I tend to agree with you about the objects not being innately magical, with the exception of the horn.
I wonder if we need to look separately at the portals and the objects that were used.

Susan is told by Father Christmas that if she puts it to her lips - presumably blowing - then "help of some kind" would come. In the same way he has said that the bow "does not easily miss".

The other presents may be endowed with some hint of magical quality: Lucy's cordial did heal. But the horn is not a portal, merely a summons. In LWW Peter hurries to defend his sisters against a wolf attack, when Susan blows it.

The magic rings in MN are definitely magical. The whole history of them is told in MN. No debate.

There are at least two mentions of portals to the world of men.  The White Witch says, "A door from the world of men. I have heard of such things."  (LWW ch4)  Aslan tells Caspian about the cave his ancestors went into to hide, "But it was one of the magical places of that world, one of the chinks or chasms between worlds in old times, but they have grown rarer.  This was one of the last: I do not say the last." (PC ch 15)
Polly tells Jadis when they first meet, "We've come from another world; by Magic." (MN ch 5) Jadis soon instructs them to prepare the magic to take her back to their world. She doesn't hear that they are using rings until they all arrive in dawning Narnia. I would have expected her to know of portals between worlds, but she seems to have used her magic only for controlling and destroying others. 

It is not until the final page of MN that Lewis refers to 'the magic qualities of the wardrobe'. And since it doesn't lead to Narnia every time someone opens the door in LWW must we conclude it is only a magical object, and not a portal? And if we have read the last two pages of MN, we know how it became magical. Aslan actually made use of magic from another era in human history (or legend).

Posted by: @col-klink

The painting in The Voyage of the Dawn Treader. I admit I'm conflicted on this one. A painting of a ship in our world that happens to look exactly like the Dawn Treader would be a pretty ridiculous coincidence. But none of the characters describe it as magical as they do the wardrobe or the horn. I think it's possible C. S. Lewis just meant it to be a normal painting that Aslan used to bring Lucy, Edmund and Eustace to Narnia. 

I think it's one of those things that Lewis deliberately left as mysterious and didn't try to explain how it worked or why it happened, perhaps because he never thought up a clear answer to that himself. The painting does seem to be an image of the actual Dawn Treader, with a prow shaped like the head of a dragon, and a single mast with a sail of a rich purple colour. Not the kind of ship you would generally have expected to see in a painting in 1940s England, I'm pretty sure. Edmund and Lucy both describe her as a "Narnian ship" as they look at the painting — "And she is such a very Narnian ship," Lucy emphasises.

All this is before the painting comes to life and becomes a portal into Narnia, so even to begin with, we're given the clear impression that this is no ordinary painting and it's somehow not really a coincidence that it's there. But where the painting came from — who painted it, and how it comes to be an exact likeness of a specific Narnian ship — we're just not told. The only background information Lewis gives us is the cryptic comment that Aunt Alberta "couldn't get rid of it [the painting] because it had been a wedding present from someone she did not want to offend." Who could that have been, and how did this person get hold of a painting of a Narnian ship, and why did he or she give it to Alberta and Harold (of all people!) as a wedding present, and what is this mysterious giver's relationship to the Scrubbs, that it's a person they don't want to offend??? There's just no indication anywhere, so we're left to wonder.

I recall reading a theory somewhere that it could have been Professor Kirke (Digory), since he's the only other person we know of at this point in the series who has apparently been to Narnia (Lewis hadn't yet written The Magician's Nephew when he wrote this story). But there's nothing to suggest that the Professor knows Eustace's parents, let alone that he would be close enough to them to give them a wedding present, or that he is the sort of person they would be afraid of offending. And even if Digory was indeed the giver, how did he get the painting in the first place, and why did he see it as a suitable present for Harold and Alberta?

Those mysteries aside, I do think we can say Lewis saw the picture itself as magical, even though it's not described specifically as such in the story. One of the few surviving pages of Lewis's notes relating to Narnia — quoted in full in Past Watchful Dragons by Walter Hooper (p. 54) — is a list of possible plots for future stories, jotted down after he'd completed LWW, and it includes this concept:

PICTURE. A magic picture. One of the children gets thro' the frame into the picture and one of the creatures gets out of the picture into our world.

The same page of notes ("written in what looks like a very hurried hand, as if it were dashed off the moment it came into his [Lewis's] head," according to Hooper) also includes the idea that two children from our world "somehow got on board a ship of ancient build", which is sailing backwards in time "to islands that have not existed for millennia", with the object of saving the life of a "sick king [who] needs blood of a boy in the far future." That's presumably the earliest notion he had of what would become VDT — he didn't go along with those original ideas for the nature and purpose of the journey, but kept the concept of a sea voyage and visits to islands as the basis for a story.

It's not quite clear whether Lewis envisioned the "ship" idea (first on the page) and the "picture" idea (directly underneath it but starting a new paragraph) as both part of the same story at this stage, but they apparently came to him almost simultaneously. I'm bringing this up mainly because in these very sketchy initial notes, he does specifically write "A magic picture", so I think it's safe to assume that he was still thinking of it in that way by the time it got into Aunt Alberta's spare room in the story as we know it!  

@courtenay I get what you're saying. I really do. It's just that the painting is handled very similarly to how the door in SC and the train in LB are handled. It serves its purpose and then the characters never bring it up again. By contrast, they talk about the mechanics of the wardrobe, the rings and Susan's horn repeatedly and in some detail. (They learn that the wardrobe apparently only works once a day and Professor Kirke says at the end, that it will never work again-not for the Pevensies anyway. We learn the yellow rings take people to the Wood Between the Worlds and the green ones take them to the worlds themselves. Peter and Edmund express disconcertion that they have no choice but to appear in Narnia whenever the horn is blown.) It feels weird to me that C. S. Lewis wouldn't have explored the painting's implications a bit more if it were innately magical. Then again, it feels weird to me if the painting isn't innately magical if it just happens to feature a Narnian ship and... pretty much everything you wrote in your post. That one's a question mark for me. Definitely the most mysterious of the entrances to Narnia. (Well, maybe the cave in Prince Caspian is equally mysterious but since that wasn't manmade, it doesn't strike me as needing an explanation for its magic like the painting.)

@col-klink Good points — and I agree, the painting is the most mysterious and unexplained of all the portals into Narnia that we see in the series.We can only speculate why Lewis didn't explain it at all. Perhaps he didn't think of it as relevant to the story once the adventure gets underway — especially since Edmund and Lucy have been to Narnia twice before, each time by totally different means, so maybe it's just implied they're not really surprised by anything of that sort anymore? Or maybe Lewis just liked this idea of a picture becoming a portal, but couldn't think of any convincing reasons why there would be a painting of the Dawn Treader in our world, and why someone gave it to Alberta and Harold, and why it became a portal to Narnia at just that crucial moment, so he didn't bother to even try explaining it??

There's one thing, I've been thinking, that enables Lewis to get away with not explaining all the magical (or possibly magical) elements in the Narnia stories. In these books, magic is only wielded by a few beings who have special knowledge of it — whereas the main (POV) characters are always "ordinary" people (usually children from our world) who have no innate knowledge of magic and are never taught anything much about it, let alone how to use it themselves. So there's far less of a need for magic things to be explained in detail and no need for a fully worked-out system of what magic can or can't do in this fantasy world. That's in contrast to, say, the Harry Potter books, where the main characters do learn and use magic and there has to be a lot more explanation (not always convincing) of what it can do and how it works. 

@courtenay I've read a fan theory that Polly pained the picture and gave it to Harold and Alberta. There are a lot of holes that could be poked in that theory though. The Magician's Nephew never describes Polly as an artist. It would make her a jerk if she gave Eustace's parents a painting they wouldn't like for their wedding. Polly never saw any ship designs when she was in Narnia that could have inspired her, so it'd be just as huge a coincidence if a random artist happened to draw a Narnian ship. And, honestly, if she just happened to know the Scrubbs and just happened to give them the painting that would also be a huge coincidence.  

That being said, I don't necessarily mind coincidences in fiction. One of my favorite authors is Charles Dickens after all. I know modern people consider them a storytelling crutch but what am I supposed to do? Pretend I don't enjoy Dickens when I really do just because of what modern people say? So if anyone likes the theory of Polly being the painting's artist, more power to them!  

I wonder if anyone is going to argue that the door in The Silver Chair really was a magic door, not just a door that Aslan used one time. I'd be interested in their reasoning. I suspect though that their argument would be heavily based on them believing the painting of the Dawn Treader was magic and my argument for that thing not being magic is that I don't believe the door was magic...so we'll probably just be going in circles.  

@col-klink I think it's pretty clear that the painting, and then the door in the school's garden wall, must be understood as ordinary objects that Aslan uses as portals just once. 

@coracle I definitely agree about the door in SC. But the painting — well, yes, as far as we know, it only acts as a portal that one time. Yet we still don't have any explanation of how a painting of "a very Narnian ship" came to be in our world in the first place, let alone how it happened to be given to the Scrubbs as a wedding present... (Of course Narnia fan art and other more generic fantasy scenes are easy to find these days, but I can't imagine a ship with a dragon-head prow would have been a common or obvious subject for someone to just decide to paint in early- to mid-20th-century England!)

@col-klink As for coincidences in stories, they're not really problematic when an author is writing from the standpoint that there's a higher power in charge of everything. Which Lewis certainly was doing (quite explicitly), and Tolkien as well (more subtly)... I can't speak for Dickens on that subject, mind you.  

The painting in The Voyage of the Dawn Treader may have been of a ship that was much like the real one in Narnia. It doesn’t really matter that much if it was of a real ship in our world or one that was imagined by the artist. It is Narnian looking and the picture of it is enough for transporting people into Narnia. The painting was capable of working magic.  I think it may be nitpicking to find fault with the book for not having a work of art that is exactly representing a ship in the story.  

I never thought much about this. To me it's clear, as a writer myself, the idea of the portals is a magical macguffin, used by the writer to fulfill a need in the plot -- the transport of other people into this magical world. But, if you want to apply logic to it, it's also clear the portals are ultimately controlled by Aslan, except when they're not, as in the case of the cave the ancestral Telmarines used, or the rings that give entrance to the Wood Between the Worlds. Then again, Aslan is capable of summoning a person from Earth to Narnia by merely his will (as in the case of Frank's wife Helen) so... hmmmm?

I’m not sure if the people and animals living in Narnia had wardrobes in their homes like the one that the Pevensies used to enter Narnia.  Mr. Tumnus mistook the wardrobe for another country so he probably did not know that the wardrobe existed or what it exactly was. The wardrobe is only mentioned as something in our world which provided a way into Narnia. The train station in Prince Caspian was the entrance in that book. But railroads probably did not exist in Narnia.  The portals to other worlds do not always exist in the worlds to which they transport people.

Did anyone have a problem with believing the rings transporting people in The Magician’s Nephew?  The rings were magical, but perhaps it may seem too incredible that they could move people into another world whether they do it in pools or in Uncle Andrew’s study in the attic.  Could Lewis have used something better to transport his characters? 

@narnian78 Andrew spent much of his life and health finding out what the dust in the box was, and then making the rings. It has a feeling of a late Victorian fairy tale, a bit Nesbit too. But I don't have any problem with believing it, in the story; it's suitable for what an old man might have done in such a context. And it's no more more amazing than the children seeing a lion creating a new world. 

I wonder what Tolkien thought of his use of rings in this story. 

@coracle 

I think Tolkien read the complete Chronicles of Narnia even though he did not like them. When he read The Magician’s Nephew he might have thought the magic rings were something like those in his own book but not as powerful. But the story may have made him angry so that he did not like anything in it that was used as a portal to another world.  I think he was too much of a perfectionist to like Narnia. 🙂

I always thought the cliff where Jill and Eustace meet Aslan in The Silver Chair was rather magical since it was on the edge of Narnia. I think Aslan used it as a portal to transport the children to other parts of Narnia. So it was more magical than the door in the schoolyard, which was mainly a means of escape from the bullies. The place where the deceased King Caspian is resurrected before the return of the children to our world had a similar magical atmosphere.  I don’t think the story would have been the same without the transition places and the wonderful encounters with Aslan that occurred there. 🙂