[414] How does one train thus: 'Concentrating the mind (citta), I shall breathe out'; how does one train thus: 'Concentrating the mind, I shall breathe in'?
And what is the concentration faculty (samādhindriya)? One-pointedness of mind and non-distraction by way of the long out-breath is concentration (samādhi); one-pointedness of mind and non-distraction by way of the long in-breath is concentration; one-pointedness of mind and non-distraction by way of concentrating the mind while breathing out is concentration. That which is the mind's steadiness, settledness, steadfastness, non-wavering, non-distraction, the state of a mind unwavering, calm, the concentration faculty, the power of concentration, right concentration — this [is concentration].
By way of concentrating the mind while breathing out, [there is] the consciousness-mind (viññāṇacitta). Establishing (upaṭṭhāna) is mindfulness; contemplation is knowledge (ñāṇa). The mind is the [object of] establishing, not the mindfulness; mindfulness is both the establishing and mindfulness itself. With that mindfulness and that knowledge one contemplates that mind. Therefore it is said: 'the development of the establishment of mindfulness (satipaṭṭhāna) as contemplation of the mind in the mind.'
'Contemplates': how does one contemplate that mind? … Thus one contemplates that mind.
'Development' (bhāvanā): there are four kinds of development … development in the sense of repeated practice. The out-breaths and in-breaths while concentrating the mind are purification of virtue in the sense of restraint … For one who understands the one-pointedness of mind and non-distraction by way of concentrating the mind while breathing out … understanding [this], one brings the faculties together; therefore it is said: 'and one penetrates the meaning of calm.'
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.