[401] How, breathing in short, does he know "I breathe in short"; breathing out short, does he know "I breathe out short"?
He breathes in a short in-breath reckoned as brief; he breathes out a short out-breath reckoned as brief; he both breathes in and breathes out short in-breaths and out-breaths reckoned as brief. As he both breathes in and breathes out short in-breaths and out-breaths reckoned as brief, zeal (chanda) arises. Through the influence of zeal, he breathes in a short in-breath, more subtle than before, reckoned as brief; through the influence of zeal, he breathes out a short out-breath, more subtle than before, reckoned as brief; through the influence of zeal, he both breathes in and breathes out short in-breaths and out-breaths, more subtle than before, reckoned as brief. As, through the influence of zeal, he both breathes in and breathes out short in-breaths and out-breaths, more subtle than before, reckoned as brief, gladness (pāmojja) arises. Through the influence of gladness, he breathes in a short in-breath, more subtle than before, reckoned as brief; through the influence of gladness, he breathes out a short out-breath, more subtle than before, reckoned as brief; through the influence of gladness, he both breathes in and breathes out short in-breaths and out-breaths, more subtle than before, reckoned as brief. As, through the influence of gladness, he both breathes in and breathes out short in-breaths and out-breaths, more subtle than before, reckoned as brief, the mind (citta) turns away from the short in-breaths and out-breaths, and equanimity becomes established.
By these nine aspects, the body of short in-breaths and out-breaths is the establishing (upaṭṭhāna); mindfulness is the knowledge (ñāṇa) of contemplation. The body is the establishing, but it is not the mindfulness; mindfulness is both the establishing and the mindfulness itself. 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English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.