That Hideous Strength is cited:
Description IN A LETTER CONCERNING C.S. LEWIS’S WORKS, his dear friend J.R.R. Tolkien makes an observation about the prevalence of dualism in Lewis’s fiction: “I noticed, for the first time consciously, how dualistic Lewis’ mind and imagination [were], though as a philosopher his reason entirely rejected this. So the pun Hierarchy/Lowerarchy. And of course the ‘Miserific Vision’ is rationally nonsense, not to say theologically blasphemous” (371). In this letter, however, Tolkien blurs the distinction between two different types of dualism: a philosophical dualism, the dualism that Tolkien says Lewis’s reason rejects, and narrative dualism (a term of my own coinage and defined in the following paragraphs), which serves as a literary device. Although Lewis rejects philosophical dualism, he employs narrative dualism in his fiction, namely in That Hideous Strength; there Lewis uses the device paradoxically to lead Mark and Jane, the novel’s two protagonists, to a unity of purpose and marital harmony by means of their separate experiences in the camps of Logres and the N.I.C.E. In Mere Christianity, Lewis defines philosophical dualism as “the belief that there are two equal and independent powers at the back if everything, one of them good and the other bad, and that this universe is the battlefield in which they fight out an endless war” (42). He goes on to say that “[t]he two powers, or spirits, or gods—the good one and the bad one—are supposed to be quite independent. [...] Neither of them made the other, neither of them has the right to call itself God” (42). With this philosophical dualism, as Tolkien states, Lewis did not agree; he believed that the opposing forces, good and evil, right and wrong, were neither matching in power nor did they equally deserve to exist. He believed, as he says in Mere Christianity, that “one of the two powers is actually wrong and the other actually right,” and “what we mean by calling them good and bad turns out to be that one of them is in a right relation to the real ultimate God and the other is a wrong relation to Him” (43). One should note, however, that although Lewis did not believe in dualism as a religion in itself or as part of his own Christianity, he maintains that dualism is almost a part of Christianity.
Zoroastrian dualism in the east: Ahura Mazda vs Ahriman.
In the west, Manicaeism.
Dualism would require that there is equal capacity for evil to destroy good. Not so... evil only thrives when unopposed.