Outline of Aristotle's Theory of Tragedy in the
POETICS
Definition of Tragedy: “Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; with incidents arousing pity and fear, wherewith to accomplish its katharsis of such emotions. . . . Every Tragedy, therefore, must have six parts, which parts determine its quality—namely, Plot, Characters, Diction, Thought, Spectacle, Melody.” (translation by S. H. Butcher; click on the context links to consult the full online text)
The treatise we call the Poetics was composed at least 50 years after the death of Sophocles. Aristotle was a great admirer of Sophocles’ Oedipus the King, considering it the perfect tragedy, and not surprisingly, his analysis fits that play most perfectly. I shall therefore use this play to illustrate the following major parts of Aristotle's analysis of tragedy as a literary genre.

Tragedy is the “imitation of an action” (mimesis) according to “the law of probability or necessity.” Aristotle indicates that the medium of tragedy is drama, not narrative; tragedy “shows” rather than “tells.” According to Aristotle, tragedy is higher and more philosophical than history because history simply relates what has happened while tragedy dramatizes what may happen, “what is possibile according to the law of probability or necessity.” History thus deals with the particular, and tragedy with the universal. Events that have happened may be due to accident or coincidence; they may be particular to a specific situation and not be part of a clear cause-and-effect chain. Therefore they have little relevance for others. Tragedy, however, is rooted in the fundamental order of the universe; it creates a cause-and-effect chain that clearly reveals what mayhappen at any time or place because that is the way the world operates. Tragedy therefore arouses not only pity but also fear, because the audience can envision themselves within this cause-and-effect chain (context).

Plot is the “first principle,” the most important feature of tragedy. Aristotle defines plot as “the arrangement of the incidents”: i.e., not the story itself but the way the incidents are presented to the audience, the structure of the play. According to Aristotle, tragedies where the outcome depends on a tightly constructed cause-and-effect chain of actions are superior to those that depend primarily on the character and personality of the protagonist. Plots that meet this criterion will have the following qualities (context). See Freytag's Triangle for a diagram that illustrates Aristotle's ideal plot structure, and Plot of Oedipus the King for an application of this diagram to Sophocles’ play.
1. The plot must be “a whole,” with a beginning, middle, and end. The beginning, called by modern critics the incentive moment, must start the cause-and-effect chain but not be dependent on anything outside the compass of the play (i.e., its causes are downplayed but its effects are stressed). The middle, or climax, must be caused by earlier incidents and itself cause the incidents that follow it (i.e., its causes and effects are stressed). The end, or resolution, must be caused by the preceding events but not lead to other incidents outside the compass of the play (i.e., its causes are stressed but its effects downplayed); the end should therefore solve or resolve the problem created during the incentive moment (context). Aristotle calls the cause-and-effect chain leading from the incentive moment to the climax the “tying up” (desis), in modern terminology the complication. He therefore terms the more rapid cause-and-effect chain from the climax to the resolution the “unravelling” (lusis), in modern terminology the dénouement (context).
2. The plot must be “complete,” having “unity of action.” By this