[861] Furthermore, Ānanda, when a bhikkhu cognizes a mental object (dhamma) with the mind, there arises what is agreeable, there arises what is disagreeable, there arises what is both agreeable and disagreeable. He understands thus: 'There has arisen in me this agreeable, there has arisen this disagreeable, there has arisen this both agreeable and disagreeable; and that is conditioned, gross, dependently arisen. But this is peaceful, this is sublime, namely, equanimity (upekkhā).' In him that arisen agreeable, that arisen disagreeable, that arisen both agreeable and disagreeable ceases, and equanimity becomes established.
Just as, Ānanda, a strong man might let two or three drops of water fall onto an iron pan heated throughout the day — slow, Ānanda, would be the falling of the drops of water, and yet they would quickly reach exhaustion and evaporation — even so, Ānanda, in whomever the arisen agreeable, the arisen disagreeable, and the arisen both agreeable and disagreeable cease thus swiftly, thus quickly, thus without difficulty, and equanimity becomes established — this, Ānanda, is called, in the discipline of the Noble One, the supreme development of the faculties (indriyabhāvanā) regarding mental objects cognizable by the mind.
Thus indeed, Ānanda, is the supreme development of the faculties in the discipline of the Noble One.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.