[727] How does one dwell contemplating the body in the body (kāye kāyānupassī)?
Here someone contemplates the earth-body (paṭhavīkāya) as impermanent, not as permanent; contemplates it as suffering (dukkha), not as pleasant; contemplates it as not-self, not as self; he becomes disenchanted, does not delight; he becomes dispassionate, is not impassioned; he causes cessation (nirodha), does not cause origination; he relinquishes, does not grasp. Contemplating it as impermanent, he abandons the perception of permanence; contemplating it as suffering, he abandons the perception of pleasure; contemplating it as not-self, he abandons the perception of self; becoming disenchanted, he abandons delight; becoming dispassionate, he abandons lust; causing cessation, he abandons the origin; relinquishing, he abandons grasping. In these seven ways he contemplates the body. The body is the establishment (upaṭṭhāna), but it is not mindfulness (sati); mindfulness is both the establishment and mindfulness. With that mindfulness and that knowledge (ñāṇa) he contemplates that body. Therefore it is called "the development (bhāvanā) of the establishment of mindfulness (satipaṭṭhāna) that is contemplating the body in the body."
Development: there are four kinds of development — development in the sense that the dhammas arisen therein do not exceed one another; development in the sense that the faculties (indriya) have a single function; development in the sense of carrying forward the energy appropriate thereto; development in the sense of repeated practice. Here someone contemplates the water-body, the fire-body, the air-body, the head-hair-body, the body-hair-body, the outer-skin-body, the skin-body, the flesh-body, the sinew-body, the bone-body, the bone-marrow-body as impermanent, not as permanent; contemplates it as suffering, not as pleasant; contemplates it as not-self, not as self; he becomes disenchanted, does not delight; he becomes dispassionate, is not impassioned; he causes cessation, does not cause origination; he relinquishes, does not grasp. Contemplating it as impermanent, he abandons the perception of permanence; contemplating it as suffering, he abandons the perception of pleasure; contemplating it as not-self, he abandons the perception of self; becoming disenchanted, he abandons delight; becoming dispassionate, he abandons lust; causing cessation, he abandons the origin; relinquishing, he abandons grasping. In these seven ways he contemplates the body. The body is the establishment, but it is not mindfulness; mindfulness is both the establishment and mindfulness. With that mindfulness and that knowledge he contemplates that body. Therefore it is called "the development of the establishment of mindfulness that is contemplating the body in the body."
Development: there are four kinds of development … development in the sense of repeated practice … Thus he dwells contemplating the body in the body.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.