[374] The strengthening of equanimity (upekkhā) is the middle of the first jhāna. How many characteristics does the middle have?
The middle has three characteristics: one looks on with equanimity at the mind (citta) that is purified; one looks on with equanimity at it as having entered upon serenity (samatha); one looks on with equanimity at its establishment in unity (ekattupaṭṭhāna). In that one looks on with equanimity at the purified mind, in that one looks on with equanimity at it as having entered upon serenity, and in that one looks on with equanimity at its establishment in unity — the strengthening of equanimity is the middle of the first jhāna. These are the three characteristics of the middle. Therefore it is said that the first jhāna is beautiful in the middle and endowed with characteristics.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.