Of simple Plots and actions the episodic are the worst. I call a Plot episodic when there is neither probability nor necessity in the sequence of episodes. Actions of this sort bad poets construct through their own fault, and good ones on account of the players. His work being for public performance, a good poet often stretches out a Plot beyond its capabilities, and is thus obliged to twist the sequence of incident.
Tragedy, however, is an imitation not only of a complete action, but also 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; there is more of the marvellous in them then than if they happened of themselves or by mere chance. Even matters of chance seem most marvellous if there is an appearance of design as it were in them; as for instance the statue of Mitys at Argos killed the author of Mitys' death by falling down on him when a looker-on at a public spectacle; for incidents like that we think to be not without a meaning. A Plot, therefore, of this sort is necessarily finer than others.
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Plots are either simple or complex, since the actions they represent are naturally of this twofold description. The action, proceeding in the way defined, as one continuous whole, I call simple, when the change in the hero's fortunes takes place without Peripety 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 antecedents. There is a great difference between a thing happening propter hoc and post hoc.
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A Peripety is the change from one state of things within the play to its opposite of the kind described, and that too in the way we are saying, in the probable or necessary sequence of events; 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. And in Lynceus: just as he is being led off for execution, with Danaus at his side to put him to death, the incidents preceding this bring it about that he is saved and Danaus put to death. 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 inanimate things, even things of a very casual kind; and it is also possible to discover whether some one has done or not done something. But the form most directly connected with the Plot and the action of the piece 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 be that of one party only to the other, the latter being already known; or both the parties may have to discover themselves. Iphigenia, for instance, was discovered to Orestes by sending the letter; and another Discovery was required to reveal him to Iphigenia.
Two parts of the Plot, then, Peripety and Discovery, are on matters of this sort. A third part is Suffering; which we may define as an action of a destructive or painful nature, such as murders on the stage, tortures, woundings, and the like. The other two have been already explained.