Is Lewis's fiction allegorical?

C. S. Lewis denied that his Christian fiction was allegorical. Some people find that puzzling, because, on one definition, some of his fiction is allegorical. Clearly he was operating with a specialized definition of allegory. In a letter to a correspondent, he gives a fairly detailed explanation of what he means:

-----------------------------------------------------------------------------------------

By an allegory I mean a composition (whether pictorial or literary) in which immaterial realities are represented by feigned physical objects e.g. a pictured Cupid allegorically represents erotic love (which in reality is an experience, not an object occupying a given area of space) or, in Bunyan, a giant that represents Despair.

If Aslan represented the immaterial Deity in the same way in which Giant Despair represents despair, he would be an allegorical figure. In reality however he is an invention giving an imaginary answer to the question, "What might Christ become like if there really were a world like Narnia, and He chose to be incarnate and die and rise again in that world as He actually has done in ours?" This is not an allegory at all So in Perelandra. This also works out a supposition. ("Suppose, even now, in some other planet there were a first couple undergoing the same that Adam and Eve underwent here, but successfully.")

Allegory and such supposals differ because they mix the real and the unreal in different ways. Bunyan's picture of Giant Despair does not start from a supposal at all. It is not a supposition but a fact that despair can capture and imprison a human soul. What is unreal or (fictional) is the giant, the castle, and the dungeon. The Incarnation of Christ in another world is mere supposal: but granted the supposition, He would really have been a physical object in that world as He was in Palestine and His death on the Stone Table would have been a physical event no less than his death on Calvary.

Similarly, if the angels (who I believe to be real beings in the actual universe) have that relation to the Pagan gods which they are assumed to have in Perelandra, they might really manifest themselves in real form as they did to Ransom. 

Again, Ransom (to some extent) plays the role of Christ not because he allegorically represents him (as Cupid represents falling in love) but because in reality every real Christian is really called upon in some measure to enact Christ. Of course Ransom does this rather more spectacularly than most. But that does not mean that he does it allegorically. It only means that fiction (at any rate my kind of fiction) choose extreme cases. The Collected Letters of C. S. Lewis, Volume lll: Narnia, Cambridge, and Joy 1950-1963 (HarperOne, 2007),1004-05.