“What is Primary has no opposite.” Aristotle, Metaphysics
“Dualism has not yet reached the ground of being.” CS Lewis, Evil and God
God’s goodness, according to Christianity, is maximally perfect. ‘God is light’ St. John says, ‘and in Him there is no darkness at all.’ So while it is true that there can be no evil without good, the opposite – that there can no good without evil – is not true. This is because if God is the supreme, self-existent being, his goodness is such that it does not need anything else other than itself to exist. It simply is, and is good, through and through. God is himself perfectly good; and since perfectly good, in him there is absolutely no evil at all. If we were to imagine God’s goodness as a color it would be absolutely solid white. In the same way that whiteness as such does not need or ‘depend’ on blackness to be white, so God does not need or depend on evil in order to be what it is.
If we deny this and if we say that God’s goodness does in fact depend on evil, then this entails Dualism. If God “needs” evil, then Goodness, as such, needs Evil. Good and Evil are then on some level or equal ground. I am dependent on food and water and air because without them I cannot exist. But the Christian God does not “need” evil in this way. The good as such would still be just as good if evil never existed. As said above, apart from creation, God existing in His own right has no evil in Him at all. Evil, however, is parasitic. It can only exist if there is something good there first that it can live off of.
Although we live in a world of contrasts, and we very often experience certain goods otherwise impossible without certain evil, this, I believe, is not because God’s nature is dualistic in this way. It is because His omnipotence is such that He can draw forth good even out of evil. To suppose that good needs evil would be equivalent to supposing a marriage needed adultery, or that a beautiful face needed some grotesque deformity, in order to be maximally good. A marriage may, in the long run, be better after adultery has occurred, but it is not something that must happen to have a maximally great marriage.
What the Christian eventually gets to is the idea that God is essentially a triune unity. He is three persons existing in a maximally perfect relation. But a maximally perfect relation can involve nothing evil in it, or it would stand the position of potentially being more perfect than it is. God, therefore, as source and ground of all that exists, cannot contain anything evil in himself.
So, that is the first argument against Dualism: God’s maximal perfection entails that evil cannot exist in him, and thus evil cannot be a metaphysically absolute value.
A closely connected argument against Dualism is this. Any sort of division implies an overarching whole capable of containing the division. Matter can be divided from itself because it exists in something that contains it – namely space. Likewise ideas exist in something larger that can contain then – namely minds. Now, if you push this idea to the distinction between Good and Evil, and if you make them equal metaphysical absolutes, then you must posit something further back which itself is capable of containing them. Further, since neither the good nor the evil explain themselves, neither one is ultimate. Thus you have two equally self-existent powers, neither of which is more ultimate than the other, existing somehow within some further overarching power that allows them to co-exist together. But once you posit that reality, good and evil cease to be metaphysical ultimates. They are replaced by whatever ultimate contains them. In short, there must be a single absolute source and ground of being. And if it is not good, then it cannot be evil (for evil depends on good.) But if it is neither good nor evil there is nothing we can say about it and it becomes the Unknowable of the Eastern religions. But, if it is Good, we are at the Christian-Platonic-Aristotelian idea of God as equivalent to the Good as such.