(From Orthodoxy The Romance of Faith Chapt 4 The Ethics of Elfland p.48 Doubleday Image)
That is the question Lewis is addressing, not his conclusion. Lewis reminds us how careful we must be using terms like "good" and "almighty" without keeping all the intrinsics of creation in mind. Nature is relentless. Man cannot permanently persuade, move, or entreat "nature." It is "Nature" that is inexorable.