The first part of MN 19 refers to a stage prior to the Buddha’s enlightenment, when his method was in an inchoate form only beginning to be developed. It deals with the second factor of the NEP, right thought.
In MN 19 is seen the emergence of the themes of insight and serenity, and the latter’s function as a support:
“Thinking imbued with renunciation has arisen in me; and that leads neither to my own affliction, nor to the affliction of others, nor to the affliction of both. It fosters discernment" (Insight)
“ If I were to think & ponder in line with that (renunciation) even for a night… even for a day… even for a day & night, I do not envision any danger that would come from it, except that thinking & pondering a long time would tire the body. When the body is tired, the mind is disturbed; and a disturbed mind is far from concentration. So I steadied my mind right within, settled, unified, & concentrated it.” (Serenity)
Later the Buddha describes the approach from the fully developed path:
"He discerns that ‘When I exert a [physical, verbal, or mental] fabrication against this cause of stress, then from the fabrication of exertion there is dispassion. When I look on with equanimity at that cause of stress, then from the development of equanimity there is dispassion.’ So he exerts a fabrication against the cause of stress where there comes dispassion from the fabrication of exertion, and develops equanimity with regard to the cause of stress where there comes dispassion from the development of equanimity. Thus the stress coming from the cause of stress for which there is dispassion through the fabrication of exertion is exhausted & the stress resulting from the cause of stress for which there is dispassion through the development of equanimity is exhausted. "—MN 101
The Anapanasati sutta like the Satipatthana sutta contains two intertwined themes of opposite characteristic, serenity and insight. The first and second tetrads establish tranquillity of body and mind, but the second tetrad includes the development of joy (piti), part of the energy group of the seven factors of enlightenment, an insight factor (SN 46.53). It is simplistic to regard the first three tetrads as dealing exclusively with serenity.
In SN 46.53 is seen how with the insight group the association is with the fire and air elements, while the serenity group is with water and earth.