"A skillful literary artist has constructed a tale. If wise, ..." — Edgar Allan Poe quote meaning

A skillful literary artist has constructed a tale. If wise, he has not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no words written, of which the tendency, direct or indirect, is not to the one pre-established design. And by such means, with such care and skill, a picture is at length painted which leaves in the mind of him who contemplates it with a kindred art, a sense of the fullest satisfaction. The idea of the tale has been presented unblemished because undisturbed: and this is an end unattainable by the novel. Undue brevity is just as exceptionable here as in the poem; but undue length is yet more to be avoided.