Interesting idea. It does seem a little odd that Digory (and Polly) only had one Narnia experience. Lucy, Edmund, and Eustace each had three and Peter, Susan, and Jill had two, so why does Digory only get one?

I've sometimes thought of what an 8th Narnia book could have looked like and for some reason I never considered Digory having another adventure, but it makes a lot of sense when you think about it. Digory could have met King Gale or Moonwood the Hare!

Another possibility is that Digory had met someone who had had an adventure, not in Narnia, but in some other world. (If they had been to Narnia, they presumably would have been introduced to the other Friends of Narnia in The Last Battle.) After all, there were a lot of other pools in the Wood Between the Worlds. Some odd look or sentence could have tipped Digory or the other party off to each other's similar experience.

@col-klink That's an interesting idea. Even if he didn't actually meet them, he might have read about it. We know that there were other "chinks and chasms" between worlds (based on what Aslan said), and if Uncle Andrew was right, the people of Atlantis had contact with other worlds at least once. It doesn't seem unreasonable that, in the world of Narnia, there are credible accounts of people traveling the worlds (even if no one else recognized them as such), and Professor Kirke would have a good idea of how to find those accounts.

I also agree with you that it seems unlikely there were other people from our world that ended up in Narnia that we don't know about. I suppose it's possible there were other Friends of Narnia that our heroes just didn't know and therefore couldn't identify in TLB, but... I also just get a sense that the seven books are the seven adventures of crossings between our world and Narnia, specifically. Certainly, I expect there are adventures between worlds and in just Narnia that we never heard about, but in my mind, there's just the seven crossovers between Earth and Narnia. That's based on pure speculation, of course. 😛

As for how Digory could've known about the rules for Narnia, specifically... well, I'm not sure I have a satisfying answer. My best guess is that Digory got to know Aslan "by another name" in our world as he grew older, and between his studies and just knowing Aslan's character, he made some assumptions.

My last wild guess is about why Digory would assume the children would go back, when he never did. My guess is that Digory and Polly were not King or Queen of Narnia, but the children were. I think he just recognized that such a title wouldn't be withdrawn just because they came home, and Kings and Queens would have responsibilities. After all, the last King and Queen of Narnia he met stayed there permanently.

Interestingly, there's one piece of extended fan-fiction I'm aware of that DOES work on the premise that Digory and Polly had a second visit to Narnia while they were still children. I hadn't heard about it until someone brought it up in another discussion on NarniaWeb, but there's a British author who a year or two ago wrote an "eighth Narnia book" and had it privately printed for friends and family, and was rumoured to be approaching Lewis's publishers for permission to publish it officially, but that hasn't come about (not very surprising!). There's an earlier thread here where it was mentioned and I found and linked to a few articles about it.

From the very little I could gather, it sounds like the story is based on the idea that Digory and Polly are called back to Narnia as the tree of protection is dying (presumably some centuries after Digory planted it in The Magician's Nephew), which one can guess means that Jadis would be able to return to Narnia. It's supposed to fill in the gaps between The Magician's Nephew and The Lion, the Witch and the Wardrobe by explaining how the Stone Table came to be made.

I really wouldn't mind a new Narnian story on that topic, as there are HUGE inconsistencies between what we're told about the White Witch in LWW and what Lewis later wrote as her "origin story" as Jadis in MN — it'd take a very skilled author and a lot of imagination to reconcile them. And I hadn't thought of this before, but @wanderer makes a really good point that at the end of LWW, it sounds like the Professor might have been to Narnia more than once, or that he knows others who have. Of course Lewis hadn't thought up the plots of all the later Narnia books at the time when he finished writing the first one (which is probably why a lot of those inconsistencies are there in the first place), but it's an intriguing thought...

Posted by: @ryadian

I also agree with you that it seems unlikely there were other people from our world that ended up in Narnia that we don't know about. I suppose it's possible there were other Friends of Narnia that our heroes just didn't know and therefore couldn't identify in TLB, but... I also just get a sense that the seven books are the seven adventures of crossings between our world and Narnia, specifically. Certainly, I expect there are adventures between worlds and in just Narnia that we never heard about, but in my mind, there's just the seven crossovers between Earth and Narnia. That's based on pure speculation, of course. ?

We learn from Aslan near the end of Prince Caspian that the Telmarines were originally pirates from our world who found their way into Narnia — or the neighbouring land of Telmar, rather — through a cave that turned out to be one of those "chinks and chasms" between worlds. Off the top of my head, I think that's the only other crossing between Earth and Narnia that we're told about, apart from the ones involving the "Seven Friends". But it obviously could and did happen, so maybe it happened at other times as well and those stories just don't come into the books Lewis wrote?

Of course, though, if the Professor (Digory) had met other people who'd been to Narnia, that raises the question of why they didn't become "Friends of Narnia" as well. Maybe he met them during his youth or young adulthood and didn't stay in contact with them — apart from Polly, of course — but it was only when the Pevensies started to talk with him further about their own adventures that he introduced them to Polly and they all (except for Susan) agreed to keep in touch regularly from then on? There's so much that's a bit vague about it all and I think it comes down to Lewis not planning it out very clearly himself as he continued the series. But it's fun to wonder.  

Didn’t Lewis model Digory after one of his own professors?  It would seem he would want Digory to have more than one experience in Narnia even as a child. In The Lion, The Witch and the Wardrobe the Professor tells the Pevensie children that there are other ways to reach Narnia besides the wardrobe. I think it implies that he may have visited Narnia at different times even if the books don’t say when he went there.

In some ways he was like Lewis, the age and occupation. 

But his actual model was his father's retired headmaster (school principal), Mr Kirkpatrick.  Both Jack and his older brother Warnie had live-in tutoring with this man and his wife to prepare for important exams. 
https://apilgriminnarnia.com/tag/the-great-knock/

(Last year I visited the town of Great Bookham, where he stayed at the Kirkpatricks' house, which I was told was called Garstons; I found a street called The Garstons, which I am sure is the same location - here's a picture from the 1910s. ).

Posted by: @col-klink

Another possibility is that Digory had met someone who had had an adventure, not in Narnia, but in some other world. (If they had been to Narnia, they presumably would have been introduced to the other Friends of Narnia in The Last Battle.) After all, there were a lot of other pools in the Wood Between the Worlds. Some odd look or sentence could have tipped Digory or the other party off to each other's similar experience.

 Interesting idea! That also begs the question...did the person Digory meet have an “Aslan experience” in their own world, too?  We know from VDT that Aslan has another name here on earth, so does that mean that that he also has other names in other worlds (though that question presents other issues)?

Posted by: @ryadian

Certainly, I expect there are adventures between worlds and in just Narnia that we never heard about, but in my mind, there's just the seven crossovers between Earth and Narnia. That's based on pure speculation, of course. ?

The Chronicles do seem to document just the seven “crossovers,” and it would be strange for another Son of Adam or a Daughter is Eve to come to Narnia and not have any of the other characters mention it.  However, there is a very large gap between MN and LWW, likewise with LWW and PC, and I like to think that somewhere along the way we got at least one more crossing.  In-universe, everyone who crosses over is exclusively from England, and there’s a part of me that wants to believe that Aslan called people to Narnia from other parts of the world, too.  Although the fact that I’m even saying these things is a testament to how well-written and inspiring Lewis’s work is!  

As a final note, Aslan only tells the characters, their own stories and what they are “meant to know,” so maybe these seven books are all that we are “meant to know”?  

Even though it's a bit odd that Digory and Polly only had one trip to Narnia, I feel like this couldn't be true based on the canon because I feel sure the Seven Friends would have discussed it, and we'd have heard about it. I'm thinking of the conversation between Jewel and Jill in The Last Battle:

"Oh, this is nice!" said Jill. "Just walking along like this. I wish there could be more of this sort of adventure. It's a pity there's always so much happening in Narnia."

But the Unicorn explained to her that she was quite mistaken. He said that the Sons and Daughters of Adam and Eve were brought out of their own strange world into Narnia only at times when Narnia was stirred and upset, but she mustn't think it was always like that. In between their visits there were hundreds and thousands of years when peaceful King followed peaceful King till you could hardly remember their names or count their numbers, and there was really hardly anything to put into the History Books.

That said, the quote in LWW is curious and I'm really curious as to what Lewis had in mind at the time he was writing it. The line about "once a King, always a King" is especially strange, seeing as I can't think of a reference point in Narnian history... although I wonder if it's possible that Lewis is alluding to 1 Corithians 24-25.

If he's referring to someone else having visited Narnia, though, there is one person hiding in plain sight that comes to my mind: the Narrator!

I've never really bought the idea that the Narrator is Professor Kirke, and whoever they are, they seem to know an awful lot about Narnia. The lines at the end of SC, for instance, are written in such a way as though the Narrator had sailed in the caves himself, but as far as we know, that couldn't be Digory... or could it?

That is an interesting thing to think about considering in the books, Digory only visits once. To tell the truth, I don't think he visits multiple times. I think all the experiences that any of the children throughout the books were learning experiences and made them better people. Digory went to Narnia once to learn from the experience as did (Susan, Peter, Edmund, Lucy, Jill, Eustace, etc.) I also  think the other children went more then once and Digory went only once was because they needed to be there and they needed to come more than once and learn and help with the things they can do. Digory was needed once to come to Narnia to amend his mistakes. He was only needed to help Narnia once because Aslan needed him that one time. I also think that Digory knew that he could only come once, because in the Last Battle they asked Jill and Eustace to try to go to Tirian to help him, the rest of them know they can't go because Aslan told them they couldn't come back to Narnia. Yet Aslan never told Digory or Polly they couldn't come back, they seemed to accept that they couldn't or didn't. Overall, I think Digory knew that he came to Narnia for one purpose and that he wouldn't be able to return.