Contemplating the body as empty, observing mind as empty, seeing no impermanence and no impurity in emptiness--such items of the path lead directly to the magic citadel.
When contemplating the body as impermanent, you see impermanence is identical to emptiness. Contemplating the reality nature of the body, you see it is not permanent, not impermanent, not empty, not nonempty. Contemplating the mind is also like this. Such items of the path lead indirectly to the abode of treasure.
Contemplating the real nature of the body as neither pure nor impure, yet aware of both purity and impurity, and so on, contemplating the real nature of the mind as neither permanent nor impermanent, yet aware of both permanence and impermanence, such items of the path lead directly to the abode of treasure.
Chih-i, Stopping and Seeing, p. 32