One significant factor in the shaping of Tolkien’s Middle-earth legendarium, and one that begins to address more fully the role of metaphysics in his fiction, was Tolkien’s desire to do what he believed all fairy-stories must do, namely fashion a coherent “secondary world” into which the reader can imaginatively enter and even “escape.” As Tolkien argues in his programmatic essay “On Fairy-Stories,” one of the fundamental criteria of any sub-created, secondary world is that it must possess what he calls the “inner consistency of reality.” Fantasy on Tolkien’s definition involves the “making or glimpsing of Other-worlds” which, like our own world, must have a logic or order of their own, so that
a successful ‘sub-creator’… makes a Secondary World which your mind can enter. Inside it, what he relates is ‘true’: it accords with the laws of that world. You therefore believe it, while you are, as it were, inside. The moment disbelief arises, the spell is broken; the magic, or rather art, has failed. You are then out in the Primary World again, looking at the little abortive Secondary World from outside. (TR 60)
This particular criterion was so important for Tolkien that, in first developing and then afterward explaining and revising his own extended fairy-story, he was often preoccupied—one might almost say obsessed—with ensuring and defending the rational coherence of his mythical world. Aware of his own compulsive tendencies in this regard, and after having momentarily indulged them while writing to one correspondent, Tolkien concluded his letter with this apology: “I am sorry if this all seems dreary and ‘pompose’. But so do all attempts to ‘explain’ the images and events of a mythology. Naturally the stories come first. But it is, I suppose, some test of the consistency of a mythology as such, if it is capable of some sort of rational or rationalized explanation” (L 260). In Tolkien’s mind, the artistic success of his fiction hinged on his ability to demonstrate the rational coherence of his mythology. It was thus an authentic unity of artistic and mythic imagination (mythos) on the one hand and philosophical rationality (logos) on the other that Tolkien sought to achieve in his own mythology and which he even represented symbolically in one of the central images of his legendarium, the Light of the Two Trees of Valinor, from which the primary sources of light in Tolkien’s fictional world—the sun, moon, stars, Silmaril jewels, and even the phial of Galadrial which Frodo takes with him into Mordor—have their origin. As Tolkien writes, “The Light of Valinor (derived from light before any fall) is the light of art undivorced from reason, that sees things both scientifically (or philosophically) and imaginatively (or subcreatively) and says that they are good—as beautiful” (L 148n). For Tolkien, good fairy-stories may be as much a matter of reason as they are of the imagination, so that good fantasy necessitates good philosophy, and good mythology requires a good metaphysics.
 After waxing metaphysical in another letter, Tolkien similarly confessed to his correspondent: “Now (you will reasonably say) I am taking myself even more seriously than you did, and making a great song and oration about a good tale, which admittedly owes its similitude to mere craft. It is so. But the things I have scribbled about, arise in some form or another from all writing (or art) that is not careful to dwell within the walls of ‘observed fact’” (L 196). Even Tolkien had second thoughts about his own metaphysical digressions, as the letter ultimately went unfinished and unsent. He wrote in explanation at the top of the draft, “It seemed to be taking myself too importantly” (L 196).