There's a lot of necessary legal analysis missing here, and independent creation of an idea plays no part in it. Copyright protects creative expressions (i.e.,

There's a lot of necessary legal analysis missing here, and independent creation of an idea plays no part in it. Copyright protects creative expressions (i.e., artistic works), not mere ideas. I think what you’re trying to say is that independent creation would be a basis to deny a prima facie copyright infringement claim: there is no infringement because no copy has occurred. But that’s a different thing than an affirmative defense. A defense is something you raise after admitting the truth of the claim: you’re basically saying, “yes, I did it, but I’m justified because XYZ.”