坐禪三昧經 1.13

1.13

復次，觀亦有三品：或初習行、或已習行、或久習行。若初習行，當教言作破皮想，除却不淨，當觀赤骨人，繫意觀行，不令外念；外念諸緣，攝念令還。若已習行，當教言想却皮肉，盡觀頭骨，不令外念；外念諸緣，攝念令還。若久習行，當教言身中一寸心却皮肉，繫意五處：頂、額、眉間、鼻端、心處。如是五處住意觀骨，不令外念；外念諸緣，攝念令還，常念觀心，心出制持。

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.