It probably won't come as a surprise to you that Tolkien's creation story for Middle Earth, "Ainulindale," and C.S. Lewis' novel about the beginning of Narnia, The Magician's Nephew closely resemble each other. Like many creation myths, both Narnia and Middle-earth start out in a sort of 'nothingness' and both tales explain how the material and spiritual world come into being.
There are, however, important differences in each creation story. As the narrator in The Magician's Nephew is quick to point out, “For what you see and hear depends a good deal on where your standing” (125). Although there are clear similarities in the way each author describes his world's creation, there is an important and fundamental difference in “where one is standing” in each text. Tolkien's “Ainulindale” places the reader among Illuvatar and the Ainur, and internal growth within the world's creators plays a significant role. In Lewis' The Magician's Nephew however, the reader is placed alongside Digory and Polly who visit Narnia as outsides and are thus external to its creation.
Narnia, however, is fundamentally different than Middle Earth in that it is but one world among many. As such, Digory and Polly themselves are external to it. Unlike the Ainur who come to know much of illuvatar and are told much of “what is, what was, and is to come” (18) in the world, the children and their party enter Narnia quite literally in the dark. They do not take an active part in creating Narnia, but rather hear a voice singing that “seemed to come from all directions at once” (98) and watch as “a thousand, thousand points of light leaped out” (99). Illuvatar may have direct contact with the Ainur before Middle Earth is created, but Aslan lets the children witness Narnia's creation from the outside, revealing himself only after the world of Narnia takes shape. This has two effects that are very different from what we see in “Ainulindale:"The first is that unlike the Ainur, the children can see that Narnia is being brought to life with music. For, just as the singing voice creates “the mightiest and most glorious sound it had yet produced, the sun arose” (101).
Digory and Polly not only learn of Aslan and Narnia differently than the Ainur learn of Illuvatar and Middle Earth, but they have an equally different experience with the creation of evil. The children expose Narnia to evil by bringing in Jadis. But in “Ainulindale” the evil of Melkor is weaved within the creation of the world itself. As the music progresses, Melkor seeks “to increase the power and glory of the part assigned to himself” (16), creating immediate discord among the Ainur. Illuvatar allows for within its very conception.