The Poetics by Aristotle
Every tragedy falls into two parts,--Complication and Unravelling or Denouement. Incidents extraneous to the action are frequently combined with a portion of the action proper, to form the Complication; the rest is the Unravelling. By the Complication I mean all that extends from the beginning of the action to the part which marks the turning-point to good or bad fortune. The Unravelling is that which extends from the beginning of the change to the end. Thus, in the Lynceus of Theodectes, the Complication consists of the incidents presupposed in the drama, the seizure of the child, and then again The Unravelling extends from the accusation of murder to the end.
There are four kinds of Tragedy, the Complex, depending entirely on Reversal of the Situation and Recognition; the Pathetic (where the motive is passion),--such as the tragedies on Ajax and Ixion; the Ethical (where the motives are ethical),--such as the Phthiotides and the Peleus. The fourth kind is the Simple We here exclude the purely spectacular element, exemplified by the Phorcides, the Prometheus, and scenes laid in Hades. The poet should endeavour, if possible, to combine all poetic elements; or failing that, the greatest number and those the most important; the more so, in face of the cavilling criticism of the day. For whereas there have hitherto been good poets, each in his own branch, the critics now expect one man to surpass all others in their several lines of excellence.
In speaking of a tragedy as the same or different, the best test to take is the plot. Identity exists where the Complication and Unravelling are the same. Many poets tie the knot well, but unravel it ill. Both arts, however, should always be mastered.
Again, the poet should remember what has been often said, and not make an Epic structure into a Tragedy--by an Epic structure I mean one with a multiplicity of plots--as if, for instance, you were to make a tragedy out of the entire story of the Iliad. In the Epic poem, owing to its length, each part assumes its proper magnitude. In the drama the result is far from answering to the poet's expectation. The proof is that the poets who have dramatised the whole story of the Fall of Troy, instead of selecting portions, like Euripides; or who have taken the whole tale of Niobe, and not a part of her story, like Aeschylus, either fail utterly or meet with poor success on the stage. Even Agathon has been known to fail from this one defect. In his Reversals of the Situation, however, he shows a marvellous skill in the effort to hit the popular taste,--to produce a tragic effect that satisfies the moral sense. This effect is produced when the clever rogue, like Sisyphus, is outwitted, or the brave villain defeated. Such an event is probable in Agathon's sense of the word: 'it is probable,' he says, 'that many things should happen contrary to probability.'
The Chorus too should be regarded as one of the actors; it should be an integral part of the whole, and share in the action, in the manner not of Euripides but of Sophocles. As for the later poets, their choral songs pertain as little to the subject of the piece as to that of any other tragedy. They are, therefore, sung as mere interludes, a practice first begun by Agathon. Yet what difference is there between introducing such choral interludes, and transferring a speech, or even a whole act, from one play to another?