“Now to the content of the divine order, the fixed quality, the measurelessness and so forth—there is opposed the content of the evil principle, its unfixedness, measurelessness and so forth: total is opposed to total. The existence of the one genus is a falsity, primarily, essentially, a falseness; the other genus has Essence-Authentic; the opposition is of truth to lie. … But why does the existence of the Principle of Good necessarily comport the existence of a Principle of Evil? Is it because the All necessarily comports the existence of matter? Yes, … the nature of this cosmos is, therefore, a blend; … what comes into it from God is good; evil is from the Ancient Kind which, we read, is the underlying matter not yet brought to order by the Ideal-Form.” ~Plotinus
One of the hardest things for many to accept is that God and evil both exist. They think that if there is a God then everything that exists must be His creations. And since God is, by definition, Good, nothing evil can come from Her. And that is quite true as far as it goes: evil cannot have its source in God, and an evil creation cannot be created by God. Where they err is in the assumption that everything was created by God.
Another reason why opposites must exist is because of the creation by the fallen angels of matter. God created spirit. And only spirit. The fallen one created matter by taking some of the spirit created by God and changing it, splitting its essences into two. Because matter is spirit that has been split, divided, opposites exist in the realm of matter. It cannot be otherwise. Others have also pointed out that it is hard to know what is good if there isn’t something to contrast it with. It would be like painting a canvas solid black and calling it; “Lovers Kissing at Midnight”. It could be just what you called it, but nobody can really tell because they just see solid black. Or a solid white canvas labeled “A snowman in a blizzard”. The label is meaningless because there is no contrast.
It should also be obvious to anyone who thinks about it that the much loved concept of free will could not exist if there were not two opposite sides (with various mixtures in between) to choose from. How can you say you have free will if you can only choose good or good? Behind curtain number one is good. Behind curtain number two is good. Choose one. There is no real choice in that, is there. So in order for free will to exist, two opposites must exist to choose from, not just two minor variations of the same thing.
The important thing to remember, though, is that this is ONLY true on the material plane of existence. This duality does not and cannot exist on the spiritual level. I think that may be where many of those who oppose the idea of duality get confused. They think that if duality exists on the material plane, it must also exist on the spiritual levels, but that is not true. It must be true on the material level because the spiritual entity or object must be divided into two material beings or objects in order to slow them down enough to make them solid, to make them into matter. Spirits are not good or bad, hot or cold, male or female; they just are. But the material must be dual in order to exist. And matter was not created by God, so the idea that God cannot create evil is true, but evil exists because it was created at a lower level by the fallen angels. Note also that, as illustrated in the Yin-Yang symbol, nothing is ever totally good or totally bad, totally male or totally female.