[416] How does one train thus: 'Contemplating impermanence, I shall breathe in'; how does one train thus: 'Contemplating impermanence, I shall breathe out'?
'Impermanent' (aniccaṃ): what is impermanent? The five aggregates (khandha) are impermanent. In what sense are they impermanent? They are impermanent in the sense of arising and passing away.
One seeing the arising of the five aggregates — how many characteristics does he see? One seeing their passing away — how many characteristics does he see? One seeing their arising and passing away — how many characteristics does he see? One seeing the arising of the five aggregates sees twenty-five characteristics … One seeing the arising and passing away of the five aggregates sees these fifty characteristics.
He trains thus: 'Contemplating impermanence in form, I shall breathe in'; he trains thus: 'Contemplating impermanence in form, I shall breathe out'. He trains thus: 'Contemplating impermanence in feeling (vedanā), in perception, in volitional formations, in consciousness, in the eye … in aging and death, I shall breathe in'; he trains thus: 'Contemplating impermanence in aging and death, I shall breathe out'.
Through the in-breaths and out-breaths of one contemplating impermanence, dhammas are the establishment (upaṭṭhāna) — [and this] is mindfulness; the contemplation is knowledge (ñāṇa). Dhammas are the establishment, not the mindfulness; mindfulness is both the establishment and the mindfulness itself. With that mindfulness and with that knowledge he contemplates those dhammas. Therefore it is said: 'the development of the establishment of mindfulness (satipaṭṭhāna) consisting in contemplation of dhammas among dhammas.'
'He contemplates': how does he contemplate those dhammas? … Thus he contemplates those dhammas.
'Development' (bhāvanā): there are four kinds of development … development in the sense of repeated cultivation.
For the in-breaths and out-breaths of one contemplating impermanence, there is purification of virtue in the sense of restraint … For one who understands the unification and non-distraction of the mind (citta) through the in-breaths and out-breaths of one contemplating impermanence … Understanding, he brings the faculties together; therefore it is said: 'and he penetrates the meaning of peace.'
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.