On Fairy-Stories was subsequently published with Leaf by Niggle in Tree and Leaf, as well as in The Tolkien Reader, published in 1966. The length of the essay, as it appears in Tree and Leaf, is 60 pages, including about ten pages of notes.
The essay is significant because it contains Tolkien's explanation of his philosophy on fantasy and thoughts on mythopoiesis. Moreover, the essay is an early analysis of speculative fiction by one of the most important authors in the genre.
The essay On Fairy-Stories is an attempt to explain and defend the fairy-story genre, which Tolkien names Fairy Stories, and is careful to distinguish from "traveler's tales" (such as Gulliver's Travels), science fiction (such as H.G. Wells' The Time Machine), beast tales (such as Aesop's Fables and Peter Rabbit), and dream stories (such as Alice in Wonderland). One touchstone of the authentic fairy tale is that it is presented as wholly credible. "It is at any rate essential to a genuine fairy-story, as distinct from the employment of this form for lesser or debased purposes, that it should be presented as 'true.' ...But since the fairy-story deals with 'marvels,' it cannot tolerate any frame or machinery suggesting that the whole framework in which they occur is a figment or illusion."
Tolkien observes that "an essential power of Faerie is thus the power of making immediately by the will the visions of 'fantasy.'" He defines fairy stories as not stories about fairies or other supernatural beings, but stories about the interaction between humans and those beings as representative of the numinous. Second, he emphasizes that through the use of fantasy, which he equates with fancy and imagination, the author can bring the reader to experience a world which is consistent and rational, yet utterly strange as well. He calls this “a rare achievement of Art,” and notes that it was important to him as a reader: "It was in fairy-stories that I first divined the potency of the words, and the wonder of things, such as stone, and wood, and iron; tree and grass; house and fire; bread and wine."
Tolkien suggests that fairy stories allow the reader to review his or her own world from the "perspective" of a different world. This concept, which shares much in common with phenomenology, Tolkien calls "recovery," in the sense that one's unquestioned assumptions might be recovered and changed by an outside perspective. Second, he defends fairy stories as offering escapist pleasure to the reader. And third, Tolkien suggests that fairy stories (can) provide moral or emotional consolation, through their happy ending, which he terms a "eucatastrophe."
In conclusion and as expanded upon in an epilogue, Tolkien asserts that a truly good and representative fairy story is marked by joy: "Far more powerful and poignant is the effect [of joy] in a serious tale of Faerie. In such stories, when the sudden turn comes, we get a piercing glimpse of joy, and heart's desire, that for a moment passes outside the frame, rends indeed the very web of story, and lets a gleam come through." Tolkien sees Christianity as partaking in and fulfilling the overarching mythological nature of the cosmos: "I would venture to say that approaching the Christian story from this perspective, it has long been my feeling (a joyous feeling) that God redeemed the corrupt making-creatures, men, in a way fitting to this aspect, as to others, of their strange nature. The Gospels contain a fairy-story, or a story of a larger kind which embraces all the essence of fairy-stories. ...and among its marvels is the greatest and most complete conceivable eucatastrophe. The Birth of Christ is the eucatastrophe of Man's history. The Resurrection is the eucatastrophe of the story of the Incarnation."