Against Manichaean dualism: evil's deficient cause
If with the Gnostics, Manichaeans, and Encratites you object that evil mixed with good cannot be from the good God, and gather that another God from whom evils arise must be posited: we answer,
1. that no efficient cause of evil is to be sought, but a deficient one, which is not God — from whom nothing but good can be — but the creature itself falling from its goodness, because evil's nature consists in privation.
2. If God were the cause of evils, He would be the highest evil; but there cannot be a highest evil, because it destroys itself. 3. If this God is the highest evil, He will neither act nor know nor live nor be at all, because action, knowledge, and life are goods
Latin
Si malum bono permixtum cum Gnosticis, Manichaeis & Encratitis objicias, quod a bono Deo esse nequit, & alium Deum a quo mala oriantur statuendum colligas: respondemus, 1. mali causam efficientem nullam quaerendam esse, sed deficientem, quae non est Deus, a quo nil nisi bonum esse potest, sed ipsa creatura bonitate sua excidens, quia mali natura consistit in privatione. 2. Si Deus sit malorum causa, erit summum malum; summum malum esse nequit, quia seipsum destruit. 3. Si summum malum est Deus, neque aget, neque cognoscet, neque vivet, neque erit omnino, quia agere, cognoscere, vivere bona sunt.