坐禪三昧經 1.13

1.13

Furthermore, contemplation also has three grades: beginning practitioners, those who have already practiced, and those who have long practiced. If one is a beginning practitioner, one should be instructed to form the perception of broken skin, remove the impurities, and contemplate a red-skeleted person, binding the attention to the contemplation practice and not letting thoughts stray outward; when thoughts stray outward to various conditions, gather the thoughts and bring them back. If one has already practiced, one should be instructed to imagine removing the skin and flesh and fully contemplate the skull, not letting thoughts stray outward; when thoughts stray outward to various conditions, gather the thoughts and bring them back. If one has long practiced, one should be instructed to mentally remove the skin and flesh for one inch within the body, binding the attention to five places: the crown of the head, the forehead, between the eyebrows, the tip of the nose, and the heart area. In these five places, anchor the attention and contemplate the bone, not letting thoughts stray outward; when thoughts stray outward to various conditions, gather the thoughts and bring them back, constantly mindful of observing the mind, subduing and restraining the mind as it emerges.