Aristotle on the Oidipous Tyrannos
Excerpts from the Poetica
A reversal (peripeteia) is a change of the situation into its opposite (from good fortune to bad or from bad to good), as has been said, and in a way that is either plausible or inevitable, as has also been said. In the Oidipous, for example, the messenger who comes to gladden Oidipous and release him from his fears about his mother, actually produces just the opposite effect through revealing the secret of his birth.
A discovery (ana-gnorisis), as the word itself indicates, is a change from ignorance to knowledge, and thereby to either friendship or enmity, according as the characters involved are marked for good or evil fortune. The best kind of discovery is one combined with reversals, like the reversal in Oidipous. There no doubt other kinds. It is possible to "discover," within our meaning of the word, whether a person has done something or not, and there may even be discoveries most intimately connected with the plot and the dramatic action is such as we have been describing. This kind of discovery, combined with reversal, will arouse either pity or fear; and according to out theory, it is actions productive of such effects that are properly represented in tragedy.
Tragedy at its best, as we have seen, will have not a simple but a complex type of construction, and also it will represent actions arousing pity and fear. Since this latter operation is the distinctive function of tragic mimesis, it evidently follows that there are three types of plot to be avoided. First a thoroughly good man should not be shown passing from prosperity to misfortune, for such a situation does not arouse either pity or fear, but merely offends us by its brutality. Nor, on the other hand, should a bad man be shown passing form adversity to prosperity--a situation neither our pity and fear nor our moral sympathies. Finally, it must not be an utterly wicked man who is shown passing from prosperity to adversity. Such a situation may excite our moral sympathies but it will not arouse our pity nor our fear: pity is aroused by undeserved misfortune, and fear by the misfortune of someone like ourselves, so that an event of the sort described is neither piteous nor fear-inspiring. The remaining case is that of man who is a mean between these extremes: who, though not outstandingly virtuous and just, yet falls into misfortune not through vice or depravity but through some "tragic flaw" (hamartia); and moreover he should be drawn form the ranks of men who have enjoyed great reputation and prosperity, like Oidipous....
Of all discoveries the best is that which arises from the action itself, where the shock of surprise is the outcome of a plausible succession of events. Such is the discovery in Sophocles' Oidipous.