a single moment you can return at once to the state of real awakeness.
(Trans. Thomas Cleary, 2001)
Excerpts from Guidepost of Silent Illumination by Hongzhi Zhengjue (1091 - 1157 CE)
Silent and serene, forgetting words,
Bright clarity appears before you.
When you reflect it you become vast,
Where you embody it you are spiritually uplifted.Spiritually solitary and shining,
Inner illumination restores wonder...When wonder exists in serenity,
all achievement is forgotten in illumination....Alertly seeing through confusion
Is the way of silent illumination
And the origin of subtle radiance.
Vision penetrating into subtle radiance
Is weaving gold on a jade loom....When silent illumination is fulfilled,
The lotus blossoms, the dreamer awakens.