Digha Nikaya 2:120
SUJATOSantaṁ vā ajjhattaṁ vicikicchaṁ ‘atthi me ajjhattaṁ vicikicchā’ti pajānāti, asantaṁ vā ajjhattaṁ vicikicchaṁ ‘natthi me ajjhattaṁ vicikicchā’ti pajānāti, yathā ca anuppannāya vicikicchāya uppādo hoti tañca pajānāti, yathā ca uppannāya vicikicchāya pahānaṁ hoti tañca pajānāti, yathā ca pahīnāya vicikicchāya āyatiṁ anuppādo hoti tañca pajānāti.
When they have doubt in them, they understand: ‘I have doubt in me.’ When they don’t have doubt in them, they understand: ‘I don’t have doubt in me.’ They understand how doubt 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.