Edgar Allan Poe says

Keeping originality always in view — for he is false to himself who ventures to dispense with so obvious and so easily attainable a source of interest — I say to myself, in the first place, “Of the innumerable effects, or impressions, of which the heart, the intellect, or (more generally) the soul is susceptible, what one shall I, on the present occasion, select?” Having chosen a novel, first, and secondly a vivid effect, I consider whether it can best be wrought by incident or tone — whether by ordinary incidents and peculiar tone, or the converse, or by peculiarity both of incident and tone — afterward looking about me (or rather within) for such combinations of event, or tone, as shall best aid me in the construction of the effect. 

I have often thought how interesting a magazine paper might be written by any author who would — that is to say, who could — detail, step by step, the processes by which any one of his compositions attained its ultimate point of completion. Why such a paper has never been given to the world, I am much at a loss to say — but, perhaps, the autorial vanity has had more to do with the omission than any one other cause. Most writers — poets in especial — prefer having it understood that they compose by a species of fine frenzy — an ecstatic intuition — and would positively shudder at letting the public take a peep behind the scenes, at the elaborate and vacillating crudities of thought — at the true purposes seized only at the last moment — at the innumerable glimpses of idea that arrived not at the maturity of full view — at the fully matured fancies discarded in despair as unmanageable — at the cautious selections and rejections — at the painful erasures and interpolations — in a word, at the wheels and pinions — the tackle for scene-shifting — the step-ladders and demon-traps — the cock’s feathers, the red paint and the black patches, which, in ninety-nine cases out of the hundred, constitute the properties of the literary histrio…. 

It is my design to render it manifest that no one point in … composition is referrible either to accident or intuition — that the work proceeded, step by step, to its completion with the precision and rigid consequence of a mathematical problem….

If any literary work is too long to be read at one sitting, we must be content to dispense with the immensely important effect derivable from unity of impression — for, if two sittings be required, the affairs of the world interfere, and every thing like totality is at once destroyed. But since, ceteris paribus, no poet can afford to dispense with any thing that may advance his design, it but remains to be seen whether there is, in extent, any advantage to counterbalance the loss of unity which attends it. Here I say no, at once. What we term a long poem is, in fact, merely a succession of brief ones — that is to say, of brief poetical effects…..

It appears evident, then, that there is a distinct limit, as regards length, to all works of literary art — the limit of a single sitting — and that, although in certain classes of prose composition, such as “Robinson Crusoe,” (demanding no unity,) this limit may be advantageously overpassed, it can never properly be overpassed in a poem. Within this limit, the extent of a poem may be made to bear mathematical relation to its merit — in other words, to the excitement or elevation — again in other words, to the degree of the true poetical effect which it is capable of inducing; for it is clear that the brevity must be in direct ratio of the intensity of the intended effect: — this, with one proviso — that a certain degree of duration is absolutely requisite for the production of any effect at all….

Now I designate Beauty as the province of the poem, merely because it is an obvious rule of Art that effects should be made to spring from direct causes — that objects should be attained through means best adapted for their attainment — no one as yet having been weak enough to deny that the peculiar elevation alluded to, is most readily attained in the poem. Now the object, Truth, or the satisfaction of the intellect, and the object Passion, or the excitement of the heart, are, although attainable, to a certain extent, in poetry, far more readily attainable in prose. Truth, in fact, demands a precision, and Passion, a homeliness (the truly passionate will comprehend me) which are absolutely antagonistic to that Beauty which, I maintain, is the excitement, or pleasurable elevation, of the soul. It by no means follows from any thing here said, that passion, or even truth, may not be introduced, and even profitably introduced, into a poem — for they may serve in elucidation, or aid the general effect, as do discords in music, by contrast — but the true artist will always contrive, first, to tone them into proper subservience to the predominant aim, and, secondly, to enveil them, as far as possible, in that Beauty which is the atmosphere and the essence of the poem….

But in subjects so handled, however skilfully, or with however vivid an array of incident, there is always a certain hardness or nakedness, which repels the artistical eye. Two things are invariably required — first, some amount of complexity, or more properly, adaptation; and, secondly, some amount of suggestiveness — some undercurrent, however indefinite of meaning. It is this latter, in especial, which imparts to a work of art so much of that richness (to borrow from colloquy a forcible term) which we are too fond of confounding with the ideal. It is the excess of the suggested meaning — it is the rendering this the upper instead of the undercurrent of the theme — which turns into prose (and that of the very flattest kind) the so called poetry of the so called transcendentalists.