“Inconceivable events and conditions form a class apart from all other story elements, and cannot be made convincing by any mere process of casual narration. They have the handicap of incredibility to overcome; and this can be accomplished only through a careful realism in every other phase of the story, plus a gradual atmospheric or emotional build-up of the utmost subtlety. The emphasis, too, must be kept right—hovering always over the wonder of the central abnormality itself. It must be remembered that any violation of what we know as natural law is in itself a far more tremendous thing than any other event or feeling which could possibly affect a human being. Therefore in a story dealing with such a thing we cannot expect to create any sense of life or illusion of reality if we treat the wonder casually and have the characters moving about under ordinary motivations. The characters, though they must be natural, should be subordinated to the central marvel around which they are grouped. The true "hero" of a marvel tale is not any human being, but simply a set of phenomena. Over and above everything else should tower the stark, outrageous monstrousness of the one chosen departure from Nature. The characters should react to it as real people would react to such a thing if it were suddenly to confront them in daily life; displaying the almost soul-shattering amazement which anyone would naturally display instead of the mild, tame, quickly-passed-over emotions prescribed by cheap popular convention. Even when the wonder is one to which the characters are assumed to be used, the sense of awe, marvel, and strangeness which the reader would feel in the presence of such a thing must somehow be suggested by the author.... Atmosphere, not action, is the thing to cultivate in the wonder story. We cannot put stress on the bare events, since the unnatural extravagance of these events makes them sound hollow and absurd when thrown into too high relief. Such events, even when theoretically possible or conceivable in the future, have no counterpart or basis in existing life and human experience, hence can never form the groundwork of an adult tale. All that a marvel story can ever be, in a serious way, is a vivid picture of a certain type of human mood. The moment it tries to be anything else it becomes cheap, puerile, and unconvincing. Therefore a fantastic author should see that his prime emphasis goes into subtle suggestion—the imperceptible hints and touches of selective and associative detail which express shadings of moods and build up a vague illusion of the strange reality of the unreal—instead of into bald catalogues of incredible happenings which can have no substance or meaning apart from a sustaining cloud of colour and mood-symbolism. A serious adult story must be true to something in life. Since marvel tales cannot be true to the events of life, they must shift their emphasis toward something to which they can be true; namely, certain wistful or restless moods of the human spirit, wherein it seeks to weave gossamer ladders of escape from the galling tyranny of time, space, and natural laws.”

"Some Notes on Interplanetary Fiction", Californian 3, No. 3 (Winter 1935): 39-42. Published in Collected Essays, Volume 2: Literary Criticism edited by S. T. Joshi, p. 178
Non-Fiction