[390] How, when breathing out long, does he know, "I breathe out long"; when breathing in long, does he know, "I breathe in long"?
He breathes out a long out-breath reckoned as an extent [of time]; he breathes in a long in-breath reckoned as an extent; he breathes out and breathes in long out-breaths and in-breaths reckoned as an extent. As he breathes out and breathes in long out-breaths and in-breaths reckoned as an extent, zeal (chanda) arises. Through the influence of zeal, he breathes out a long out-breath, more subtle than before, reckoned as an extent; through the influence of zeal, he breathes in a long in-breath, more subtle than before, reckoned as an extent; through the influence of zeal, he breathes out and breathes in long out-breaths and in-breaths, more subtle than before, reckoned as an extent. As, through the influence of zeal, he breathes out and breathes in long out-breaths and in-breaths, more subtle than before, reckoned as an extent, gladness (pāmujja) arises. Through the influence of gladness, he breathes out a long out-breath, more subtle than before, reckoned as an extent; through the influence of gladness, he breathes in a long in-breath, more subtle than before, reckoned as an extent; through the influence of gladness, he breathes out and breathes in long out-breaths and in-breaths, more subtle than before, reckoned as an extent. As, through the influence of gladness, he breathes out and breathes in long out-breaths and in-breaths, more subtle than before, reckoned as an extent, the mind (citta) turns away from the long out-breaths and in-breaths, and equanimity becomes established.
Through these nine aspects, the long out-breaths and in-breaths are the body; the establishment (upaṭṭhāna) is mindfulness; the contemplation is knowledge (ñāṇa). The body is the establishment, but it is not the mindfulness; mindfulness is both the establishment and the mindfulness. With that mindfulness and with that knowledge he contemplates that body. Therefore it is called "the development of the establishment of mindfulness (satipaṭṭhāna) consisting in contemplating the body in the body."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.