I've heard of that theory too and wouldn't mind looking into it some more, although I'm inclined to think it's a little far-fetched and reading a bit too much into things — again, because it doesn't appear that Lewis had such an overarching plan in mind when he wrote the first few books, at least. I gather he once stated that when he wrote The Voyage of the Dawn Treader he "felt sure it would be the last" in the series, but it turned out not to be.
(Apologies for the way I keep making vague references and semi-quotes with no sources, by the way... I have several books on C.S. Lewis and Narnia, but left them all back in Australia when I moved to England a few years ago, so I don't have much on hand that I can quote directly!! )
Anyway, going by that, it seems Lewis didn't start out with the intention of writing seven Narnia books, which would mean he couldn't consciously have been following the kind of scheme that that theory suggests. But it's still an interesting area to speculate about, I agree.
I have read some of Planet Narnia, but I wonder about it too, though the book is very well researched, it builds a lot on an a few comments Lewis made to a former student. And Lewis isn't around to 'blow the gaff' if one has been made. The Planets are very much part of The Cosmic Trilogy however.
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