We have moved this to Talk About Narnia, as you see. Thanks, King_Erlian!
Interesting topic: discuss!

Well, I don't think that having Kirke visit Narnia as a boy weakens his character as the rational one at all. When reading LWW first, we don't really know who the professor is - he is mysterious, but we trust him and know that he is wise. I think the same idea carries over even when we know he has been to Narnia himself, only it's more effective. Even if this hadn't been the case, I think he'd still believe that Lucy had been to Narnia, that he was open to all reason of thought. It's an element of his character. Think of The Last Battle, "it's all in Plato", Kirke is a professor after all.

I don't think anything is lost on the character at all. If anything, I think there's a deeper understanding when we know that Digory has been to Narnia before.

My point was that I think Lewis' original intention in The Lion, The Witch And The Wardrobe was that the Professor be a character who was "neutral" - he had no reason to either believe or disbelieve in Narnia, yet it strengthened the case for Narnia being real if such a neutral character could come forward and say, "Actually, the evidence we have so far points in favour, even if one's gut reaction is to say this is absurd."

Later, when he began to write The Magician's Nephew, Lewis needed a boy character living a generation or two earlier than the Pevensies, and the Professor was an obvious choice. However that means he lost his neutrality; he was already one of the Friends of Narnia, and so the idea of a neutral character supporting the side of belief on the grounds of the available evidence was lost.

If you are reading in publication order, you don't learn that the Professor has been to Narnia until much later. If Lewis had revealed in LWW that the Professor had been to Narnia (or anywhere else) it would have affected our perception of the scene. But he doesn't. There are several books in between to settle the idea of Narnia, adding that the Pevensies continue to be in contact with him, before we return to the Professor, which I think is why it doesn't harm his credibility as a voice of reason.

[...] Lewis needed a boy character living a generation or two earlier than the Pevensies, and the Professor was an obvious choice. However that means he lost his neutrality; he was already one of the Friends of Narnia, and so the idea of a neutral character supporting the side of belief on the grounds of the available evidence was lost.

I agree that the neutral character was lost. Rereading the arguments after having read that Digory Kirke is a Friend of Narnia may give a small sense of cheating - he pretends just to use logical arguments, but he's actually read the answer book ...

But I still think it made sense to use Professor Kirke as the boy who saw the beginning of Narnia. Someone from his generation was needed, and it might have been a bit much to introduce yet another person.

It makes sense for Kirke to advise the Pevensies to be cautious about who they talk to about Narnia, and not to do it too much. We find later that he has lived by this advice himself - he didn't blurt out that he knew about Narnia, he waited till later before he even admitted that he had been there. So he can use Reason even though he's not completely neutral.

This is a really interesting topic. But, even if he went to Narnia before, his adventure did not teach him the logical reasoning that he used in LWW.

Also, as seen later in the Last Battle, Susan thinks they were playing games as children and stops being a friend of Narnia. Although, Digory did not live as long in Narnia as Susan did or go back to Narnia until the Last Battle, he still did not forget it. (Granted he had the wardrobe and his mother lived.)

Not to mention one of my favorite lines of his is:"What do they teach them at these schools?”

I think this line in itself says why I believe he is still a very powerful character in being the voice of reason despite also having gone to Narnia before.

This is a really interesting topic. But, even if he went to Narnia before, his adventure did not teach him the logical reasoning that he used in LWW.
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Not to mention one of my favorite lines of his is:"What do they teach them at these schools?”

Another typical line is "It's all in Plato"

He's a Professor, he's clearly had a classical education, and that would be where he has learnt the theory of logic and how to use it for logical reasoning.

If he is the voice of reason, that would explain why he's one of my favorite characters.
His appeal to Plato seems to reinforce that idea, but I think he only functions as a Voice of Reason in the stories occurring later in his life- I doubt that the voice of reason would ring a mysterious bell, in spite of all common sense. If he is supposed to be Reason, it would seem he grew into that role.
Hmmm... at the moment I'm writing a fantasy novel involving a voice-of-reason kind of character... perhaps Professor K. could serve as inspiration??? I must think on this...

I think that the scene with Peter, Susan, and the Professor is weakened if Professor Kirke has been to Narnia before and already knows that it exists. However, I'm not sure that MN is entirely to blame because I think there are already strong hints at the end of LWW that Professor Kirke has had previous experiences with Narnia.

And that would have been the very end of the story if it hadn’t been that they felt they really must explain to the Professor why four of the coats out of his wardrobe were missing. And the Professor, who was a very remarkable man, didn’t tell them not to be silly or not to tell lies, but believed the whole story. “No,” he said, “I don’t think it will be any good trying to go back through the wardrobe door to get the coats. You won’t get into Narnia again by that route. Nor would the coats be much use by now if you did! Eh? What’s that? Yes, of course you’ll get back to Narnia again some day. Once a King in Narnia, always a King in Narnia. But don’t go trying to use the same route twice. Indeed, don’t try to get there at all. It’ll happen when you’re not looking for it. And don’t talk too much about it even among yourselves. And don’t mention it to anyone else unless you find that they’ve had adventures of the same sort themselves. What’s that? How will you know? Oh, you’ll know all right. Odd things they say — even their looks — will let the secret out. Keep your eyes open. Bless me, what do they teach them at these schools?”

I have a hard time believing that the Professor had reasoned all of this out without having actually been to Narnia, or at least having known people who have been.

Also, as seen later in the Last Battle, Susan thinks they were playing games as children and stops being a friend of Narnia. Although, Digory did not live as long in Narnia as Susan did or go back to Narnia until the Last Battle, he still did not forget it. (Granted he had the wardrobe and his mother lived.)

This is interesting because, if Susan can convince herself that Narnia doesn't exist after years of reigning as a queen, I could easily imagine Digory beginning to doubt his own experiences and turning to philosophy to make sense of them. Especially if memories of Narnia become like memories of a dream in the same way memories of our world become when you are in Narnia. I could even see his doubts and questions about his childhood experience becoming the catalyst for Digory becoming a professor and making the pursuit of truth one of the main goals in his life.

I read the books in Chronological order. So I did not see Digory, or Professor Kirke as the voice of reason but instead as the voice of belief and trust. Him having already been to Narnia made it seem very clear to the reader that Digory is in the right and the logical explanations he uses are only to pursuade Peter and Susan to believe in Lucy.  I did not think that took away from his character at all, because he seemed mysterious in the first place. Even if he believed in Lucy just by logic, that would be very questionable by itself, and he would need some sort of background knowledge or experience to say he so surely believed her.  In fact, reading MN before LWW helped to clear up any doubts, knowing who he was already and why he believed Lucy. It helped it all make sense. 

I read LWW before MN and I didn't find Professor Kirke's character was weakened at all when I discovered — or rather, confirmed — that he himself had been to Narnia before. In fact, it made so much sense. He IS the "Voice of Reason" in LWW (and when we see him again in LB), but it's not the kind of "reason" most children would expect from adults in our world, nor is it supposed to be — which I think is a big part of the point of the story.

We see that by the fact that Peter and Susan are surprised (and a little bit disconcerted) when the Professor not only doesn't accept either of the two explanations they can think of — that Lucy is either lying or mad — but puts forth a third one that they hadn't even considered: that she is telling the truth. He makes quite clear, in that famous conversation with the two older children, that he's completely willing to accept that there are "other worlds" out there, and that indeed "nothing is more probable". This is NOT the kind of answer most grown-ups would give to this question, and clearly not the kind of answer that Peter and Susan were expecting.

Even as a very young first-time reader, I picked up instantly from that conversation that the Professor knows something that most people don't know — and that was more than confirmed by the speech he gives at the end, which Reepicheep775 has quoted above. I've always felt it's made quite obvious in LWW that the Professor has either been to Narnia himself, or knows other people who have been.

And since the Professor knows that Narnia is real, it's perfectly logical, surely, that he should steer Susan and Peter towards accepting that it at least might be real. He must know instinctively, during that first conversation, that this is not the time to tell them straight out that there ARE other worlds and he knows it firsthand because he's been to one. There's no way they would believe him, at this point, if he did tell them. (And that's why he warns them, after they have been to Narnia, not to tell anyone else about it unless they too have had similar adventures.) But if he can at least open their minds to the possibility that these things might be true... well, I doubt he knows what will happen next, but I'm sure he can already guess that it will only do them good.

As I was saying, I definitely picked up this much from reading LWW on its own, so when I read MN (it was the second of the Chronicles I read) some time later, everything just slotted into place — so the Professor really DID go to Narnia himself, and so that's why he obviously knew more than he was letting on in LWW!! I don't know what my response would have been if I'd read MN first, but reading it after LWW didn't diminish the role of the Professor for me at all. It just confirmed that he really did know what he was talking about!  

Of course the professor’s telling the other children that they should believe Lucy when she had been to Narnia should be in the first book.  It is so critical to the realism of the beginning of the story.  The logic of the fantasy is that you must first believe people who have the reputation of being truthful. If that is the premise then the stories which follow will be more convincing.