You chased all the order over to good vs. evil.
Evil, if it is real, will have a form and a substance. Example: The usual model is spirit in body, which is lame. More like, *I* am a thing with a soul and a body, but in myself different from either. I am a bridge. And so two things are completed by a third.
A thing can have a form but no substance (a triangle) or a substance without form (eternity) but it isn't real unless experience reveals the opposite, and the two ideas live in your mind at once in a sort of superposition. That is the same thing as the bridge between body and spirit. The strange thing is, in a sense evil is an unawareness of the split between form and substance - the kind of awareness you get from the bridge. And so given the three part nature of all this, good is possible because it is aware of evil.