Majjhima Nikaya 2:116
SUJATOIdha, bhikkhave, bhikkhu santaṁ vā ajjhattaṁ kāmacchandaṁ ‘atthi me ajjhattaṁ kāmacchando’ti pajānāti, asantaṁ vā ajjhattaṁ kāmacchandaṁ ‘natthi me ajjhattaṁ kāmacchando’ti pajānāti; yathā ca anuppannassa kāmacchandassa uppādo hoti tañca pajānāti, yathā ca uppannassa kāmacchandassa pahānaṁ hoti tañca pajānāti, yathā ca pahīnassa kāmacchandassa āyatiṁ anuppādo hoti tañca pajānāti.
It’s when a mendicant who has sensual desire in them understands: ‘I have sensual desire in me.’ When they don’t have sensual desire in them, they understand: ‘I don’t have sensual desire in me.’ They understand how sensual desire arises; how, when it’s already arisen, it’s given up; and how, once it’s given up, it doesn’t arise again in the future.