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.