[413] How does one train thus: "Gladdening the mind (citta), I shall breathe out"; how does one train thus: "Gladdening the mind, I shall breathe in"?
And what is the gladdening (abhippamoda) of the mind? For one who knows the one-pointedness and non-distraction of the mind by way of the long out-breath, there arises gladdening of the mind — that which is the mind's rejoicing, gladness, mirth, merriment, elation of mind, satisfaction of mind. For one who knows the one-pointedness and non-distraction of the mind by way of the long in-breath, there arises gladdening of the mind — that which is the mind's rejoicing, gladness, mirth, merriment, elation of mind, satisfaction of mind … For one who knows the one-pointedness and non-distraction of the mind by way of experiencing the mind while breathing out, by way of experiencing the mind while breathing in, there arises gladdening of the mind — that which is the mind's rejoicing, gladness, mirth, merriment, elation of mind, satisfaction of mind. This is the gladdening of the mind.
By way of the out-breaths and in-breaths while gladdening the mind, there is the mind as consciousness (viññāṇa-citta); the establishment (upaṭṭhāna) is mindfulness; the contemplation is knowledge (ñāṇa). The mind is the establishment, but it is not the mindfulness; mindfulness is both the establishment and the mindfulness itself. With that mindfulness and with that knowledge he contemplates that mind. Hence it is said: "development of the establishment of mindfulness (satipaṭṭhāna) by contemplating the mind in the mind."
"He contemplates": how does he contemplate that mind? … Thus he contemplates that mind.
"Development" (bhāvanā): there are four kinds of development … development in the sense of repeated cultivation (āsevana). The out-breaths and in-breaths while gladdening the mind are purification of virtue (sīlavisuddhi) in the sense of restraint … For one who knows the one-pointedness and non-distraction of the mind by way of the out-breaths and in-breaths while gladdening the mind … knowing, he brings the faculties (indriya) together; therefore it is said: "and he penetrates the meaning of calm (samattha)."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.