Perelandra & Theological Debate

I have in my notes a response to some superficial/snarky characterization of the fact that Ransom ultimately takes his "debate" with the Un-man to the physical plane in Perelandra.   (As best I can reconstruct it, the claim must have been that Lewis seems to suggest that Christianity cannot be defended by logic and reason, but only by physical violence.)   I've adapted my thoughts here for sharing.

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Preliminarily, I would acknowledge that few believers of any faith are capable of winning philosophical debates with outsiders in real life. 

That said, Ransom is never really engaged in a philosophical debate with the Un-man; both of them are addressing their arguments to the Lady, and the purpose is to create or prevent a Fall on her planet.  Critically, they are not discussing or debating Christianity, which has absolutely no relevance to unfallen Perelandra.  Indeed, Ransom believes that a Fall on Perelandra would prompt divine grace in some terrifyingly different, unimaginable form.†

Rather, the Un-man is looking to tempt the Lady to rebel against Maleldil, and it is not limiting itself to logic and reason.  Appeals to emotion and self-image are entirely fair game.  For example, at one point, the Un-man starts telling seemingly interminable stories which Ransom eventually realizes are designed to play on the Lady's altruism and nobility of spirit to trigger dangerous pride or vanity.  The Un-man seeks to manipulate the Lady by any means available.

Ransom is playing defense.  He is trying to expose and guard against the Un-man's temptations and thus prevent a new Fall.  But he simply cannot win the contest for persuasion of the Lady, because he's human.  He, unlike the diabolical agent animating the Un-man, needs food and sleep, and has only a few decades' experience to draw on.

I would also note that the Un-man literally can't be bothered to debate Ransom.  It doesn't need to persuade him of anything, so it's perfectly content to torment him by repeating his name incessantly at intervals for no reason.*  This passage from chapter 10 explains the peculiar relationship between the Un-man and rationality itself:  

[The Un-man] showed plenty of subtlety and intelligence when talking to the Lady; but Ransom soon perceived that it regarded intelligence simply and solely as a weapon, which it had no more wish to employ in its off-duty hours than a soldier has to do bayonet practice when he is on leave.  Thought was for it a device necessary to certain ends, but thought in itself did not interest it.  It assumed reason as externally and inorganically as it had assumed Weston's body.  The moment the Lady was out of sight it seemed to relapse. 

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FN† From chapter 11: 

If [Ransom] now failed, this world also would hereafter be redeemed.  If he were not the ransom, Another would be.  Yet nothing was ever repeated.  Not a second crucifixion: perhaps--who knows--not even a second Incarnation . . . some act of even more appalling love, some glory of yet deeper humility.  For he had seen already how the pattern grows and how from each world it sprouts into the next through some other dimension.  The small external evil which Satan had done in Malacandra was only as a line: the deeper evil he had done in Earth was as a square: if Venus fell, her evil would be a cube – her Redemption beyond conceiving.  Yet redeemed she would be. 

FN* Luckily for readers, Lewis explains the concept in a few pages rather than reporting the entire monologue journalistically.