I just finished reading all the Narnia books to my kids and ran across this little bit from the professor Digory Kirk—my lifelong role model—towards the end of The Last Battle:
Listen, Peter. When Aslan said you could never go back to Narnia, he meant the Narnia you were thinking of. But that was not the real Narnia. That had a beginning and an end. It was only a shadow or a copy of the real Narnia which has always been here and always will be here: just as our world, England and all, is only a shadow or copy of something in Aslan’s real world. You need not mourn over Narnia, Lucy. All of the old Narnia that mattered, all the dear creatures, have been drawn into the real Narnia through the Door. And of course it is different; as different as a real thing is from a shadow or as waking life is from a dream…It’s all in Plato, all in Plato: bless me, what do they teach them at these schools!
Of course, if you have ever taken my ancient and medieval philosophy class you will know that I try to resist the urge of thinking of the material world in Plato as a copy of some higher material world populated by the Forms. That is to say, the Form of White (if there are Forms of sensible qualities for Plato) is not a perfectly white object up in Platonic heaven. Rather, it is no color at all since it is not an object that can be seen.
That being said, I think Lewis rightly latches onto several key features of Platonism in the Narnia books that historically have had and ought to have had a grip on Christian thinkers and we see coming out in a particularly lovely way in this section of The Last Battle: (i) there are gradations of being, i.e. such a thing as being more or less real, (ii) our familiar world is not the most real, (iii) but that does not make it altogether something to be despised either, since what we find good and beautiful in this world is a reflection or echo of something deeper and more real, (iv) even though this world is good, we should not mourn its final loss because all that is good and beautiful in it is preserved at a higher level. If you ever catch me saying that I am a Platonist (I do on occasion) it is usually these features which I, like Lewis, mean.