In this passage, written in 1925, writer Edith Wharton distinguishes between subjects suitable for short stories and those suitable for novels.

It is sometimes said that a “good subject” for a short story should always be capable of being expanded into a novel. The principle may be defendable in special cases; but it is certainly a misleading one on which to build any general theory. Every “subject” (in the novelist’s sense of the term) must necessarily contain within itself its own dimensions; and one of the fiction-writer’s essential gifts is that of discerning whether the subject which presents itself to him, asking for incarnation, is suited to the proportions of a short story or of a novel. If it appears to be adapted to both the chances are that it is inadequate to either. It would be a great mistake, however, to try to base a hard-and-fast theory on the denial of the rule as on its assertion. Instances of short stories made out of subjects that could have been expanded into a novel, and that are yet typical short stories and not mere stunted novels, will occur to everyone. General rules in art are useful chiefly as a lamp in a mine, or a handrail down a black stairway; they are necessary for the sake of the guidance they give, but it is a mistake, once they are formulated, to be too much in awe of them. There are at least two reasons why a subject should find expression in novel-form rather than as a tale; but neither is based on the number of what may be conveniently called incidents, or external happenings, which the narrative contains. There are novels of action which might be condensed into short stories without the loss of their distinguishing qualities. The marks of the subject requiring a longer development are, first, the gradual unfolding of the inner life of its characters, and secondly the need of producing in the reader’s mind the sense of the lapse of time. Outward events of the most varied and exciting nature may without loss of probability be crowded into a few hours, but moral dramas usually have their roots deep in the soul, their rise far back in time; and the suddenest-seeming clash in which they culminate should be led up to step by step if it is to explain and justify itself. There are cases, indeed, when the short story may make use of the moral drama at its culmination. If the incident dealt with be one which a single retrospective flash sufficiently lights up, it is qualified for use as a short story; but if the subject be so complex, and its successive phases so interesting, as to justify elaboration, the lapse of time must necessarily be suggested, and the novel-form becomes appropriate. The effect of compactness and instantaneity sought in the short story is attained mainly by the observance of two “unities”—the old traditional one of time, and that other, more modern and complex, which requires that any rapidly enacted episode shall be seen through only one pair of eyes . . . . One thing more is needful for the ultimate effect of probability; and that is, never let the character who serves as reflector record anything not naturally within his register. It should be the storyteller’s first care to choose this reflecting mind deliberately, as one would choose a building-site, or decide upon the orientation of one’s house, and when this is done, to live inside the mind chosen, trying to feel, see and react exactly as the latter would, no more, no less, and, above all, no otherwise. Only thus can the writer avoid attributing incongruities of thought and metaphor to his chosen interpreter.