VI.2 A tragedy, then, is the imitation of an action that is serious...in a dramatic, not in a narrative form; with incidents arousing pity and fear, with which to accomplish its catharsis of such emotions...
  
IX.3-4...poetry is something more philosophic and of graver import than history: for poetry tends to express the universal, history the particular. By the universal I mean how a person of a certain type will probably or necessarily speak or act--which is the aim of poetry.
  
IX.11-12 Tragedy is an imitation ...of incidents arousing pity and fear. Such incidents have the very greatest effect on the mind when they occur unexpectedly and at the same time in consequence of one another; they arouse more awe than if they happened accidentally and by chance...
  
X.1-3 Plots are either simple or complex, since the actions they represent naturally are characterized by a similar difference. The action, proceeding in the way defined, as one continuous whole, I call simple, when the change in the hero's fortune takes place without peripety ('reversal of fortune') or discovery; and complex, when it involves one or the other, or both. These should each of them arise out of the structure of the plot itself, so as to be the consequence, necessary or probable, of the preceding action. It makes a great difference whether what happens is caused by the preceding action of just follows it.
  
XI.1-5 A peripety is the change of the kind described from one state of things within the plot to its opposite, and that too in the way we are saying, in accordance with probability or necessity; as it is for instance in Oedipus; here the opposite state of things is produced by the Messenger, who, coming to gladden Oedipus and to remove his fears as to his mother, reveals the secret of his birth...A discovery is, as the very word implies, a change from ignorance to knowledge, and thus to either love or hate, in the personages marked for good or evil fortune. The finest form of discovery is one attended by peripeties, like that which goes with the discovery in Oedipus. There are no doubt other forms of it; what we have said may happen in a way in reference to discover whether some one has done or not done something. But the form most directly connected with the plot and the action is the first mentioned. This, with a peripety, will arouse either pity or fear--actions of that nature being what Tragedy is assumed to represent; and it will also serve to bring about the happy or unhappy ending. The discovery, then, being of persons, it may happen that one person only is recognized by the other, the latter being already known, or it may be necessary that the recognition take place on both sides...
  
XIII.2-3...there are three forms of plot to be avoided. (1) A good man must not be seen passing from happiness to misery, or (2) a bad man from misery to happiness. The first situation is not fear-inspiring or piteous, but simply odious to us. The second is the most untragic that can be; it has no one of the requisites of Tragedy; it does not appeal to the human feeling in us, or to our pity, or to our fears. Nor, on the other hand, should (3) an extremely bad man be seen falling from happiness into misery. Such a story may arouse the human feeling in us, but it will not move us to either fear or pity; we feel pity for a man who does not deserve his misfortune; we fear for someone like ourselves; neither feeling is involved here. There remains, then, the intermediate kind of personage, a man not preeminently virtuous or just, whose misfortune, however, is brought upon him not by vice or depravity but by some error of judgment... He should be famous and prosperous like Oedipus...and the noted men of such noble families.